From cb73547af5178dc2d9c4353cce6c51130219fee9 Mon Sep 17 00:00:00 2001
From: shankar0123 <skreddy040@gmail.com>
Date: Sun, 10 May 2026 23:18:23 +0000
Subject: [PATCH] =?UTF-8?q?harden(oidc):=20pre-login=20UA/IP=20binding=20(?=
 =?UTF-8?q?MED-16)=20=E2=80=94=20RFC=209700=20=C2=A74.7.1?=
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Content-Transfer-Encoding: 8bit

Audit 2026-05-10 MED-16 closure.

WHAT.

Binds the OIDC pre-login row to the (clientIP, userAgent) tuple of
the /auth/oidc/login request, and enforces a constant-time compare
against the /auth/oidc/callback request at consume time. Defeats
replay of a stolen pre-login cookie by a different browser /
source — the secondary defense layer recommended by RFC 9700 §4.7.1
when the primary layer (HMAC integrity + Path=/ + SameSite=Lax on
the cookie) is bypassed via CSRF / XSS / TLS-termination leak.

WHY.

Pre-fix, the pre-login cookie's HMAC verified only that 'some'
caller of /auth/oidc/login was talking to /auth/oidc/callback; it
did not verify that the SAME browser / source was on both sides.
An attacker who exfiltrated the cookie value via any vector could
replay the bytes through their own user-agent and ride the victim's
authorization. RFC 9700 §4.7.1 calls out the gap explicitly and
recommends binding state to a user-agent fingerprint + source IP.

HOW.

Migration:
  migrations/000044_prelogin_uaip.up.sql
    ALTER TABLE oidc_pre_login_sessions
      ADD COLUMN IF NOT EXISTS client_ip   TEXT,
      ADD COLUMN IF NOT EXISTS user_agent  TEXT;
  Both nullable for in-flight rolling-deploy compat — the consume-
  side check only enforces when both row AND request carry non-empty
  values for the leg in question.

Domain:
  internal/repository/oidc.go (PreLoginSession) — adds ClientIP +
    UserAgent fields.

Repository:
  internal/repository/postgres/oidc_prelogin.go — Create persists
    via sql.NullString (empty → NULL); LookupAndConsume reads back.
    Re-uses package-local nullableString from discovery.go.

Service:
  internal/auth/oidc/service.go
    - PreLoginStore.CreatePreLogin signature takes (clientIP,
      userAgent) as positions 5–6.
    - PreLoginStore.LookupAndConsume returns (clientIP, userAgent)
      as positions 5–6.
    - HandleAuthRequest signature gains (clientIP, userAgent),
      threaded to the store.
    - HandleCallback adds Step 1.5 — UA / IP constant-time compare
      between stored row and incoming request. Per-leg toggles via
      preLoginRequireUA / preLoginRequireIP service fields. Empty
      values on either side pass through (rolling-deploy + headless-
      proxy compat).
    - New sentinels ErrPreLoginUAMismatch, ErrPreLoginIPMismatch.
    - SetPreLoginBindingRequirements(requireUA, requireIP) helper
      for main.go config wiring.

Adapter:
  internal/auth/oidc/prelogin.go — PreLoginAdapter passes the new
    fields through to the repo row.

Handler:
  internal/api/handler/auth_session_oidc.go
    - OIDCAuthHandshaker.HandleAuthRequest signature updated.
    - LoginInitiate captures clientIPFromRequest + r.UserAgent()
      and passes to the service.
    - classifyOIDCFailure adds errors.Is dispatch for the two new
      sentinels → prelogin_ua_mismatch / prelogin_ip_mismatch
      audit categories.

Config:
  internal/config/config.go
    + AuthConfig.OIDCPreLoginRequireUA (default true)
      env CERTCTL_OIDC_PRELOGIN_REQUIRE_UA
    + AuthConfig.OIDCPreLoginRequireIP (default true)
      env CERTCTL_OIDC_PRELOGIN_REQUIRE_IP
  cmd/server/main.go calls oidcService.SetPreLoginBindingRequirements
    from cfg.Auth.OIDCPreLoginRequire{UA,IP}.

Tests (internal/auth/oidc/service_test.go):
  - TestService_HandleCallback_MED16_UAMismatchRejected
  - TestService_HandleCallback_MED16_IPMismatchRejected
  - TestService_HandleCallback_MED16_BothMatch_Succeeds
  - TestService_HandleCallback_MED16_LegacyRowEmptyValues  (rolling-
    deploy compat — empty stored values pass through)
  - TestService_HandleCallback_MED16_RequireUAFalse_AllowsMismatch
    (operator escape-hatch — UA mismatch silently allowed)

Mechanical fan-out:
  - stubPreLogin / stubPreLoginRepo signatures updated.
  - All existing call sites in service_test.go (~40), prelogin_test.go,
    bench_test.go, logging_test.go, provider_enabled_test.go,
    integration_keycloak_test.go, integration_okta_smoke_test.go,
    auth_session_oidc_test.go updated to pass empty strings for the
    new params — pre-existing tests do not exercise UA/IP binding
    semantics.

VERIFY.

- go vet ./internal/auth/oidc/... ./internal/api/handler/...
  ./internal/config/...                                       PASS
- go test -short -count=1 -run MED16 ./internal/auth/oidc/... PASS (5/5)
- go test -short -count=1 ./internal/auth/oidc/...            PASS (4.6s)
- go test -short -count=1 ./internal/api/handler/...          PASS (4.3s)
- go test -short -count=1 ./internal/config/...               PASS

Refs: cowork/auth-bundles-audit-2026-05-10.md MED-16
      cowork/auth-bundles-fixes-2026-05-10/HANDOFF.md item 6
      RFC 9700 §4.7.1 — OAuth 2.0 Security Best Current Practice
---
 CHANGELOG.md                                  |  15 ++
 cmd/server/main.go                            |   6 +
 internal/api/handler/auth_session_oidc.go     |  21 +-
 .../api/handler/auth_session_oidc_test.go     |   2 +-
 internal/auth/oidc/bench_test.go              |   2 +-
 .../auth/oidc/integration_keycloak_test.go    |  10 +-
 .../auth/oidc/integration_okta_smoke_test.go  |   2 +-
 internal/auth/oidc/logging_test.go            |   4 +-
 internal/auth/oidc/prelogin.go                |  28 ++-
 internal/auth/oidc/prelogin_test.go           |  40 ++--
 internal/auth/oidc/provider_enabled_test.go   |   4 +-
 internal/auth/oidc/service.go                 |  86 +++++++-
 internal/auth/oidc/service_test.go            | 199 +++++++++++++-----
 internal/config/config.go                     |  21 ++
 internal/repository/oidc.go                   |   9 +
 internal/repository/postgres/oidc_prelogin.go |  44 +++-
 migrations/000044_prelogin_uaip.down.sql      |   4 +
 migrations/000044_prelogin_uaip.up.sql        |  47 +++++
 18 files changed, 441 insertions(+), 103 deletions(-)
 create mode 100644 migrations/000044_prelogin_uaip.down.sql
 create mode 100644 migrations/000044_prelogin_uaip.up.sql

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b9ae390..8e4d4c4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -34,6 +34,21 @@
   RFC-9207 discovery. Providers that don't advertise support (the majority
   today) keep pre-fix behavior — back-compat is preserved.
 
+- **Pre-login UA / source-IP binding on OIDC callback (Audit 2026-05-10
+  MED-16).** RFC 9700 §4.7.1 defense against stolen-pre-login-cookie replay
+  by a different browser / source. Migration `000044_prelogin_uaip` adds
+  `client_ip` + `user_agent` to `oidc_pre_login_sessions`; values captured at
+  `/auth/oidc/login` are constant-time compared at `/auth/oidc/callback`.
+  Mismatches return HTTP 400 with audit `failure_category` =
+  `prelogin_ua_mismatch` or `prelogin_ip_mismatch`. Two operator escape
+  hatches: `CERTCTL_OIDC_PRELOGIN_REQUIRE_UA` and
+  `CERTCTL_OIDC_PRELOGIN_REQUIRE_IP` (both default `true`) — operators on
+  enterprise proxies that rewrite UA, or dual-stack v4/v6 environments where
+  source IP routinely flips, can disable the affected leg. The binding column
+  is persisted even when enforcement is off, so retroactive forensics remain
+  possible. Empty values on either side pass through (rolling-deploy +
+  headless-proxy compat).
+
 ## v2.1.0 - Auth Bundles 1 + 2: RBAC primitive + OIDC SSO + sessions ⚠️
 
 > **SECURITY: AUDIT YOUR API KEYS.**
diff --git a/cmd/server/main.go b/cmd/server/main.go
index dc8ada4..100c633 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -421,6 +421,12 @@ func main() {
 		preLoginAdapter,
 		cfg.Encryption.ConfigEncryptionKey,
 	)
+	// Audit 2026-05-10 MED-16 — apply per-leg pre-login UA / IP
+	// binding enforcement toggles from config.
+	oidcService.SetPreLoginBindingRequirements(
+		cfg.Auth.OIDCPreLoginRequireUA,
+		cfg.Auth.OIDCPreLoginRequireIP,
+	)
 	// SameSite resolution from CERTCTL_SESSION_SAMESITE (default Lax;
 	// "Strict" for high-security environments at the cost of breaking
 	// inbound deep-links from external apps).
diff --git a/internal/api/handler/auth_session_oidc.go b/internal/api/handler/auth_session_oidc.go
index b87414a..0a41013 100644
--- a/internal/api/handler/auth_session_oidc.go
+++ b/internal/api/handler/auth_session_oidc.go
@@ -55,7 +55,10 @@ import (
 // OIDCAuthHandshaker is the slice of *oidc.Service the OIDC HTTP path
 // consumes. Phase 3's *oidc.Service satisfies this directly.
 type OIDCAuthHandshaker interface {
-	HandleAuthRequest(ctx context.Context, providerID string) (authURL, cookieValue, preLoginID string, err error)
+	// Audit 2026-05-10 MED-16 — clientIP + userAgent persist into the
+	// pre-login row so HandleCallback can reject mismatches at consume
+	// time (RFC 9700 §4.7.1 binding).
+	HandleAuthRequest(ctx context.Context, providerID, clientIP, userAgent string) (authURL, cookieValue, preLoginID string, err error)
 	// Audit 2026-05-10 MED-17 — callbackIss carries the value of the
 	// RFC 9207 `iss` query parameter on /auth/oidc/callback (empty
 	// string when the IdP doesn't send it). The service enforces the
@@ -233,7 +236,14 @@ func (h *AuthSessionOIDCHandler) LoginInitiate(w http.ResponseWriter, r *http.Re
 		Error(w, http.StatusBadRequest, "missing required query parameter `provider`")
 		return
 	}
-	authURL, cookieValue, _, err := h.oidcSvc.HandleAuthRequest(r.Context(), providerID)
+	// Audit 2026-05-10 MED-16 — capture clientIP + UA at /auth/oidc/login
+	// so HandleCallback can reject a stolen pre-login cookie replayed
+	// from a different browser/source. clientIPFromRequest already
+	// honours the LOW-5 trusted-proxy gating; r.UserAgent() reads the
+	// header verbatim.
+	loginIP := clientIPFromRequest(r)
+	loginUA := r.UserAgent()
+	authURL, cookieValue, _, err := h.oidcSvc.HandleAuthRequest(r.Context(), providerID, loginIP, loginUA)
 	if err != nil {
 		// Provider not found is the most common case; map to 404.
 		if errors.Is(err, repository.ErrOIDCProviderNotFound) {
@@ -1178,6 +1188,9 @@ func classifyOIDCFailure(err error) string {
 		return "ok"
 	}
 	// Audit 2026-05-10 MED-17 — typed dispatch for the iss family.
+	// Audit 2026-05-10 MED-16 — typed dispatch for the UA/IP binding
+	// family (no substring guarantees because UA strings are operator
+	// data and could match anything).
 	switch {
 	case errors.Is(err, oidcsvc.ErrIssParamMissing):
 		return "iss_param_missing"
@@ -1185,6 +1198,10 @@ func classifyOIDCFailure(err error) string {
 		return "iss_param_mismatch"
 	case errors.Is(err, oidcsvc.ErrIssuerMismatch):
 		return "id_token_iss_mismatch"
+	case errors.Is(err, oidcsvc.ErrPreLoginUAMismatch):
+		return "prelogin_ua_mismatch"
+	case errors.Is(err, oidcsvc.ErrPreLoginIPMismatch):
+		return "prelogin_ip_mismatch"
 	}
 	msg := strings.ToLower(err.Error())
 	switch {
diff --git a/internal/api/handler/auth_session_oidc_test.go b/internal/api/handler/auth_session_oidc_test.go
index 9017016..4e2c406 100644
--- a/internal/api/handler/auth_session_oidc_test.go
+++ b/internal/api/handler/auth_session_oidc_test.go
@@ -43,7 +43,7 @@ type stubOIDCSvc struct {
 	refreshErr  error
 }
 
-func (s *stubOIDCSvc) HandleAuthRequest(_ context.Context, _ string) (string, string, string, error) {
+func (s *stubOIDCSvc) HandleAuthRequest(_ context.Context, _, _, _ string) (string, string, string, error) {
 	return s.authURL, s.cookie, s.preLoginID, s.authReqErr
 }
 func (s *stubOIDCSvc) HandleCallback(_ context.Context, _, _, _, _, _, _ string) (*oidcsvc.CallbackResult, error) {
diff --git a/internal/auth/oidc/bench_test.go b/internal/auth/oidc/bench_test.go
index fe636d2..158599e 100644
--- a/internal/auth/oidc/bench_test.go
+++ b/internal/auth/oidc/bench_test.go
@@ -94,7 +94,7 @@ func BenchmarkOIDC_SteadyState(b *testing.B) {
 		// Each iteration needs a fresh pre-login row (HandleCallback
 		// consumes the row atomically + single-use). State + nonce +
 		// verifier are stable; the cookie value is unique per call.
-		cookie, _, err := pl.CreatePreLogin(ctx, "op-bench", "bench-state", "test-nonce-fixed", "verifier-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+		cookie, _, err := pl.CreatePreLogin(ctx, "op-bench", "bench-state", "test-nonce-fixed", "verifier-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 		if err != nil {
 			b.Fatalf("CreatePreLogin: %v", err)
 		}
diff --git a/internal/auth/oidc/integration_keycloak_test.go b/internal/auth/oidc/integration_keycloak_test.go
index 7dca852..1139177 100644
--- a/internal/auth/oidc/integration_keycloak_test.go
+++ b/internal/auth/oidc/integration_keycloak_test.go
@@ -420,7 +420,7 @@ func TestKeycloakIntegration_AuthCodeFlow_HappyPath(t *testing.T) {
 	defer cancel()
 
 	// HandleAuthRequest produces the IdP redirect URL + pre-login cookie.
-	authURL, preLoginCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID)
+	authURL, preLoginCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID, "", "")
 	if err != nil {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
@@ -486,7 +486,7 @@ func TestKeycloakIntegration_LogoutRevokesSession(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 	defer cancel()
 
-	authURL, preLoginCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID)
+	authURL, preLoginCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID, "", "")
 	if err != nil {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
@@ -529,7 +529,7 @@ func TestKeycloakIntegration_JWKSRotation_RefreshKeysPicksUpNewKey(t *testing.T)
 	defer cancel()
 
 	// Pre-rotate baseline login.
-	preAuthURL, preCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID)
+	preAuthURL, preCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID, "", "")
 	if err != nil {
 		t.Fatalf("pre-rotate HandleAuthRequest: %v", err)
 	}
@@ -548,7 +548,7 @@ func TestKeycloakIntegration_JWKSRotation_RefreshKeysPicksUpNewKey(t *testing.T)
 
 	// Post-rotate login: Keycloak signs the new token under the new
 	// key (higher priority); the service must validate it.
-	postAuthURL, postCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID)
+	postAuthURL, postCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID, "", "")
 	if err != nil {
 		t.Fatalf("post-rotate HandleAuthRequest: %v", err)
 	}
@@ -573,7 +573,7 @@ func TestKeycloakIntegration_UnmappedGroupsFailsClosed(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 	defer cancel()
 
-	authURL, preCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID)
+	authURL, preCookie, _, err := svc.HandleAuthRequest(ctx, fx.Provider.ID, "", "")
 	if err != nil {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
diff --git a/internal/auth/oidc/integration_okta_smoke_test.go b/internal/auth/oidc/integration_okta_smoke_test.go
index 87ef000..512d125 100644
--- a/internal/auth/oidc/integration_okta_smoke_test.go
+++ b/internal/auth/oidc/integration_okta_smoke_test.go
@@ -121,7 +121,7 @@ func TestOktaSmoke_DiscoveryAndRefreshKeys(t *testing.T) {
 	// the configured Okta issuer. We don't drive the browser login
 	// here — the Keycloak fixture covers full auth-code; this test
 	// only confirms the wire setup against a real Okta tenant.
-	authURL, _, _, err := svc.HandleAuthRequest(ctx, prov.ID)
+	authURL, _, _, err := svc.HandleAuthRequest(ctx, prov.ID, "", "")
 	if err != nil {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
diff --git a/internal/auth/oidc/logging_test.go b/internal/auth/oidc/logging_test.go
index 10e23f9..f3b8ecc 100644
--- a/internal/auth/oidc/logging_test.go
+++ b/internal/auth/oidc/logging_test.go
@@ -50,7 +50,7 @@ func TestLoggingHygiene_HandleAuthRequest_LeaksNothing(t *testing.T) {
 	buf, restore := captureLogger(t)
 	defer restore()
 
-	authURL, cookieValue, _, err := svc.HandleAuthRequest(context.Background(), "op-leak-1")
+	authURL, cookieValue, _, err := svc.HandleAuthRequest(context.Background(), "op-leak-1", "", "")
 	if err != nil {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
@@ -83,7 +83,7 @@ func TestLoggingHygiene_HandleCallback_LeaksNothing(t *testing.T) {
 
 	// Pre-login row with a known verifier we can grep for after.
 	verifier := "test-verifier-do-not-leak-aaaaaaaaaaaaa"
-	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-leak-2", "the-state", "test-nonce-fixed", verifier)
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-leak-2", "the-state", "test-nonce-fixed", verifier, "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
diff --git a/internal/auth/oidc/prelogin.go b/internal/auth/oidc/prelogin.go
index e2f6759..669f7f0 100644
--- a/internal/auth/oidc/prelogin.go
+++ b/internal/auth/oidc/prelogin.go
@@ -87,9 +87,12 @@ func (a *PreLoginAdapter) SetRandReaderForTest(r func([]byte) (int, error)) {
 // value under the active SessionSigningKey, persists the row, and
 // returns the cookie value + the row id.
 //
+// Audit 2026-05-10 MED-16 — clientIP + userAgent are persisted into
+// the row for the callback-time UA/IP binding check.
+//
 // Implements the Phase 3 OIDCService.PreLoginStore.CreatePreLogin
 // interface signature.
-func (a *PreLoginAdapter) CreatePreLogin(ctx context.Context, providerID, state, nonce, verifier string) (cookieValue, sessionID string, err error) {
+func (a *PreLoginAdapter) CreatePreLogin(ctx context.Context, providerID, state, nonce, verifier, clientIP, userAgent string) (cookieValue, sessionID string, err error) {
 	active, err := a.keys.GetActive(ctx, a.tenantID)
 	if err != nil {
 		return "", "", fmt.Errorf("pre-login: get active signing key: %w", err)
@@ -110,6 +113,8 @@ func (a *PreLoginAdapter) CreatePreLogin(ctx context.Context, providerID, state,
 		State:          state,
 		Nonce:          nonce,
 		PKCEVerifier:   verifier,
+		ClientIP:       clientIP,
+		UserAgent:      userAgent,
 	}
 	if err := a.repo.Create(ctx, row); err != nil {
 		return "", "", fmt.Errorf("pre-login: persist row: %w", err)
@@ -132,25 +137,28 @@ func (a *PreLoginAdapter) CreatePreLogin(ctx context.Context, providerID, state,
 //   - Row found but past 10-minute TTL -> ErrPreLoginExpired (row is
 //     deleted at the repo layer regardless).
 //
+// Audit 2026-05-10 MED-16 — also returns the row's stored clientIP +
+// userAgent so the service-layer caller can enforce the UA/IP binding.
+//
 // Implements the Phase 3 OIDCService.PreLoginStore.LookupAndConsume
 // interface signature.
-func (a *PreLoginAdapter) LookupAndConsume(ctx context.Context, cookieValue string) (providerID, state, nonce, verifier string, err error) {
+func (a *PreLoginAdapter) LookupAndConsume(ctx context.Context, cookieValue string) (providerID, state, nonce, verifier, clientIP, userAgent string, err error) {
 	plID, signingKeyID, providedHMAC, perr := session.ParseCookieValue(cookieValue, "pl-")
 	if perr != nil {
-		return "", "", "", "", ErrPreLoginNotFound
+		return "", "", "", "", "", "", ErrPreLoginNotFound
 	}
 
 	signingKey, kerr := a.keys.Get(ctx, signingKeyID)
 	if kerr != nil {
-		return "", "", "", "", ErrPreLoginNotFound
+		return "", "", "", "", "", "", ErrPreLoginNotFound
 	}
 	hmacKey, derr := session.DecryptKeyMaterial(signingKey.KeyMaterialEncrypted, a.encryptionKey)
 	if derr != nil {
-		return "", "", "", "", ErrPreLoginNotFound
+		return "", "", "", "", "", "", ErrPreLoginNotFound
 	}
 	expectedHMAC := session.ComputeCookieHMAC(plID, signingKeyID, hmacKey)
 	if subtle.ConstantTimeCompare(expectedHMAC, providedHMAC) != 1 {
-		return "", "", "", "", ErrPreLoginNotFound
+		return "", "", "", "", "", "", ErrPreLoginNotFound
 	}
 
 	row, lerr := a.repo.LookupAndConsume(ctx, plID)
@@ -159,15 +167,15 @@ func (a *PreLoginAdapter) LookupAndConsume(ctx context.Context, cookieValue stri
 		// the OIDC service consumes; the audit row distinguishes via
 		// the wrapped error from the repo (which the handler logs).
 		if errors.Is(lerr, repository.ErrPreLoginNotFound) {
-			return "", "", "", "", ErrPreLoginNotFound
+			return "", "", "", "", "", "", ErrPreLoginNotFound
 		}
 		if errors.Is(lerr, repository.ErrPreLoginExpired) {
-			return "", "", "", "", ErrPreLoginNotFound
+			return "", "", "", "", "", "", ErrPreLoginNotFound
 		}
-		return "", "", "", "", fmt.Errorf("pre-login: lookup_and_consume: %w", lerr)
+		return "", "", "", "", "", "", fmt.Errorf("pre-login: lookup_and_consume: %w", lerr)
 	}
 
-	return row.OIDCProviderID, row.State, row.Nonce, row.PKCEVerifier, nil
+	return row.OIDCProviderID, row.State, row.Nonce, row.PKCEVerifier, row.ClientIP, row.UserAgent, nil
 }
 
 // newID returns `pl-<base64url-no-pad>` with 16 bytes of entropy.
diff --git a/internal/auth/oidc/prelogin_test.go b/internal/auth/oidc/prelogin_test.go
index 1097765..ea3d2ac 100644
--- a/internal/auth/oidc/prelogin_test.go
+++ b/internal/auth/oidc/prelogin_test.go
@@ -196,7 +196,7 @@ func TestPreLoginAdapter_CreatePreLogin_GetActiveFailure(t *testing.T) {
 	keys := newStubSigningKeyLookup(nil)
 	keys.getActErr = errors.New("postgres unavailable")
 	a := NewPreLoginAdapter(repo, keys, "t-default", "")
-	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err == nil || !strings.Contains(err.Error(), "get active signing key") {
 		t.Errorf("err = %v, want wrapped 'get active signing key'", err)
 	}
@@ -210,7 +210,7 @@ func TestPreLoginAdapter_CreatePreLogin_DecryptFailure(t *testing.T) {
 	key.KeyMaterialEncrypted = []byte{0x03, 0x00, 0x01, 0x02} // bogus v3 blob
 	keys := newStubSigningKeyLookup(key)
 	a := NewPreLoginAdapter(repo, keys, "t-default", "passphrase-set")
-	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err == nil || !strings.Contains(err.Error(), "decrypt active key") {
 		t.Errorf("err = %v, want wrapped 'decrypt active key'", err)
 	}
@@ -223,7 +223,7 @@ func TestPreLoginAdapter_CreatePreLogin_RNGFailure(t *testing.T) {
 	a.SetRandReaderForTest(func(_ []byte) (int, error) {
 		return 0, errors.New("RNG drained")
 	})
-	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err == nil || !strings.Contains(err.Error(), "generate id") {
 		t.Errorf("err = %v, want wrapped 'generate id'", err)
 	}
@@ -234,7 +234,7 @@ func TestPreLoginAdapter_CreatePreLogin_PersistFailure(t *testing.T) {
 	repo.createErr = errors.New("FK violation")
 	keys := newStubSigningKeyLookup(activeKeyForTest(t, "sk-1"))
 	a := NewPreLoginAdapter(repo, keys, "t-default", "")
-	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	_, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err == nil || !strings.Contains(err.Error(), "persist row") {
 		t.Errorf("err = %v, want wrapped 'persist row'", err)
 	}
@@ -247,7 +247,7 @@ func TestPreLoginAdapter_CreatePreLogin_HappyPath(t *testing.T) {
 	repo := newStubPreLoginRepo()
 	keys := newStubSigningKeyLookup(activeKeyForTest(t, "sk-1"))
 	a := NewPreLoginAdapter(repo, keys, "t-default", "")
-	cookie, sid, err := a.CreatePreLogin(context.Background(), "op-x", "the-state", "the-nonce", "verifier-xxx")
+	cookie, sid, err := a.CreatePreLogin(context.Background(), "op-x", "the-state", "the-nonce", "verifier-xxx", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -279,7 +279,7 @@ func TestPreLoginAdapter_CreatePreLogin_HappyPath(t *testing.T) {
 func TestPreLoginAdapter_LookupAndConsume_MalformedCookie(t *testing.T) {
 	a := NewPreLoginAdapter(newStubPreLoginRepo(),
 		newStubSigningKeyLookup(activeKeyForTest(t, "sk-1")), "t-default", "")
-	_, _, _, _, err := a.LookupAndConsume(context.Background(), "definitely-not-a-cookie")
+	_, _, _, _, _, _, err := a.LookupAndConsume(context.Background(), "definitely-not-a-cookie")
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("err = %v, want ErrPreLoginNotFound", err)
 	}
@@ -292,14 +292,14 @@ func TestPreLoginAdapter_LookupAndConsume_UnknownSigningKey(t *testing.T) {
 	createKey := activeKeyForTest(t, "sk-1")
 	createKeys := newStubSigningKeyLookup(createKey)
 	createAdapter := NewPreLoginAdapter(repo, createKeys, "t-default", "")
-	cookie, _, err := createAdapter.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	cookie, _, err := createAdapter.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
 
 	emptyKeys := newStubSigningKeyLookup(nil) // sk-1 is not in this lookup
 	consumeAdapter := NewPreLoginAdapter(repo, emptyKeys, "t-default", "")
-	_, _, _, _, err = consumeAdapter.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err = consumeAdapter.LookupAndConsume(context.Background(), cookie)
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("err = %v, want ErrPreLoginNotFound (unknown signing key)", err)
 	}
@@ -312,7 +312,7 @@ func TestPreLoginAdapter_LookupAndConsume_DecryptKeyFailure(t *testing.T) {
 	createKey := activeKeyForTest(t, "sk-1")
 	createKeys := newStubSigningKeyLookup(createKey)
 	createAdapter := NewPreLoginAdapter(repo, createKeys, "t-default", "")
-	cookie, _, err := createAdapter.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	cookie, _, err := createAdapter.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -322,7 +322,7 @@ func TestPreLoginAdapter_LookupAndConsume_DecryptKeyFailure(t *testing.T) {
 	corruptedKey.KeyMaterialEncrypted = []byte{0x03, 0x00, 0x01, 0x02} // bogus v3
 	corruptedKeys := newStubSigningKeyLookup(&corruptedKey)
 	consumeAdapter := NewPreLoginAdapter(repo, corruptedKeys, "t-default", "passphrase-set")
-	_, _, _, _, err = consumeAdapter.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err = consumeAdapter.LookupAndConsume(context.Background(), cookie)
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("err = %v, want ErrPreLoginNotFound (decrypt failure → uniform sentinel)", err)
 	}
@@ -336,7 +336,7 @@ func TestPreLoginAdapter_LookupAndConsume_HMACMismatch(t *testing.T) {
 	createKey := activeKeyForTest(t, "sk-1")
 	createKeys := newStubSigningKeyLookup(createKey)
 	createAdapter := NewPreLoginAdapter(repo, createKeys, "t-default", "")
-	cookie, _, err := createAdapter.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	cookie, _, err := createAdapter.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -349,7 +349,7 @@ func TestPreLoginAdapter_LookupAndConsume_HMACMismatch(t *testing.T) {
 	swapped.KeyMaterialEncrypted = swappedMaterial
 	swappedKeys := newStubSigningKeyLookup(&swapped)
 	consumeAdapter := NewPreLoginAdapter(repo, swappedKeys, "t-default", "")
-	_, _, _, _, err = consumeAdapter.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err = consumeAdapter.LookupAndConsume(context.Background(), cookie)
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("err = %v, want ErrPreLoginNotFound (HMAC mismatch)", err)
 	}
@@ -368,7 +368,7 @@ func TestPreLoginAdapter_LookupAndConsume_RepoNotFound(t *testing.T) {
 	plID := "pl-orphan-id"
 	cookie := session.SignCookieValue(plID, keys.active.ID, hmacKey)
 
-	_, _, _, _, err := a.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err := a.LookupAndConsume(context.Background(), cookie)
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("err = %v, want ErrPreLoginNotFound (repo miss)", err)
 	}
@@ -378,12 +378,12 @@ func TestPreLoginAdapter_LookupAndConsume_RepoExpired(t *testing.T) {
 	repo := newStubPreLoginRepo()
 	keys := newStubSigningKeyLookup(activeKeyForTest(t, "sk-1"))
 	a := NewPreLoginAdapter(repo, keys, "t-default", "")
-	cookie, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	cookie, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
 	repo.expireOnNext = true
-	_, _, _, _, err = a.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err = a.LookupAndConsume(context.Background(), cookie)
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("err = %v, want ErrPreLoginNotFound (expired → uniform sentinel)", err)
 	}
@@ -393,13 +393,13 @@ func TestPreLoginAdapter_LookupAndConsume_RepoOtherError(t *testing.T) {
 	repo := newStubPreLoginRepo()
 	keys := newStubSigningKeyLookup(activeKeyForTest(t, "sk-1"))
 	a := NewPreLoginAdapter(repo, keys, "t-default", "")
-	cookie, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v")
+	cookie, _, err := a.CreatePreLogin(context.Background(), "op-x", "s", "n", "v", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
 	// Inject a non-NotFound, non-Expired error to exercise the wrap branch.
 	repo.wrappedErr = errors.New("postgres dropped connection")
-	_, _, _, _, err = a.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err = a.LookupAndConsume(context.Background(), cookie)
 	if errors.Is(err, ErrPreLoginNotFound) {
 		t.Error("err must NOT be ErrPreLoginNotFound for non-sentinel repo failure")
 	}
@@ -412,11 +412,11 @@ func TestPreLoginAdapter_LookupAndConsume_HappyPath(t *testing.T) {
 	repo := newStubPreLoginRepo()
 	keys := newStubSigningKeyLookup(activeKeyForTest(t, "sk-1"))
 	a := NewPreLoginAdapter(repo, keys, "t-default", "")
-	cookie, _, err := a.CreatePreLogin(context.Background(), "op-okta", "the-state-42", "the-nonce-42", "the-verifier-42")
+	cookie, _, err := a.CreatePreLogin(context.Background(), "op-okta", "the-state-42", "the-nonce-42", "the-verifier-42", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
-	pid, st, nn, vf, err := a.LookupAndConsume(context.Background(), cookie)
+	pid, st, nn, vf, _, _, err := a.LookupAndConsume(context.Background(), cookie)
 	if err != nil {
 		t.Fatalf("LookupAndConsume: %v", err)
 	}
@@ -425,7 +425,7 @@ func TestPreLoginAdapter_LookupAndConsume_HappyPath(t *testing.T) {
 	}
 
 	// Single-use: second consume returns ErrPreLoginNotFound.
-	_, _, _, _, err = a.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err = a.LookupAndConsume(context.Background(), cookie)
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("second consume err = %v, want ErrPreLoginNotFound (single-use violated)", err)
 	}
diff --git a/internal/auth/oidc/provider_enabled_test.go b/internal/auth/oidc/provider_enabled_test.go
index 613091b..4a8dfd4 100644
--- a/internal/auth/oidc/provider_enabled_test.go
+++ b/internal/auth/oidc/provider_enabled_test.go
@@ -23,7 +23,7 @@ func TestService_HandleAuthRequest_DisabledProvider_RejectsWithErrProviderDisabl
 	// to simulate the operator toggling the provider offline. The next
 	// HandleAuthRequest hits the disabled-check before the cached entry
 	// is reused.
-	if _, _, _, err := svc.HandleAuthRequest(context.Background(), "op-disabled"); err != nil {
+	if _, _, _, err := svc.HandleAuthRequest(context.Background(), "op-disabled", "", ""); err != nil {
 		t.Fatalf("warm HandleAuthRequest: %v", err)
 	}
 	if entry, ok := svc.cache["op-disabled"]; ok && entry.cfgRow != nil {
@@ -32,7 +32,7 @@ func TestService_HandleAuthRequest_DisabledProvider_RejectsWithErrProviderDisabl
 		t.Fatal("expected cache entry for op-disabled after warmup")
 	}
 
-	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-disabled")
+	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-disabled", "", "")
 	if !errors.Is(err, ErrProviderDisabled) {
 		t.Errorf("HandleAuthRequest(disabled provider) err = %v; want ErrProviderDisabled", err)
 	}
diff --git a/internal/auth/oidc/service.go b/internal/auth/oidc/service.go
index b3dc657..03b83de 100644
--- a/internal/auth/oidc/service.go
+++ b/internal/auth/oidc/service.go
@@ -85,6 +85,17 @@ type Service struct {
 	// resolution + user upsert; on grantAdmin=true the user's resolved
 	// role IDs are extended with r-admin. See bootstrap_hook.go.
 	adminBootstrapHook AdminBootstrapHook
+
+	// Audit 2026-05-10 MED-16 — Per-leg toggles for the pre-login UA/IP
+	// binding check. Both default to true; operators on enterprise
+	// proxies (UA rewrite) or dual-stack v4/v6 (IP flip) flip the
+	// affected leg false via CERTCTL_OIDC_PRELOGIN_REQUIRE_UA /
+	// CERTCTL_OIDC_PRELOGIN_REQUIRE_IP. Even when both are false,
+	// the binding values are still persisted so audit forensics can
+	// detect mismatches retroactively — only the in-band reject is
+	// suppressed.
+	preLoginRequireUA bool
+	preLoginRequireIP bool
 }
 
 // providerEntry caches the go-oidc Provider + the OAuth2 config + the
@@ -126,16 +137,28 @@ type PreLoginStore interface {
 	// CreatePreLogin persists a row with the given identifiers.
 	// providerID is the configured op-... id; state, nonce, verifier
 	// are server-generated random strings the callback will validate.
+	// clientIP + userAgent (Audit 2026-05-10 MED-16) are the
+	// /auth/oidc/login request's source IP (post LOW-5 XFF gating) +
+	// User-Agent header, persisted into the row so HandleCallback can
+	// reject mismatches at consume time. Empty strings are tolerated
+	// (rolling-deploy compat + headless / proxy contexts) — the
+	// consume-side check only enforces when both sides carry non-empty
+	// values for the leg in question.
 	// Returns the opaque cookie value the handler sets, plus the
 	// session ID (used as the audit trail anchor).
-	CreatePreLogin(ctx context.Context, providerID, state, nonce, verifier string) (cookieValue, sessionID string, err error)
+	CreatePreLogin(ctx context.Context, providerID, state, nonce, verifier, clientIP, userAgent string) (cookieValue, sessionID string, err error)
 
 	// LookupAndConsume reads the pre-login row by cookie value AND
 	// deletes it atomically. Single-use: a second call with the same
 	// cookie value returns ErrPreLoginNotFound. Returns the stored
 	// state/nonce/verifier/providerID for the caller to validate
 	// against the callback parameters.
-	LookupAndConsume(ctx context.Context, cookieValue string) (providerID, state, nonce, verifier string, err error)
+	//
+	// Audit 2026-05-10 MED-16 — also returns the row's persisted
+	// clientIP + userAgent so HandleCallback can defeat replay of a
+	// stolen pre-login cookie by a different browser. Empty values are
+	// returned for rows persisted before migration 000044.
+	LookupAndConsume(ctx context.Context, cookieValue string) (providerID, state, nonce, verifier, clientIP, userAgent string, err error)
 }
 
 // SessionMinter wraps the post-login session creation. Phase 4's
@@ -193,6 +216,20 @@ var (
 	// RFC 9207 §2.3. HTTP 400.
 	ErrIssParamMismatch = errors.New("oidc: callback iss parameter does not match provider issuer URL")
 
+	// ErrPreLoginUAMismatch: pre-login row's User-Agent doesn't match
+	// the request hitting /auth/oidc/callback. Audit 2026-05-10 MED-16
+	// closure — RFC 9700 §4.7.1 binding-state recommendation. HTTP 400.
+	// Operators on enterprise proxies that rewrite UA may set
+	// CERTCTL_OIDC_PRELOGIN_REQUIRE_UA=false to disable.
+	ErrPreLoginUAMismatch = errors.New("oidc: pre-login row User-Agent does not match callback request")
+
+	// ErrPreLoginIPMismatch: pre-login row's client IP doesn't match
+	// the request hitting /auth/oidc/callback. Audit 2026-05-10
+	// MED-16. HTTP 400. Operators on dual-stack v4/v6 environments
+	// where source IP routinely flips may set
+	// CERTCTL_OIDC_PRELOGIN_REQUIRE_IP=false to disable.
+	ErrPreLoginIPMismatch = errors.New("oidc: pre-login row client IP does not match callback request")
+
 	// ErrAudienceMismatch: ID token `aud` doesn't include the
 	// configured client_id. HTTP 400.
 	ErrAudienceMismatch = errors.New("oidc: audience mismatch")
@@ -322,6 +359,11 @@ func NewService(
 		encryptionKey: encryptionKey,
 		cache:         make(map[string]*providerEntry),
 		clockNow:      time.Now,
+		// MED-16 defaults: both legs ON. cmd/server/main.go reads
+		// CERTCTL_OIDC_PRELOGIN_REQUIRE_UA / _IP and calls
+		// SetPreLoginBindingRequirements to override.
+		preLoginRequireUA: true,
+		preLoginRequireIP: true,
 	}
 }
 
@@ -331,6 +373,16 @@ func (s *Service) SetClockForTest(now func() time.Time) {
 	s.clockNow = now
 }
 
+// SetPreLoginBindingRequirements wires the MED-16 UA/IP enforcement
+// toggles. Both default to true; set false to log-only behaviour for
+// a given leg (the binding is still persisted + audited; only the
+// in-band reject is suppressed). Called by cmd/server/main.go from
+// the config layer.
+func (s *Service) SetPreLoginBindingRequirements(requireUA, requireIP bool) {
+	s.preLoginRequireUA = requireUA
+	s.preLoginRequireIP = requireIP
+}
+
 // =============================================================================
 // HandleAuthRequest: kicks off the OIDC handshake.
 //
@@ -346,7 +398,14 @@ func (s *Service) SetClockForTest(now func() time.Time) {
 
 // HandleAuthRequest builds the IdP redirect URL + persists the
 // pre-login session row holding state + nonce + PKCE verifier.
-func (s *Service) HandleAuthRequest(ctx context.Context, providerID string) (authURL, cookieValue, preLoginID string, err error) {
+//
+// Audit 2026-05-10 MED-16 — clientIP + userAgent are persisted into
+// the pre-login row so HandleCallback can reject a stolen cookie
+// replayed by a different browser. Empty values are tolerated for
+// headless / proxy callers; the consume-side check only enforces
+// when both row and request carry non-empty values on the leg in
+// question.
+func (s *Service) HandleAuthRequest(ctx context.Context, providerID, clientIP, userAgent string) (authURL, cookieValue, preLoginID string, err error) {
 	entry, err := s.getOrLoad(ctx, providerID)
 	if err != nil {
 		return "", "", "", err
@@ -371,7 +430,7 @@ func (s *Service) HandleAuthRequest(ctx context.Context, providerID string) (aut
 	// (well within the RFC 7636 43-128 character bound).
 	verifier := oauth2.GenerateVerifier()
 
-	cookieValue, preLoginID, err = s.preLogin.CreatePreLogin(ctx, providerID, state, nonce, verifier)
+	cookieValue, preLoginID, err = s.preLogin.CreatePreLogin(ctx, providerID, state, nonce, verifier, clientIP, userAgent)
 	if err != nil {
 		return "", "", "", fmt.Errorf("oidc: pre-login store: %w", err)
 	}
@@ -428,11 +487,28 @@ func (s *Service) HandleCallback(
 	preLoginCookie, code, callbackState, callbackIss, ip, userAgent string,
 ) (*CallbackResult, error) {
 	// Step 1: consume the pre-login row (single-use).
-	providerID, storedState, storedNonce, verifier, err := s.preLogin.LookupAndConsume(ctx, preLoginCookie)
+	providerID, storedState, storedNonce, verifier, storedIP, storedUA, err := s.preLogin.LookupAndConsume(ctx, preLoginCookie)
 	if err != nil {
 		return nil, ErrPreLoginNotFound
 	}
 
+	// Step 1.5: Audit 2026-05-10 MED-16 — UA / IP binding compare.
+	// Enforced only when (a) the leg's toggle is on, (b) the row
+	// carries a non-empty stored value (legacy rows pre-migration
+	// 000044 have NULL → empty string), and (c) the incoming request
+	// carries a non-empty value too. Constant-time compares for both
+	// legs to avoid leaking UA/IP length differences via timing.
+	if s.preLoginRequireUA && storedUA != "" && userAgent != "" {
+		if subtle.ConstantTimeCompare([]byte(userAgent), []byte(storedUA)) != 1 {
+			return nil, ErrPreLoginUAMismatch
+		}
+	}
+	if s.preLoginRequireIP && storedIP != "" && ip != "" {
+		if subtle.ConstantTimeCompare([]byte(ip), []byte(storedIP)) != 1 {
+			return nil, ErrPreLoginIPMismatch
+		}
+	}
+
 	// Step 2: state constant-time compare.
 	if subtle.ConstantTimeCompare([]byte(callbackState), []byte(storedState)) != 1 {
 		return nil, ErrStateMismatch
diff --git a/internal/auth/oidc/service_test.go b/internal/auth/oidc/service_test.go
index 971270e..4850138 100644
--- a/internal/auth/oidc/service_test.go
+++ b/internal/auth/oidc/service_test.go
@@ -420,26 +420,30 @@ type stubPreLogin struct {
 
 type preLoginRow struct {
 	providerID, state, nonce, verifier string
+	// Audit 2026-05-10 MED-16 — UA/IP binding captured at
+	// CreatePreLogin so LookupAndConsume can surface them for the
+	// service-layer compare.
+	clientIP, userAgent string
 }
 
 func newStubPreLogin() *stubPreLogin {
 	return &stubPreLogin{rows: make(map[string]preLoginRow)}
 }
-func (s *stubPreLogin) CreatePreLogin(_ context.Context, providerID, state, nonce, verifier string) (string, string, error) {
+func (s *stubPreLogin) CreatePreLogin(_ context.Context, providerID, state, nonce, verifier, clientIP, userAgent string) (string, string, error) {
 	if s.createErr != nil {
 		return "", "", s.createErr
 	}
 	cookieVal := fmt.Sprintf("pl-%d", len(s.rows)+1)
-	s.rows[cookieVal] = preLoginRow{providerID, state, nonce, verifier}
+	s.rows[cookieVal] = preLoginRow{providerID, state, nonce, verifier, clientIP, userAgent}
 	return cookieVal, "ses-" + cookieVal, nil
 }
-func (s *stubPreLogin) LookupAndConsume(_ context.Context, cookie string) (string, string, string, string, error) {
+func (s *stubPreLogin) LookupAndConsume(_ context.Context, cookie string) (string, string, string, string, string, string, error) {
 	r, ok := s.rows[cookie]
 	if !ok {
-		return "", "", "", "", ErrPreLoginNotFound
+		return "", "", "", "", "", "", ErrPreLoginNotFound
 	}
 	delete(s.rows, cookie)
-	return r.providerID, r.state, r.nonce, r.verifier, nil
+	return r.providerID, r.state, r.nonce, r.verifier, r.clientIP, r.userAgent, nil
 }
 
 // =============================================================================
@@ -465,14 +469,14 @@ func TestService_PKCEPlainRejectedSentinel(t *testing.T) {
 // a second call with the same cookie returns ErrPreLoginNotFound.
 func TestService_StateReplayDeniedByConsumeOnce(t *testing.T) {
 	pl := newStubPreLogin()
-	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-x", "the-state", "the-nonce", "verifier-xxx")
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-x", "the-state", "the-nonce", "verifier-xxx", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
-	if _, _, _, _, err := pl.LookupAndConsume(context.Background(), cookie); err != nil {
+	if _, _, _, _, _, _, err := pl.LookupAndConsume(context.Background(), cookie); err != nil {
 		t.Fatalf("first LookupAndConsume: %v", err)
 	}
-	_, _, _, _, err = pl.LookupAndConsume(context.Background(), cookie)
+	_, _, _, _, _, _, err = pl.LookupAndConsume(context.Background(), cookie)
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("second LookupAndConsume err = %v; want ErrPreLoginNotFound (single-use violated)", err)
 	}
@@ -490,7 +494,7 @@ func TestService_HandleCallback_RejectsForgedPreLoginCookie(t *testing.T) {
 // Test 4: state mismatch (cookie matches but the callback state doesn't).
 func TestService_HandleCallback_RejectsStateMismatch(t *testing.T) {
 	svc, pl := newServiceForUnitTestWithPL(t)
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-test", "real-state", "real-nonce", "verifier-xxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-test", "real-state", "real-nonce", "verifier-xxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "wrong-state", "", "ip", "ua")
 	if !errors.Is(err, ErrStateMismatch) {
 		t.Errorf("err = %v; want ErrStateMismatch", err)
@@ -642,7 +646,7 @@ func TestService_HandleCallback_HappyPath(t *testing.T) {
 	idp := newMockIdP(t)
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-happy")
 
-	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-happy", "happy-state", "test-nonce-fixed", "verifier-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-happy", "happy-state", "test-nonce-fixed", "verifier-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -668,7 +672,7 @@ func TestService_HandleCallback_RejectsWrongAudience(t *testing.T) {
 	idp.overrideAudience = []string{"some-other-client"}
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-aud")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-aud", "s", "test-nonce-fixed", "v-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-aud", "s", "test-nonce-fixed", "v-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	// gooidc.Verify catches this first; its wrap reaches us as a wrapped error.
 	// Either ErrAudienceMismatch (our re-check) OR a wrapped verify error is acceptable.
@@ -684,7 +688,7 @@ func TestService_HandleCallback_RejectsNonceMismatch(t *testing.T) {
 	idp.overrideNonce = "wrong-nonce-from-idp"
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-nonce")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-nonce", "s", "expected-nonce", "v-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-nonce", "s", "expected-nonce", "v-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrNonceMismatch) {
 		t.Errorf("err = %v; want ErrNonceMismatch", err)
@@ -697,7 +701,7 @@ func TestService_HandleCallback_RejectsExpiredToken(t *testing.T) {
 	idp.overrideExp = time.Now().Add(-2 * time.Hour) // 2 hours past
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-exp")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-exp", "s", "test-nonce-fixed", "v-cccccccccccccccccccccccccccccccccccccccccc")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-exp", "s", "test-nonce-fixed", "v-cccccccccccccccccccccccccccccccccccccccccc", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	// Either ErrTokenExpired (our re-check) or a wrapped verify error is fine.
 	if err == nil {
@@ -714,7 +718,7 @@ func TestService_HandleCallback_RejectsIATTooOld(t *testing.T) {
 	idp.overrideExp = time.Now().Add(2 * time.Hour) // exp is fine
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iat")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-iat", "s", "test-nonce-fixed", "v-dddddddddddddddddddddddddddddddddddddddddd")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-iat", "s", "test-nonce-fixed", "v-dddddddddddddddddddddddddddddddddddddddddd", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrIATTooOld) {
 		t.Errorf("err = %v; want ErrIATTooOld", err)
@@ -727,7 +731,7 @@ func TestService_HandleCallback_RejectsGroupsMissing(t *testing.T) {
 	idp.overrideGroups = []string{} // empty groups claim
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-grp")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-grp", "s", "test-nonce-fixed", "v-eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-grp", "s", "test-nonce-fixed", "v-eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
@@ -740,7 +744,7 @@ func TestService_HandleCallback_RejectsGroupsUnmapped(t *testing.T) {
 	idp := newMockIdP(t)
 	svc, pl := newServiceWithProviderAndPLNoMappings(t, idp.URL(), "op-unmap")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-unmap", "s", "test-nonce-fixed", "v-ffffffffffffffffffffffffffffffffffffffffff")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-unmap", "s", "test-nonce-fixed", "v-ffffffffffffffffffffffffffffffffffffffffff", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsUnmapped) {
 		t.Errorf("err = %v; want ErrGroupsUnmapped", err)
@@ -850,7 +854,7 @@ func TestService_HandleAuthRequest_BuildsValidIdPRedirect(t *testing.T) {
 	idp := newMockIdP(t)
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-har")
 
-	authURL, cookieValue, preLoginID, err := svc.HandleAuthRequest(context.Background(), "op-har")
+	authURL, cookieValue, preLoginID, err := svc.HandleAuthRequest(context.Background(), "op-har", "", "")
 	if err != nil {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
@@ -880,7 +884,7 @@ func TestService_HandleAuthRequest_BuildsValidIdPRedirect(t *testing.T) {
 // repo-not-found path through HandleAuthRequest.
 func TestService_HandleAuthRequest_UnknownProviderRejected(t *testing.T) {
 	svc := newServiceForUnitTest(t)
-	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-nonexistent")
+	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-nonexistent", "", "")
 	if !errors.Is(err, repository.ErrOIDCProviderNotFound) {
 		t.Errorf("err = %v; want ErrOIDCProviderNotFound", err)
 	}
@@ -900,7 +904,7 @@ func TestService_UpsertUser_UpdateExistingPath(t *testing.T) {
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	// First login creates the user.
-	cookie1, _, _ := pl.CreatePreLogin(context.Background(), "op-upd", "s1", "test-nonce-fixed", "v-1aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+	cookie1, _, _ := pl.CreatePreLogin(context.Background(), "op-upd", "s1", "test-nonce-fixed", "v-1aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "", "")
 	res1, err := svc.HandleCallback(context.Background(), cookie1, "code", "s1", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("first HandleCallback: %v", err)
@@ -913,7 +917,7 @@ func TestService_UpsertUser_UpdateExistingPath(t *testing.T) {
 	time.Sleep(10 * time.Millisecond) // ensure timestamps advance
 
 	// Second login by same subject: update path, no new user row.
-	cookie2, _, _ := pl.CreatePreLogin(context.Background(), "op-upd", "s2", "test-nonce-fixed", "v-2aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
+	cookie2, _, _ := pl.CreatePreLogin(context.Background(), "op-upd", "s2", "test-nonce-fixed", "v-2aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "", "")
 	idp.overrideEmail = "user-renamed@example.com"
 	res2, err := svc.HandleCallback(context.Background(), cookie2, "code2", "s2", "", "ip", "ua")
 	if err != nil {
@@ -1182,7 +1186,7 @@ func TestService_BootstrapHook_GrantsAdminOnMatch(t *testing.T) {
 		return true, nil // grant admin
 	})
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-bootstrap", "s", "test-nonce-fixed", "v-bootstrapxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-bootstrap", "s", "test-nonce-fixed", "v-bootstrapxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "10.0.0.1", "Mozilla/5.0")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
@@ -1205,7 +1209,7 @@ func TestService_BootstrapHook_NoMatchPreservesEmptyMappingFailClosed(t *testing
 		return false, nil // not a bootstrap match
 	})
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-no-match", "s", "test-nonce-fixed", "v-nomatchxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-no-match", "s", "test-nonce-fixed", "v-nomatchxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsUnmapped) {
 		t.Errorf("err = %v; want ErrGroupsUnmapped (no bootstrap match + empty mappings)", err)
@@ -1226,7 +1230,7 @@ func TestService_BootstrapHook_AdminAlreadyExistsFallsThroughToNormalMapping(t *
 		return false, nil
 	})
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-existing-admin", "s", "test-nonce-fixed", "v-existingxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-existing-admin", "s", "test-nonce-fixed", "v-existingxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
@@ -1248,7 +1252,7 @@ func TestService_BootstrapHook_ErrorWraps(t *testing.T) {
 	svc.SetAdminBootstrapHook(func(_ context.Context, _ string, _ []string, _ string) (bool, error) {
 		return false, fmt.Errorf("simulated AdminExists probe failure")
 	})
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-hook-err", "s", "test-nonce-fixed", "v-errxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-hook-err", "s", "test-nonce-fixed", "v-errxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err == nil || !strings.Contains(err.Error(), "admin bootstrap") {
 		t.Errorf("err = %v; want admin bootstrap wrap", err)
@@ -1269,7 +1273,7 @@ func TestService_BootstrapHook_IdempotentWhenAdminAlreadyMapped(t *testing.T) {
 		return true, nil
 	})
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-idem", "s", "test-nonce-fixed", "v-idempxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-idem", "s", "test-nonce-fixed", "v-idempxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
@@ -1324,7 +1328,7 @@ func TestService_HandleCallback_AZPRequired_OnMultiAud(t *testing.T) {
 	idp.overrideAudience = []string{"certctl", "another-relying-party"}
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-azp-req")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-azp-req", "s", "test-nonce-fixed", "v-azpreqxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-azp-req", "s", "test-nonce-fixed", "v-azpreqxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrAZPRequired) {
 		t.Errorf("err = %v; want ErrAZPRequired", err)
@@ -1338,7 +1342,7 @@ func TestService_HandleCallback_AZPMismatch(t *testing.T) {
 	idp.overrideAZP = "some-other-client" // != "certctl"
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-azp-mis")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-azp-mis", "s", "test-nonce-fixed", "v-azpmisxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-azp-mis", "s", "test-nonce-fixed", "v-azpmisxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrAZPMismatch) {
 		t.Errorf("err = %v; want ErrAZPMismatch", err)
@@ -1356,7 +1360,7 @@ func TestService_HandleCallback_ATHashMismatch(t *testing.T) {
 	idp.overrideATHash = "not-the-real-at-hash"
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-ath-mis")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ath-mis", "s", "test-nonce-fixed", "v-athmisxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ath-mis", "s", "test-nonce-fixed", "v-athmisxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrATHashMismatch) {
 		t.Errorf("err = %v; want ErrATHashMismatch", err)
@@ -1373,7 +1377,7 @@ func TestService_HandleCallback_ATHashRequired_WhenAccessTokenPresent(t *testing
 	idp.overrideATHash = "<empty>" // suppress at_hash even though access_token is returned
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-ath-req")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ath-req", "s", "test-nonce-fixed", "v-athreqxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ath-req", "s", "test-nonce-fixed", "v-athreqxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrATHashRequired) {
 		t.Errorf("err = %v; want ErrATHashRequired", err)
@@ -1389,7 +1393,7 @@ func TestService_HandleCallback_IATInFuture(t *testing.T) {
 	idp.overrideExp = time.Now().Add(2 * time.Hour)
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iat-fut")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-iat-fut", "s", "test-nonce-fixed", "v-iatfutxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-iat-fut", "s", "test-nonce-fixed", "v-iatfutxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrIATInFuture) {
 		t.Errorf("err = %v; want ErrIATInFuture", err)
@@ -1407,7 +1411,7 @@ func TestService_HandleCallback_MappingsMapError(t *testing.T) {
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-map-err", "s", "test-nonce-fixed", "v-mapxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-map-err", "s", "test-nonce-fixed", "v-mapxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err == nil || !strings.Contains(err.Error(), "group-role mapping") {
 		t.Errorf("err = %v; want group-role mapping wrap", err)
@@ -1425,7 +1429,7 @@ func TestService_HandleCallback_SessionMintError(t *testing.T) {
 	sessions := &stubSessions{mintErr: fmt.Errorf("simulated session minter failure")}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-mint-err", "s", "test-nonce-fixed", "v-mintxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-mint-err", "s", "test-nonce-fixed", "v-mintxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err == nil || !strings.Contains(err.Error(), "session mint") {
 		t.Errorf("err = %v; want session mint wrap", err)
@@ -1444,7 +1448,7 @@ func TestService_HandleCallback_UserCreateError(t *testing.T) {
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-uc-err", "s", "test-nonce-fixed", "v-ucxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-uc-err", "s", "test-nonce-fixed", "v-ucxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err == nil || !strings.Contains(err.Error(), "upsert user") {
 		t.Errorf("err = %v; want upsert user wrap", err)
@@ -1464,7 +1468,7 @@ func TestService_HandleCallback_GetByOIDCSubjectNonNotFoundError(t *testing.T) {
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-get-err", "s", "test-nonce-fixed", "v-getxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-get-err", "s", "test-nonce-fixed", "v-getxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err == nil || !strings.Contains(err.Error(), "simulated query failure") {
 		t.Errorf("err = %v; want simulated query failure unwrap", err)
@@ -1485,7 +1489,7 @@ func TestService_UpsertUser_DisplayNameFallsBackToEmail(t *testing.T) {
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-name-fb", "s", "test-nonce-fixed", "v-namxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-name-fb", "s", "test-nonce-fixed", "v-namxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
@@ -1511,7 +1515,7 @@ func TestService_FetchUserinfoGroups_HappyPath_OnEmptyIDTokenGroups(t *testing.T
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-ok", "s", "test-nonce-fixed", "v-uioxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-ok", "s", "test-nonce-fixed", "v-uioxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
@@ -1536,7 +1540,7 @@ func TestService_FetchUserinfoGroups_ReturnsErrGroupsMissing_WhenUserinfoAlsoEmp
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-empty", "s", "test-nonce-fixed", "v-uixxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-empty", "s", "test-nonce-fixed", "v-uixxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
@@ -1558,7 +1562,7 @@ func TestService_FetchUserinfoGroups_ReturnsErrGroupsMissing_WhenEndpointMissing
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-noendpoint", "s", "test-nonce-fixed", "v-uixxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-noendpoint", "s", "test-nonce-fixed", "v-uixxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
@@ -1582,7 +1586,7 @@ func TestService_HandleAuthRequest_PreLoginStoreError(t *testing.T) {
 		"",
 	)
 
-	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-pl-err")
+	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-pl-err", "", "")
 	if err == nil || !strings.Contains(err.Error(), "pre-login store") {
 		t.Errorf("err = %v; want pre-login store wrap", err)
 	}
@@ -1663,7 +1667,7 @@ func TestService_HandleAuthRequest_RandomFailureSurfaces(t *testing.T) {
 	}
 	defer func() { readRand = original }()
 
-	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-rand-fail")
+	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-rand-fail", "", "")
 	if err == nil || !strings.Contains(err.Error(), "state generate") {
 		t.Errorf("err = %v; want state generate wrap", err)
 	}
@@ -1687,7 +1691,7 @@ func TestService_HandleAuthRequest_NonceRandomFailureSurfaces(t *testing.T) {
 	}
 	defer func() { readRand = original }()
 
-	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-nonce-rand-fail")
+	_, _, _, err := svc.HandleAuthRequest(context.Background(), "op-nonce-rand-fail", "", "")
 	if err == nil || !strings.Contains(err.Error(), "nonce generate") {
 		t.Errorf("err = %v; want nonce generate wrap", err)
 	}
@@ -1702,7 +1706,7 @@ func TestService_HandleCallback_RejectsTokenResponseMissingIDToken(t *testing.T)
 	idp.suppressIDToken = true
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-no-idtok")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-no-idtok", "s", "test-nonce-fixed", "v-noidxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-no-idtok", "s", "test-nonce-fixed", "v-noidxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err == nil || !strings.Contains(err.Error(), "missing id_token") {
 		t.Errorf("err = %v; want missing id_token error", err)
@@ -1725,7 +1729,7 @@ func TestService_FetchUserinfoGroups_ReturnsErrGroupsMissing_WhenUserinfoFails(t
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-500", "s", "test-nonce-fixed", "v-uifxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-500", "s", "test-nonce-fixed", "v-uifxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
@@ -1791,7 +1795,7 @@ func TestService_HandleCallback_MED17_NoSupport_AnyIssAccepted(t *testing.T) {
 	// advertiseIssParameterSupported deliberately left false.
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iss-back-compat")
 
-	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-back-compat", "iss-bc-state", "test-nonce-fixed", "v-issbcxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-back-compat", "iss-bc-state", "test-nonce-fixed", "v-issbcxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -1815,7 +1819,7 @@ func TestService_HandleCallback_MED17_SupportButMissing(t *testing.T) {
 	idp.advertiseIssParameterSupported = true
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iss-missing")
 
-	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-missing", "iss-miss-state", "test-nonce-fixed", "v-issmsxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-missing", "iss-miss-state", "test-nonce-fixed", "v-issmsxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -1835,7 +1839,7 @@ func TestService_HandleCallback_MED17_SupportButMismatch(t *testing.T) {
 	idp.advertiseIssParameterSupported = true
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iss-mismatch")
 
-	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-mismatch", "iss-mm-state", "test-nonce-fixed", "v-issmmxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-mismatch", "iss-mm-state", "test-nonce-fixed", "v-issmmxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -1855,7 +1859,7 @@ func TestService_HandleCallback_MED17_SupportAndCorrect(t *testing.T) {
 	idp.advertiseIssParameterSupported = true
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iss-ok")
 
-	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-ok", "iss-ok-state", "test-nonce-fixed", "v-issokxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-iss-ok", "iss-ok-state", "test-nonce-fixed", "v-issokxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	if err != nil {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
@@ -1869,6 +1873,105 @@ func TestService_HandleCallback_MED17_SupportAndCorrect(t *testing.T) {
 	}
 }
 
+// =============================================================================
+// MED-16 regression tests — pre-login UA / IP binding (RFC 9700 §4.7.1).
+//
+// HandleCallback rejects a pre-login cookie whose stored client_ip or
+// user_agent doesn't match the incoming /auth/oidc/callback request's
+// values. Each leg has an independent enforcement toggle; the binding
+// is also tolerant of empty values on either side (rolling-deploy +
+// headless-proxy compat).
+// =============================================================================
+
+func TestService_HandleCallback_MED16_UAMismatchRejected(t *testing.T) {
+	idp := newMockIdP(t)
+	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-med16-ua")
+
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-med16-ua", "ua-state", "test-nonce-fixed", "verifier-med16uaxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "10.0.0.1", "MozillaLogin/1.0")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	_, err = svc.HandleCallback(context.Background(), cookie, "code", "ua-state", "", "10.0.0.1", "AttackerUA/2.0")
+	if !errors.Is(err, ErrPreLoginUAMismatch) {
+		t.Fatalf("err = %v; want ErrPreLoginUAMismatch", err)
+	}
+}
+
+func TestService_HandleCallback_MED16_IPMismatchRejected(t *testing.T) {
+	idp := newMockIdP(t)
+	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-med16-ip")
+
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-med16-ip", "ip-state", "test-nonce-fixed", "verifier-med16ipxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "10.0.0.1", "Mozilla/5.0")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	_, err = svc.HandleCallback(context.Background(), cookie, "code", "ip-state", "", "203.0.113.7", "Mozilla/5.0")
+	if !errors.Is(err, ErrPreLoginIPMismatch) {
+		t.Fatalf("err = %v; want ErrPreLoginIPMismatch", err)
+	}
+}
+
+func TestService_HandleCallback_MED16_BothMatch_Succeeds(t *testing.T) {
+	idp := newMockIdP(t)
+	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-med16-ok")
+
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-med16-ok", "ok-state", "test-nonce-fixed", "verifier-med16okxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "10.0.0.1", "Mozilla/5.0")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "ok-state", "", "10.0.0.1", "Mozilla/5.0")
+	if err != nil {
+		t.Fatalf("HandleCallback (matching UA+IP): %v", err)
+	}
+	if res == nil {
+		t.Fatal("CallbackResult nil on matching binding")
+	}
+}
+
+// TestService_HandleCallback_MED16_LegacyRowEmptyValues pins the
+// rolling-deploy compat — a pre-login row persisted before migration
+// 000044 has empty clientIP/userAgent; the consume-side check must
+// pass through (the legacy row's binding is unenforceable).
+func TestService_HandleCallback_MED16_LegacyRowEmptyValues(t *testing.T) {
+	idp := newMockIdP(t)
+	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-med16-legacy")
+
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-med16-legacy", "leg-state", "test-nonce-fixed", "verifier-med16legxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "leg-state", "", "10.0.0.1", "Mozilla/5.0")
+	if err != nil {
+		t.Fatalf("HandleCallback (legacy empty bind): %v", err)
+	}
+	if res == nil {
+		t.Fatal("CallbackResult nil for legacy-row compat path")
+	}
+}
+
+// TestService_HandleCallback_MED16_RequireUAFalse_AllowsMismatch pins
+// the operator-escape-hatch behaviour: setting requireUA=false means
+// a UA mismatch passes through silently. The binding is still
+// persisted (so audit forensics can detect it retroactively) but the
+// in-band reject is suppressed.
+func TestService_HandleCallback_MED16_RequireUAFalse_AllowsMismatch(t *testing.T) {
+	idp := newMockIdP(t)
+	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-med16-uaopt")
+	svc.SetPreLoginBindingRequirements(false, true) // UA off, IP on
+
+	cookie, _, err := pl.CreatePreLogin(context.Background(), "op-med16-uaopt", "ua-opt-state", "test-nonce-fixed", "verifier-med16optxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "10.0.0.1", "MozillaLogin/1.0")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "ua-opt-state", "", "10.0.0.1", "AttackerUA/2.0")
+	if err != nil {
+		t.Fatalf("HandleCallback (requireUA=false, UA mismatch): %v", err)
+	}
+	if res == nil {
+		t.Fatal("CallbackResult nil with requireUA=false")
+	}
+}
+
 // TestService_UpsertUser_ValidateErrorOnEmptyEmail pins the
 // User.Validate failure path. The IdP returns an empty email (missing
 // claim); the upsertUser display-name fallback resolves to "" too;
@@ -1884,7 +1987,7 @@ func TestService_UpsertUser_ValidateErrorOnEmptyEmail(t *testing.T) {
 	sessions := &stubSessions{}
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
-	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-validate-err", "s", "test-nonce-fixed", "v-valxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
+	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-validate-err", "s", "test-nonce-fixed", "v-valxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", "", "")
 	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err == nil || !strings.Contains(err.Error(), "validate") {
 		t.Errorf("err = %v; want validate wrap", err)
diff --git a/internal/config/config.go b/internal/config/config.go
index 5f5f4cd..ad574ce 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -1635,6 +1635,23 @@ type AuthConfig struct {
 	// Setting: CERTCTL_OIDC_BCL_MAX_AGE_SECONDS environment variable.
 	OIDCBCLMaxAgeSeconds int
 
+	// OIDCPreLoginRequireUA enables the RFC 9700 §4.7.1 user-agent
+	// binding check on /auth/oidc/callback. Audit 2026-05-10 MED-16.
+	// Default true. Operators on enterprise proxies that rewrite the
+	// UA header set this false; the binding value is still persisted
+	// + audited even when enforcement is off so retroactive forensics
+	// remain possible.
+	// Setting: CERTCTL_OIDC_PRELOGIN_REQUIRE_UA environment variable.
+	OIDCPreLoginRequireUA bool
+
+	// OIDCPreLoginRequireIP enables the RFC 9700 §4.7.1 source-IP
+	// binding check on /auth/oidc/callback. Audit 2026-05-10 MED-16.
+	// Default true. Operators on dual-stack v4/v6 or mobile
+	// carrier-grade NAT where source IP routinely flips set this
+	// false; persistence + audit behave the same as UA above.
+	// Setting: CERTCTL_OIDC_PRELOGIN_REQUIRE_IP environment variable.
+	OIDCPreLoginRequireIP bool
+
 	// Breakglass holds the Auth Bundle 2 Phase 7.5 break-glass admin
 	// tunables. Default-OFF; the entire surface is invisible (404
 	// instead of 403) when CERTCTL_BREAKGLASS_ENABLED is not true.
@@ -1912,6 +1929,10 @@ func Load() (*Config, error) {
 			},
 			// Audit 2026-05-10 HIGH-3 — BCL iat-skew window.
 			OIDCBCLMaxAgeSeconds: getEnvInt("CERTCTL_OIDC_BCL_MAX_AGE_SECONDS", 60),
+
+			// Audit 2026-05-10 MED-16 — pre-login UA/IP binding toggles.
+			OIDCPreLoginRequireUA: getEnvBool("CERTCTL_OIDC_PRELOGIN_REQUIRE_UA", true),
+			OIDCPreLoginRequireIP: getEnvBool("CERTCTL_OIDC_PRELOGIN_REQUIRE_IP", true),
 			// Bundle 2 Phase 7.5: break-glass admin tunables. Default-
 			// OFF; the entire surface is invisible (404 NOT 403) when
 			// Enabled=false. Threat model + recommendation in the
diff --git a/internal/repository/oidc.go b/internal/repository/oidc.go
index f66856f..35db30d 100644
--- a/internal/repository/oidc.go
+++ b/internal/repository/oidc.go
@@ -121,6 +121,15 @@ type PreLoginSession struct {
 	PKCEVerifier      string
 	CreatedAt         time.Time
 	AbsoluteExpiresAt time.Time
+
+	// Audit 2026-05-10 MED-16 — UA / IP binding (RFC 9700 §4.7.1).
+	// Persisted at /auth/oidc/login; compared on consume to defeat
+	// pre-login cookie theft. Either column may be empty for in-flight
+	// rows from a pre-deploy code path during a rolling deploy; the
+	// consume-side check only enforces when BOTH the row AND the
+	// incoming request carry non-empty values.
+	ClientIP  string
+	UserAgent string
 }
 
 // Sentinel errors for PreLoginRepository.
diff --git a/internal/repository/postgres/oidc_prelogin.go b/internal/repository/postgres/oidc_prelogin.go
index 5dd4872..bbb0c75 100644
--- a/internal/repository/postgres/oidc_prelogin.go
+++ b/internal/repository/postgres/oidc_prelogin.go
@@ -75,14 +75,23 @@ func (r *PreLoginRepository) Create(ctx context.Context, p *repository.PreLoginS
 		return fmt.Errorf("oidc_pre_login encrypt pkce_verifier: %w", verr)
 	}
 
+	// Audit 2026-05-10 MED-16 — persist UA/IP binding on Create.
+	// Empty values are inserted as NULL via sql.NullString so the
+	// schema's nullable column constraint is respected and existing
+	// integration tests that don't provide UA/IP keep working.
+	clientIP := nullableString(p.ClientIP)
+	userAgent := nullableString(p.UserAgent)
+
 	if p.CreatedAt.IsZero() && p.AbsoluteExpiresAt.IsZero() {
 		_, err := r.db.ExecContext(ctx, `
 			INSERT INTO oidc_pre_login_sessions (
 				id, tenant_id, signing_key_id, oidc_provider_id,
-				state_enc, nonce_enc, pkce_verifier_enc
-			) VALUES ($1,$2,$3,$4,$5,$6,$7)`,
+				state_enc, nonce_enc, pkce_verifier_enc,
+				client_ip, user_agent
+			) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9)`,
 			p.ID, p.TenantID, p.SigningKeyID, p.OIDCProviderID,
-			stateEnc, nonceEnc, verifierEnc)
+			stateEnc, nonceEnc, verifierEnc,
+			clientIP, userAgent)
 		if err != nil {
 			return fmt.Errorf("oidc_pre_login create: %w", err)
 		}
@@ -98,16 +107,26 @@ func (r *PreLoginRepository) Create(ctx context.Context, p *repository.PreLoginS
 	_, err := r.db.ExecContext(ctx, `
 		INSERT INTO oidc_pre_login_sessions (
 			id, tenant_id, signing_key_id, oidc_provider_id,
-			state_enc, nonce_enc, pkce_verifier_enc, created_at, absolute_expires_at
-		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9)`,
+			state_enc, nonce_enc, pkce_verifier_enc,
+			client_ip, user_agent,
+			created_at, absolute_expires_at
+		) VALUES ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11)`,
 		p.ID, p.TenantID, p.SigningKeyID, p.OIDCProviderID,
-		stateEnc, nonceEnc, verifierEnc, p.CreatedAt, p.AbsoluteExpiresAt)
+		stateEnc, nonceEnc, verifierEnc,
+		clientIP, userAgent,
+		p.CreatedAt, p.AbsoluteExpiresAt)
 	if err != nil {
 		return fmt.Errorf("oidc_pre_login create: %w", err)
 	}
 	return nil
 }
 
+// MED-16 reuses nullableString from discovery.go (same package). It
+// returns sql.NullString{Valid:false} for empty strings so the database
+// stores NULL rather than the literal empty string — avoiding ambiguity
+// at consume time between "row had no binding" and "row had an explicit
+// empty binding".
+
 // LookupAndConsume reads the row by id and atomically deletes it
 // (single-use). Returns ErrPreLoginNotFound on miss; ErrPreLoginExpired
 // when the row was found but past its TTL (the row is still deleted in
@@ -132,16 +151,19 @@ func (r *PreLoginRepository) LookupAndConsume(ctx context.Context, id string) (*
 		RETURNING id, tenant_id, signing_key_id, oidc_provider_id,
 		          state, nonce, pkce_verifier,
 		          state_enc, nonce_enc, pkce_verifier_enc,
+		          client_ip, user_agent,
 		          created_at, absolute_expires_at`,
 		id)
 
 	var p repository.PreLoginSession
 	var statePlain, noncePlain, verifierPlain sql.NullString
+	var clientIP, userAgent sql.NullString
 	var stateEnc, nonceEnc, verifierEnc []byte
 	if err := row.Scan(
 		&p.ID, &p.TenantID, &p.SigningKeyID, &p.OIDCProviderID,
 		&statePlain, &noncePlain, &verifierPlain,
 		&stateEnc, &nonceEnc, &verifierEnc,
+		&clientIP, &userAgent,
 		&p.CreatedAt, &p.AbsoluteExpiresAt,
 	); err != nil {
 		if errors.Is(err, sql.ErrNoRows) {
@@ -168,6 +190,16 @@ func (r *PreLoginRepository) LookupAndConsume(ctx context.Context, id string) (*
 		p.PKCEVerifier = verifier
 	}
 
+	// Audit 2026-05-10 MED-16 — surface the binding columns for the
+	// service-layer UA / IP compare. Empty when the row was created
+	// before this migration landed (rolling-deploy compat).
+	if clientIP.Valid {
+		p.ClientIP = clientIP.String
+	}
+	if userAgent.Valid {
+		p.UserAgent = userAgent.String
+	}
+
 	if time.Now().UTC().After(p.AbsoluteExpiresAt) {
 		return nil, repository.ErrPreLoginExpired
 	}
diff --git a/migrations/000044_prelogin_uaip.down.sql b/migrations/000044_prelogin_uaip.down.sql
new file mode 100644
index 0000000..4bfd066
--- /dev/null
+++ b/migrations/000044_prelogin_uaip.down.sql
@@ -0,0 +1,4 @@
+-- Down for 000044 — drop the pre-login UA/IP binding columns.
+ALTER TABLE oidc_pre_login_sessions
+    DROP COLUMN IF EXISTS client_ip,
+    DROP COLUMN IF EXISTS user_agent;
diff --git a/migrations/000044_prelogin_uaip.up.sql b/migrations/000044_prelogin_uaip.up.sql
new file mode 100644
index 0000000..208ae21
--- /dev/null
+++ b/migrations/000044_prelogin_uaip.up.sql
@@ -0,0 +1,47 @@
+-- =============================================================================
+-- 2026-05-10 Audit / MED-16 closure
+-- =============================================================================
+--
+-- Pre-login rows in oidc_pre_login_sessions used to carry only the OIDC state,
+-- nonce, and PKCE verifier — the binding to the user agent that initiated the
+-- handshake was implicit (the pre-login cookie's HMAC, scoped to the active
+-- SessionSigningKey, only verifies that *some* caller of /auth/oidc/login is
+-- talking to /auth/oidc/callback; it does not verify that the SAME browser /
+-- HTTP client is on both sides).
+--
+-- RFC 9700 §4.7.1 (security best current practice for OAuth 2.0) recommends
+-- binding state to a user-agent fingerprint + source IP so that a pre-login
+-- cookie leaked in transit (CSRF / XSS / TLS termination on a shared proxy)
+-- cannot be replayed by a different browser. Even with HMAC integrity, the
+-- attacker who steals the bytes could otherwise complete the handshake.
+--
+-- This migration adds:
+--   - client_ip        TEXT — captured at /auth/oidc/login from the request's
+--                             clientIPFromRequest result (post LOW-5 XFF
+--                             trusted-proxy gating, so the value is honest).
+--   - user_agent       TEXT — captured at /auth/oidc/login from r.UserAgent().
+--                             Stored verbatim; the consume path compares with
+--                             constant-time equality.
+--
+-- Both columns are nullable so in-flight pre-login rows from pre-deploy code
+-- paths still consume cleanly (the consume-side check only enforces when both
+-- the row AND the request carry non-empty values; legacy rows pass through
+-- because the row's binding columns are NULL).
+--
+-- The audit failure_category distinguishes:
+--   - prelogin_ua_mismatch  — UA changed across the redirect (most common
+--                             real-world false-positive: aggressive UA
+--                             rewriters on enterprise proxies).
+--   - prelogin_ip_mismatch  — source IP changed across the redirect (mobile
+--                             carrier-grade NAT, dual-stack v4/v6 hops, VPN
+--                             toggle).
+--   - prelogin_uaip_mismatch — both differ.
+--
+-- Operators wanting to disable the gate (e.g. dual-stack v4/v6 environments
+-- where source IP routinely flips) set the CERTCTL_OIDC_PRELOGIN_REQUIRE_UA
+-- or CERTCTL_OIDC_PRELOGIN_REQUIRE_IP env var to "false". Default true.
+-- =============================================================================
+
+ALTER TABLE oidc_pre_login_sessions
+    ADD COLUMN IF NOT EXISTS client_ip   TEXT,
+    ADD COLUMN IF NOT EXISTS user_agent  TEXT;