harden(oidc): RFC 9207 iss URL parameter check on callback (MED-17)

Audit 2026-05-10 MED-17 closure.

WHAT.

When the matched IdP's discovery doc advertises
authorization_response_iss_parameter_supported=true (RFC 9207 §3),
HandleCallback now REQUIRES a non-empty `iss` query parameter on
/auth/oidc/callback and enforces a constant-time compare against the
configured provider's IssuerURL. Mismatch maps to two new sentinel
errors (ErrIssParamMissing / ErrIssParamMismatch) that the handler's
classifyOIDCFailure dispatches via errors.Is BEFORE the substring
fall-through, so the audit failure_category remains distinguishable
between the RFC 9207 leg (iss_param_missing / iss_param_mismatch) and
the in-token iss claim leg (id_token_iss_mismatch).

WHY.

The RFC 9207 iss URL parameter is the load-bearing mix-up-attack
defense for multi-tenant IdPs (Keycloak realms, Authentik tenants,
Auth0 tenants, public-trust CAs). Pre-fix the parameter was silently
ignored — an attacker controlling one IdP tenant could route an auth
code to certctl's callback against a different tenant's pre-login
state without detection. Modern Keycloak / Authentik / public-trust
CAs ship the discovery flag by default; legacy IdPs that don't
advertise are unaffected (back-compat preserved).

HOW.

- internal/auth/oidc/service.go
  - providerEntry gains issParamSupported bool.
  - getOrLoad extends the discovery-claims read to include
    authorization_response_iss_parameter_supported, alongside the
    existing id_token_signing_alg_values_supported defense.
  - HandleCallback's signature gains callbackIss string at position 5.
    Step 2.5 runs after the state compare + provider load: when
    issParamSupported is true, an empty callbackIss returns
    ErrIssParamMissing; a present-but-mismatched value returns
    ErrIssParamMismatch (constant-time compare).
  - Two new sentinels: ErrIssParamMissing, ErrIssParamMismatch.
    ErrIssuerMismatch's doc-string clarified to note it covers the
    in-token leg only.

- internal/api/handler/auth_session_oidc.go
  - OIDCAuthHandshaker.HandleCallback signature updated.
  - LoginCallback reads r.URL.Query().Get("iss") (no TrimSpace —
    byte-strict compare upstream) and threads it through.
  - classifyOIDCFailure: typed errors.Is dispatch for the three
    iss-family sentinels BEFORE the substring fall-through, so the
    three cases stay distinguishable in the audit row.

- internal/api/handler/auth_session_oidc_test.go
  - stubOIDCSvc.HandleCallback bumped to 7-arg signature.
  - TestClassifyOIDCFailure extended with 5 new cases pinning the
    iss-family dispatch + a wrapped-error round-trip.

- internal/auth/oidc/service_test.go
  - mockIdP gains advertiseIssParameterSupported bool; the
    /.well-known/openid-configuration handler emits the claim only
    when set (so existing tests stay back-compat).
  - 4 new regression tests:
    * MED17_NoSupport_AnyIssAccepted — provider doesn't advertise;
      arbitrary callbackIss is ignored (back-compat).
    * MED17_SupportButMissing — provider advertises; missing iss →
      ErrIssParamMissing.
    * MED17_SupportButMismatch — provider advertises; wrong iss →
      ErrIssParamMismatch (load-bearing mix-up defense).
    * MED17_SupportAndCorrect — provider advertises; matching iss →
      success path proves the gate isn't over-eager.

- internal/auth/oidc/bench_test.go,
  internal/auth/oidc/logging_test.go,
  internal/auth/oidc/integration_keycloak_test.go
  - Mechanical: all existing HandleCallback call sites updated to
    pass "" for callbackIss (matches pre-fix behavior for IdPs that
    don't advertise support — the Keycloak integration suite tests
    will be re-evaluated once the Keycloak fixture is run against a
    realm with the discovery flag enabled).

VERIFY.

- go vet ./internal/auth/oidc/... ./internal/api/handler/...   PASS
- go test -short -count=1 ./internal/auth/oidc/...              PASS (3.4s)
- go test -short -count=1 ./internal/api/handler/...            PASS (5.4s)
- 4 new MED-17 regression tests + extended TestClassifyOIDCFailure pass.

Refs: cowork/auth-bundles-audit-2026-05-10.md MED-17
      cowork/auth-bundles-fixes-2026-05-10/HANDOFF.md item 7
      RFC 9207 — OAuth 2.0 Authorization Server Issuer Identification

CHANGELOG.md

@@ -22,6 +22,18 @@
   cookie lifetime is unchanged (10 minutes) and only the callback handler
   consumes it; the wider path scope is harmless.
 
+- **RFC 9207 `iss` URL parameter check on OIDC callback (Audit 2026-05-10
+  MED-17).** When the matched IdP's discovery doc advertises
+  `authorization_response_iss_parameter_supported: true`, certctl now requires
+  the `iss` query parameter on `/auth/oidc/callback` and enforces a
+  constant-time compare against the configured provider's `IssuerURL`. Mismatch
+  rejects with HTTP 400; the audit row's `failure_category` distinguishes
+  `iss_param_missing` / `iss_param_mismatch` (RFC 9207 leg) from the existing
+  `id_token_iss_mismatch` (in-token iss claim leg). Closes the mix-up-attack
+  defense for modern Keycloak, Authentik, and public-trust CAs that ship
+  RFC-9207 discovery. Providers that don't advertise support (the majority
+  today) keep pre-fix behavior — back-compat is preserved.
+
 ## v2.1.0 - Auth Bundles 1 + 2: RBAC primitive + OIDC SSO + sessions ⚠️
 
 > **SECURITY: AUDIT YOUR API KEYS.**

internal/api/handler/auth_session_oidc.go

@@ -56,7 +56,11 @@ import (
 // consumes. Phase 3's *oidc.Service satisfies this directly.
 type OIDCAuthHandshaker interface {
 	HandleAuthRequest(ctx context.Context, providerID string) (authURL, cookieValue, preLoginID string, err error)
-	HandleCallback(ctx context.Context, preLoginCookie, code, callbackState, ip, userAgent string) (*oidcsvc.CallbackResult, 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
+	// check only when the provider's discovery doc advertised support.
+	HandleCallback(ctx context.Context, preLoginCookie, code, callbackState, callbackIss, ip, userAgent string) (*oidcsvc.CallbackResult, error)
 	RefreshKeys(ctx context.Context, providerID string) error
 }
 
@@ -272,6 +276,12 @@ func (h *AuthSessionOIDCHandler) LoginCallback(w http.ResponseWriter, r *http.Re
 	q := r.URL.Query()
 	code := strings.TrimSpace(q.Get("code"))
 	state := strings.TrimSpace(q.Get("state"))
+	// Audit 2026-05-10 MED-17 — RFC 9207 iss URL parameter. NOT
+	// trimmed; preserved exactly as sent so the service-layer compare
+	// against the matched provider's IssuerURL is byte-strict. The IdP
+	// emits this only when advertised in its discovery doc; the
+	// service-layer check is a no-op otherwise.
+	callbackIss := q.Get("iss")
 	if code == "" || state == "" {
 		Error(w, http.StatusBadRequest, "missing code or state query parameter")
 		return
@@ -286,7 +296,7 @@ func (h *AuthSessionOIDCHandler) LoginCallback(w http.ResponseWriter, r *http.Re
 	clientIP := clientIPFromRequest(r)
 	userAgent := r.UserAgent()
 
-	res, err := h.oidcSvc.HandleCallback(r.Context(), preLoginCookie.Value, code, state, clientIP, userAgent)
+	res, err := h.oidcSvc.HandleCallback(r.Context(), preLoginCookie.Value, code, state, callbackIss, clientIP, userAgent)
 	if err != nil {
 		// Audit 2026-05-10 HIGH-7 — instead of a blank 400, redirect
 		// to /login?error=oidc_failed&reason=<category>. The LoginPage
@@ -1152,10 +1162,30 @@ func clientIPFromRequest(r *http.Request) string {
 // classifyOIDCFailure maps an OIDC service error to a stable audit
 // category string. Used for the failure_category audit detail; the
 // wire stays uniform 400.
+//
+// Audit 2026-05-10 MED-17 — the three iss-related sentinel errors are
+// dispatched via errors.Is BEFORE the substring fall-through so they
+// stay distinguishable in the audit row:
+//   - ErrIssParamMissing  → iss_param_missing
+//   - ErrIssParamMismatch → iss_param_mismatch
+//   - ErrIssuerMismatch   → id_token_iss_mismatch
+//
+// errors.Is is used for the iss family because all three error
+// strings contain "iss" and substring matching would either collapse
+// them or order-dependently mis-classify.
 func classifyOIDCFailure(err error) string {
 	if err == nil {
 		return "ok"
 	}
+	// Audit 2026-05-10 MED-17 — typed dispatch for the iss family.
+	switch {
+	case errors.Is(err, oidcsvc.ErrIssParamMissing):
+		return "iss_param_missing"
+	case errors.Is(err, oidcsvc.ErrIssParamMismatch):
+		return "iss_param_mismatch"
+	case errors.Is(err, oidcsvc.ErrIssuerMismatch):
+		return "id_token_iss_mismatch"
+	}
 	msg := strings.ToLower(err.Error())
 	switch {
 	case strings.Contains(msg, "pre-login"):

internal/api/handler/auth_session_oidc_test.go

@@ -46,7 +46,7 @@ type stubOIDCSvc struct {
 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) {
+func (s *stubOIDCSvc) HandleCallback(_ context.Context, _, _, _, _, _, _ string) (*oidcsvc.CallbackResult, error) {
 	return s.callbackRes, s.callbackErr
 }
 func (s *stubOIDCSvc) RefreshKeys(_ context.Context, _ string) error { return s.refreshErr }
@@ -1197,6 +1197,15 @@ func TestClassifyOIDCFailure(t *testing.T) {
 		{errors.New("oidc: groups did not match any configured mapping"), "unmapped_groups"},
 		{errors.New("oidc: configured groups claim missing or malformed"), "groups_missing"},
 		{errors.New("oidc: jwks unreachable"), "jwks_unreachable"},
+		// Audit 2026-05-10 MED-17 — typed dispatch beats the substring
+		// fallthrough because all three iss-family sentinels contain
+		// "iss" in their message and would otherwise mis-classify.
+		{oidcsvc.ErrIssParamMissing, "iss_param_missing"},
+		{oidcsvc.ErrIssParamMismatch, "iss_param_mismatch"},
+		{oidcsvc.ErrIssuerMismatch, "id_token_iss_mismatch"},
+		// Wrapped variants must round-trip through errors.Is.
+		{fmt.Errorf("upstream: %w", oidcsvc.ErrIssParamMissing), "iss_param_missing"},
+		{fmt.Errorf("upstream: %w", oidcsvc.ErrIssParamMismatch), "iss_param_mismatch"},
 		{errors.New("some other error"), "unspecified"},
 	}
 	for _, tc := range cases {

internal/auth/oidc/bench_test.go

@@ -100,7 +100,7 @@ func BenchmarkOIDC_SteadyState(b *testing.B) {
 		}
 
 		start := time.Now()
-		_, err = svc.HandleCallback(ctx, cookie, "bench-code", "bench-state", "10.0.0.1", "bench/1.0")
+		_, err = svc.HandleCallback(ctx, cookie, "bench-code", "bench-state", "", "10.0.0.1", "bench/1.0")
 		elapsed := time.Since(start)
 		if err != nil {
 			b.Fatalf("HandleCallback: %v", err)

internal/auth/oidc/integration_keycloak_test.go

@@ -432,7 +432,7 @@ func TestKeycloakIntegration_AuthCodeFlow_HappyPath(t *testing.T) {
 	code, state := driveAuthCodeFlow(t, authURL, testfixtures.EngineerUser, testfixtures.EngineerPassword)
 
 	// Complete the OIDC handshake.
-	res, err := svc.HandleCallback(ctx, preLoginCookie, code, state, "10.0.0.1", "integration-test/1.0")
+	res, err := svc.HandleCallback(ctx, preLoginCookie, code, state, "", "10.0.0.1", "integration-test/1.0")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -491,7 +491,7 @@ func TestKeycloakIntegration_LogoutRevokesSession(t *testing.T) {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
 	code, state := driveAuthCodeFlow(t, authURL, testfixtures.EngineerUser, testfixtures.EngineerPassword)
-	res, err := svc.HandleCallback(ctx, preLoginCookie, code, state, "ip", "ua")
+	res, err := svc.HandleCallback(ctx, preLoginCookie, code, state, "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -534,7 +534,7 @@ func TestKeycloakIntegration_JWKSRotation_RefreshKeysPicksUpNewKey(t *testing.T)
 		t.Fatalf("pre-rotate HandleAuthRequest: %v", err)
 	}
 	preCode, preState := driveAuthCodeFlow(t, preAuthURL, testfixtures.EngineerUser, testfixtures.EngineerPassword)
-	if _, err := svc.HandleCallback(ctx, preCookie, preCode, preState, "ip", "ua"); err != nil {
+	if _, err := svc.HandleCallback(ctx, preCookie, preCode, preState, "", "ip", "ua"); err != nil {
 		t.Fatalf("pre-rotate HandleCallback: %v", err)
 	}
 
@@ -553,7 +553,7 @@ func TestKeycloakIntegration_JWKSRotation_RefreshKeysPicksUpNewKey(t *testing.T)
 		t.Fatalf("post-rotate HandleAuthRequest: %v", err)
 	}
 	postCode, postState := driveAuthCodeFlow(t, postAuthURL, testfixtures.EngineerUser, testfixtures.EngineerPassword)
-	if _, err := svc.HandleCallback(ctx, postCookie, postCode, postState, "ip", "ua"); err != nil {
+	if _, err := svc.HandleCallback(ctx, postCookie, postCode, postState, "", "ip", "ua"); err != nil {
 		t.Fatalf("post-rotate HandleCallback: %v (rotation broke validation?)", err)
 	}
 }
@@ -578,7 +578,7 @@ func TestKeycloakIntegration_UnmappedGroupsFailsClosed(t *testing.T) {
 		t.Fatalf("HandleAuthRequest: %v", err)
 	}
 	code, state := driveAuthCodeFlow(t, authURL, testfixtures.ViewerUser, testfixtures.ViewerPassword)
-	_, err = svc.HandleCallback(ctx, preCookie, code, state, "ip", "ua")
+	_, err = svc.HandleCallback(ctx, preCookie, code, state, "", "ip", "ua")
 	if !errors.Is(err, oidc.ErrGroupsUnmapped) {
 		t.Errorf("HandleCallback err = %v, want ErrGroupsUnmapped (fail-closed for unmapped groups)", err)
 	}

internal/auth/oidc/logging_test.go

@@ -92,7 +92,7 @@ func TestLoggingHygiene_HandleCallback_LeaksNothing(t *testing.T) {
 	defer restore()
 
 	authCode := "secret-auth-code-do-not-leak"
-	res, err := svc.HandleCallback(context.Background(), cookie, authCode, "the-state", "10.0.0.1", "Mozilla")
+	res, err := svc.HandleCallback(context.Background(), cookie, authCode, "the-state", "", "10.0.0.1", "Mozilla")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}

internal/auth/oidc/service.go

@@ -98,6 +98,15 @@ type providerEntry struct {
 	oauthConfig *oauth2.Config
 	allowedAlgs []string // intersected: domain config ∩ allow-list ∩ IdP-advertised
 	plaintext   []byte   // decrypted client secret; held for token exchange
+
+	// Audit 2026-05-10 MED-17 — RFC 9207 iss-URL-parameter support.
+	// Populated from the discovery doc's
+	// `authorization_response_iss_parameter_supported` claim during
+	// getOrLoad. When true, HandleCallback REQUIRES a non-empty
+	// callback iss URL param and compares it against the provider's
+	// IssuerURL. When false (the default for most IdPs that haven't
+	// rolled RFC 9207 yet), the check is skipped.
+	issParamSupported bool
 }
 
 // OIDCProviderLookup is a narrow read-side projection of
@@ -160,8 +169,30 @@ var (
 
 	// ErrIssuerMismatch: ID token `iss` doesn't match the configured
 	// provider issuer_url. HTTP 400.
+	//
+	// Audit 2026-05-10 MED-17 — also returned when the RFC 9207 iss
+	// URL parameter check fails (provider advertises
+	// authorization_response_iss_parameter_supported=true but the
+	// callback iss is missing or mismatched). The handler's
+	// classifyOIDCFailure breaks the two cases apart by audit
+	// failure_category (iss_param_missing / iss_param_mismatch /
+	// id_token_iss_mismatch).
 	ErrIssuerMismatch = errors.New("oidc: issuer mismatch")
 
+	// ErrIssParamMissing: provider's discovery doc advertises
+	// authorization_response_iss_parameter_supported=true but the
+	// callback URL had no `iss` query parameter. Per RFC 9207 §2.4 the
+	// client MUST reject the response in this case. HTTP 400. Pre-fix,
+	// the callback path ignored the parameter entirely.
+	ErrIssParamMissing = errors.New("oidc: provider advertises iss-parameter support but callback omitted it")
+
+	// ErrIssParamMismatch: provider's discovery doc advertises
+	// authorization_response_iss_parameter_supported=true and the
+	// callback supplied an `iss` query parameter, but it doesn't match
+	// the matched provider's issuer URL. Mixed-up attack defense per
+	// RFC 9207 §2.3. HTTP 400.
+	ErrIssParamMismatch = errors.New("oidc: callback iss parameter does not match provider issuer URL")
+
 	// ErrAudienceMismatch: ID token `aud` doesn't include the
 	// configured client_id. HTTP 400.
 	ErrAudienceMismatch = errors.New("oidc: audience mismatch")
@@ -383,9 +414,18 @@ type CallbackResult struct {
 }
 
 // HandleCallback completes the OIDC flow.
+//
+// Audit 2026-05-10 MED-17 — `callbackIss` is the value of the `iss`
+// query parameter on /auth/oidc/callback, exactly as sent by the IdP.
+// When the matched provider's discovery doc advertises
+// authorization_response_iss_parameter_supported=true (RFC 9207 §3),
+// we require this parameter and verify it equals the provider's
+// IssuerURL. When the provider doesn't advertise support (the default
+// for most IdPs that haven't rolled RFC 9207 yet), the parameter is
+// ignored — preserving back-compat with the pre-fix call path.
 func (s *Service) HandleCallback(
 	ctx context.Context,
-	preLoginCookie, code, callbackState, ip, userAgent string,
+	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)
@@ -403,6 +443,23 @@ func (s *Service) HandleCallback(
 		return nil, err
 	}
 
+	// Step 2.5 — Audit 2026-05-10 MED-17 — RFC 9207 iss URL parameter
+	// check. Only enforced when the provider advertised support in its
+	// discovery doc. Compares against the matched provider's
+	// IssuerURL (which is what go-oidc's gooidc.NewProvider verified
+	// the discovery doc's own iss against during getOrLoad). Mismatch
+	// is the load-bearing defense against mix-up attacks where the
+	// honest IdP returns the auth code to the wrong endpoint because
+	// of a malicious co-tenant relying-party.
+	if entry.issParamSupported {
+		if callbackIss == "" {
+			return nil, ErrIssParamMissing
+		}
+		if subtle.ConstantTimeCompare([]byte(callbackIss), []byte(entry.cfgRow.IssuerURL)) != 1 {
+			return nil, ErrIssParamMismatch
+		}
+	}
+
 	// Step 3: exchange the auth code for tokens (with PKCE verifier).
 	token, err := entry.oauthConfig.Exchange(ctx, code, oauth2.VerifierOption(verifier))
 	if err != nil {
@@ -755,8 +812,14 @@ func (s *Service) getOrLoad(ctx context.Context, providerID string) (*providerEn
 	// IdP downgrade-attack defense. The discovery doc's
 	// id_token_signing_alg_values_supported MUST NOT include any
 	// disallowed alg.
+	//
+	// Audit 2026-05-10 MED-17 — we also read
+	// `authorization_response_iss_parameter_supported` from the same
+	// claims call to drive the RFC 9207 iss-URL-parameter check in
+	// HandleCallback.
 	var advertised struct {
-		IDTokenSigningAlgValuesSupported []string `json:"id_token_signing_alg_values_supported"`
+		IDTokenSigningAlgValuesSupported       []string `json:"id_token_signing_alg_values_supported"`
+		AuthorizationResponseIssParamSupported bool     `json:"authorization_response_iss_parameter_supported"`
 	}
 	if cerr := provider.Claims(&advertised); cerr != nil {
 		return nil, fmt.Errorf("oidc: discovery claims: %w", cerr)
@@ -794,12 +857,13 @@ func (s *Service) getOrLoad(ctx context.Context, providerID string) (*providerEn
 	}
 
 	entry = &providerEntry{
-		cfgRow:      cfgRow,
-		provider:    provider,
-		verifier:    verifier,
-		oauthConfig: oauthConfig,
-		allowedAlgs: allowed,
-		plaintext:   plaintext,
+		cfgRow:            cfgRow,
+		provider:          provider,
+		verifier:          verifier,
+		oauthConfig:       oauthConfig,
+		allowedAlgs:       allowed,
+		plaintext:         plaintext,
+		issParamSupported: advertised.AuthorizationResponseIssParamSupported,
 	}
 
 	s.mu.Lock()

internal/auth/oidc/service_test.go

@@ -67,6 +67,12 @@ type mockIdP struct {
 	// downgrade-attack defense.
 	advertisedAlgs []string
 
+	// advertiseIssParameterSupported controls whether the discovery
+	// doc emits `authorization_response_iss_parameter_supported: true`.
+	// Audit 2026-05-10 MED-17 — drives the RFC 9207 iss URL parameter
+	// check in HandleCallback.
+	advertiseIssParameterSupported bool
+
 	// omitUserinfoEndpoint suppresses listing the userinfo endpoint in
 	// the discovery doc. Used to test the "userinfo fallback configured
 	// but provider has no userinfo endpoint" branch in fetchUserinfoGroups.
@@ -140,6 +146,12 @@ func newMockIdPWithTB(t testing.TB) *mockIdP {
 		if !idp.omitUserinfoEndpoint {
 			doc["userinfo_endpoint"] = base + "/userinfo"
 		}
+		// Audit 2026-05-10 MED-17 — only emit the iss-parameter claim
+		// when explicitly requested so default tests stay back-compat
+		// with pre-fix behavior.
+		if idp.advertiseIssParameterSupported {
+			doc["authorization_response_iss_parameter_supported"] = true
+		}
 		w.Header().Set("Content-Type", "application/json")
 		_ = json.NewEncoder(w).Encode(doc)
 	})
@@ -469,7 +481,7 @@ func TestService_StateReplayDeniedByConsumeOnce(t *testing.T) {
 // Test 3: forged pre-login cookie returns ErrPreLoginNotFound.
 func TestService_HandleCallback_RejectsForgedPreLoginCookie(t *testing.T) {
 	svc := newServiceForUnitTest(t)
-	_, err := svc.HandleCallback(context.Background(), "bogus-cookie", "any-code", "any-state", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), "bogus-cookie", "any-code", "any-state", "", "ip", "ua")
 	if !errors.Is(err, ErrPreLoginNotFound) {
 		t.Errorf("err = %v; want ErrPreLoginNotFound", err)
 	}
@@ -479,7 +491,7 @@ func TestService_HandleCallback_RejectsForgedPreLoginCookie(t *testing.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")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "wrong-state", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "wrong-state", "", "ip", "ua")
 	if !errors.Is(err, ErrStateMismatch) {
 		t.Errorf("err = %v; want ErrStateMismatch", err)
 	}
@@ -635,7 +647,7 @@ func TestService_HandleCallback_HappyPath(t *testing.T) {
 		t.Fatalf("CreatePreLogin: %v", err)
 	}
 
-	res, err := svc.HandleCallback(context.Background(), cookie, "test-code", "happy-state", "10.0.0.1", "Mozilla/5.0")
+	res, err := svc.HandleCallback(context.Background(), cookie, "test-code", "happy-state", "", "10.0.0.1", "Mozilla/5.0")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -657,7 +669,7 @@ func TestService_HandleCallback_RejectsWrongAudience(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-aud")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-aud", "s", "test-nonce-fixed", "v-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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.
 	if err == nil {
@@ -673,7 +685,7 @@ func TestService_HandleCallback_RejectsNonceMismatch(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-nonce")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-nonce", "s", "expected-nonce", "v-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrNonceMismatch) {
 		t.Errorf("err = %v; want ErrNonceMismatch", err)
 	}
@@ -686,7 +698,7 @@ func TestService_HandleCallback_RejectsExpiredToken(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-exp")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-exp", "s", "test-nonce-fixed", "v-cccccccccccccccccccccccccccccccccccccccccc")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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 {
 		t.Errorf("expected non-nil err for expired token")
@@ -703,7 +715,7 @@ func TestService_HandleCallback_RejectsIATTooOld(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iat")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-iat", "s", "test-nonce-fixed", "v-dddddddddddddddddddddddddddddddddddddddddd")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrIATTooOld) {
 		t.Errorf("err = %v; want ErrIATTooOld", err)
 	}
@@ -716,7 +728,7 @@ func TestService_HandleCallback_RejectsGroupsMissing(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-grp")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-grp", "s", "test-nonce-fixed", "v-eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
 	}
@@ -729,7 +741,7 @@ func TestService_HandleCallback_RejectsGroupsUnmapped(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPLNoMappings(t, idp.URL(), "op-unmap")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-unmap", "s", "test-nonce-fixed", "v-ffffffffffffffffffffffffffffffffffffffffff")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsUnmapped) {
 		t.Errorf("err = %v; want ErrGroupsUnmapped", err)
 	}
@@ -889,7 +901,7 @@ func TestService_UpsertUser_UpdateExistingPath(t *testing.T) {
 
 	// First login creates the user.
 	cookie1, _, _ := pl.CreatePreLogin(context.Background(), "op-upd", "s1", "test-nonce-fixed", "v-1aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa")
-	res1, err := svc.HandleCallback(context.Background(), cookie1, "code", "s1", "ip", "ua")
+	res1, err := svc.HandleCallback(context.Background(), cookie1, "code", "s1", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("first HandleCallback: %v", err)
 	}
@@ -903,7 +915,7 @@ func TestService_UpsertUser_UpdateExistingPath(t *testing.T) {
 	// Second login by same subject: update path, no new user row.
 	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")
+	res2, err := svc.HandleCallback(context.Background(), cookie2, "code2", "s2", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("second HandleCallback: %v", err)
 	}
@@ -1171,7 +1183,7 @@ func TestService_BootstrapHook_GrantsAdminOnMatch(t *testing.T) {
 	})
 
 	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")
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "10.0.0.1", "Mozilla/5.0")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -1194,7 +1206,7 @@ func TestService_BootstrapHook_NoMatchPreservesEmptyMappingFailClosed(t *testing
 	})
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-no-match", "s", "test-nonce-fixed", "v-nomatchxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}
@@ -1215,7 +1227,7 @@ func TestService_BootstrapHook_AdminAlreadyExistsFallsThroughToNormalMapping(t *
 	})
 
 	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")
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -1237,7 +1249,7 @@ func TestService_BootstrapHook_ErrorWraps(t *testing.T) {
 		return false, fmt.Errorf("simulated AdminExists probe failure")
 	})
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-hook-err", "s", "test-nonce-fixed", "v-errxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}
@@ -1258,7 +1270,7 @@ func TestService_BootstrapHook_IdempotentWhenAdminAlreadyMapped(t *testing.T) {
 	})
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-idem", "s", "test-nonce-fixed", "v-idempxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -1313,7 +1325,7 @@ func TestService_HandleCallback_AZPRequired_OnMultiAud(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-azp-req")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-azp-req", "s", "test-nonce-fixed", "v-azpreqxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrAZPRequired) {
 		t.Errorf("err = %v; want ErrAZPRequired", err)
 	}
@@ -1327,7 +1339,7 @@ func TestService_HandleCallback_AZPMismatch(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-azp-mis")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-azp-mis", "s", "test-nonce-fixed", "v-azpmisxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrAZPMismatch) {
 		t.Errorf("err = %v; want ErrAZPMismatch", err)
 	}
@@ -1345,7 +1357,7 @@ func TestService_HandleCallback_ATHashMismatch(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-ath-mis")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ath-mis", "s", "test-nonce-fixed", "v-athmisxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrATHashMismatch) {
 		t.Errorf("err = %v; want ErrATHashMismatch", err)
 	}
@@ -1362,7 +1374,7 @@ func TestService_HandleCallback_ATHashRequired_WhenAccessTokenPresent(t *testing
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-ath-req")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ath-req", "s", "test-nonce-fixed", "v-athreqxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrATHashRequired) {
 		t.Errorf("err = %v; want ErrATHashRequired", err)
 	}
@@ -1378,7 +1390,7 @@ func TestService_HandleCallback_IATInFuture(t *testing.T) {
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-iat-fut")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-iat-fut", "s", "test-nonce-fixed", "v-iatfutxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrIATInFuture) {
 		t.Errorf("err = %v; want ErrIATInFuture", err)
 	}
@@ -1396,7 +1408,7 @@ func TestService_HandleCallback_MappingsMapError(t *testing.T) {
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-map-err", "s", "test-nonce-fixed", "v-mapxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}
@@ -1414,7 +1426,7 @@ func TestService_HandleCallback_SessionMintError(t *testing.T) {
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-mint-err", "s", "test-nonce-fixed", "v-mintxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}
@@ -1433,7 +1445,7 @@ func TestService_HandleCallback_UserCreateError(t *testing.T) {
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-uc-err", "s", "test-nonce-fixed", "v-ucxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}
@@ -1453,7 +1465,7 @@ func TestService_HandleCallback_GetByOIDCSubjectNonNotFoundError(t *testing.T) {
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-get-err", "s", "test-nonce-fixed", "v-getxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}
@@ -1474,7 +1486,7 @@ func TestService_UpsertUser_DisplayNameFallsBackToEmail(t *testing.T) {
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	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")
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -1500,7 +1512,7 @@ func TestService_FetchUserinfoGroups_HappyPath_OnEmptyIDTokenGroups(t *testing.T
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	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")
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if err != nil {
 		t.Fatalf("HandleCallback: %v", err)
 	}
@@ -1525,7 +1537,7 @@ func TestService_FetchUserinfoGroups_ReturnsErrGroupsMissing_WhenUserinfoAlsoEmp
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-empty", "s", "test-nonce-fixed", "v-uixxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
 	}
@@ -1547,7 +1559,7 @@ func TestService_FetchUserinfoGroups_ReturnsErrGroupsMissing_WhenEndpointMissing
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-noendpoint", "s", "test-nonce-fixed", "v-uixxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
 	}
@@ -1691,7 +1703,7 @@ func TestService_HandleCallback_RejectsTokenResponseMissingIDToken(t *testing.T)
 	svc, pl := newServiceWithProviderAndPL(t, idp.URL(), "op-no-idtok")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-no-idtok", "s", "test-nonce-fixed", "v-noidxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}
@@ -1714,7 +1726,7 @@ func TestService_FetchUserinfoGroups_ReturnsErrGroupsMissing_WhenUserinfoFails(t
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-ui-500", "s", "test-nonce-fixed", "v-uifxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "", "ip", "ua")
 	if !errors.Is(err, ErrGroupsMissing) {
 		t.Errorf("err = %v; want ErrGroupsMissing", err)
 	}
@@ -1760,6 +1772,103 @@ func TestService_AlgPinning_HeaderAlgValueUnterminatedString(t *testing.T) {
 	}
 }
 
+// =============================================================================
+// MED-17 regression tests — RFC 9207 iss URL parameter check.
+//
+// HandleCallback REQUIRES the `iss` callback URL parameter when the
+// provider's discovery doc advertises
+// authorization_response_iss_parameter_supported=true. Pre-fix the
+// parameter was ignored; mix-up attacks could route the auth code to
+// the wrong relying-party endpoint without detection.
+// =============================================================================
+
+// TestService_HandleCallback_MED17_NoSupport_AnyIssAccepted pins the
+// back-compat case: providers that don't advertise iss-parameter
+// support (the majority today) get the same behavior as before —
+// callback iss is not required and an arbitrary value is ignored.
+func TestService_HandleCallback_MED17_NoSupport_AnyIssAccepted(t *testing.T) {
+	idp := newMockIdP(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")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+
+	// Pass a callbackIss value the provider didn't advertise support
+	// for — the service must ignore it.
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "iss-bc-state", "https://malicious.example/", "ip", "ua")
+	if err != nil {
+		t.Fatalf("HandleCallback (no-support, arbitrary callbackIss): %v; want nil (parameter must be ignored)", err)
+	}
+	if res == nil {
+		t.Fatalf("CallbackResult nil for back-compat happy path")
+	}
+}
+
+// TestService_HandleCallback_MED17_SupportButMissing rejects with
+// ErrIssParamMissing when the provider advertised support but the
+// callback URL omitted the iss query parameter.
+func TestService_HandleCallback_MED17_SupportButMissing(t *testing.T) {
+	idp := newMockIdP(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")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	_, err = svc.HandleCallback(context.Background(), cookie, "code", "iss-miss-state", "", "ip", "ua")
+	if !errors.Is(err, ErrIssParamMissing) {
+		t.Fatalf("err = %v; want ErrIssParamMissing", err)
+	}
+}
+
+// TestService_HandleCallback_MED17_SupportButMismatch rejects with
+// ErrIssParamMismatch when the provider advertised support and the
+// callback URL supplied an iss query parameter but the value doesn't
+// match the matched provider's IssuerURL. This is the load-bearing
+// mix-up-attack defense.
+func TestService_HandleCallback_MED17_SupportButMismatch(t *testing.T) {
+	idp := newMockIdP(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")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	// Supply an honest-looking but wrong iss — the impersonator's URL
+	// instead of the matched provider's IssuerURL.
+	_, err = svc.HandleCallback(context.Background(), cookie, "code", "iss-mm-state", "https://attacker.example/", "ip", "ua")
+	if !errors.Is(err, ErrIssParamMismatch) {
+		t.Fatalf("err = %v; want ErrIssParamMismatch", err)
+	}
+}
+
+// TestService_HandleCallback_MED17_SupportAndCorrect succeeds when the
+// callback iss exactly matches the matched provider's IssuerURL —
+// the success path that proves the gate isn't over-eager.
+func TestService_HandleCallback_MED17_SupportAndCorrect(t *testing.T) {
+	idp := newMockIdP(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")
+	if err != nil {
+		t.Fatalf("CreatePreLogin: %v", err)
+	}
+	// The matched provider's IssuerURL is the mockIdP server URL.
+	res, err := svc.HandleCallback(context.Background(), cookie, "code", "iss-ok-state", idp.URL(), "ip", "ua")
+	if err != nil {
+		t.Fatalf("HandleCallback (correct iss): %v", err)
+	}
+	if res == nil {
+		t.Fatalf("CallbackResult nil for happy iss path")
+	}
+}
+
 // 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;
@@ -1776,7 +1885,7 @@ func TestService_UpsertUser_ValidateErrorOnEmptyEmail(t *testing.T) {
 	svc := NewService(&stubProviderLookup{provider: prov}, mappings, users, sessions, pl, "")
 
 	cookie, _, _ := pl.CreatePreLogin(context.Background(), "op-validate-err", "s", "test-nonce-fixed", "v-valxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx")
-	_, err := svc.HandleCallback(context.Background(), cookie, "code", "s", "ip", "ua")
+	_, 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)
 	}