harden(oidc): strict UA/IP binding (A-6) — close request-empty bypass in MED-16

The MED-16 closure (cb73547) added the RFC 9700 §4.7.1 pre-login
UA/IP binding but the consume-side compare at
internal/auth/oidc/service.go was gated by:

  if s.preLoginRequireUA && storedUA != "" && userAgent != "" {
      ... constant-time compare ...
  }
  if s.preLoginRequireIP && storedIP != "" && ip != "" {
      ... constant-time compare ...
  }

The `userAgent != ""` and `ip != ""` arms were intended as
rolling-deploy / headless-proxy compat ("if the request didn't supply
a value, don't try to compare against nothing"). They achieve that —
and they ALSO short-circuit the compare whenever the **attacker**
controls the request side, which is always at /auth/oidc/callback.

Threat model:
  1. Attacker acquires a pre-login cookie (HMAC-protected; requires
     RNG break OR transit leak — not implausible, that's why the
     binding exists in the first place).
  2. Attacker replays the cookie at /auth/oidc/callback from their
     own user-agent.
  3. Attacker OMITS the User-Agent header. curl doesn't send one by
     default. Many programmatic HTTP clients omit it.

Pre-A-6, step 3 trivially bypassed the binding check. The whole
RFC 9700 §4.7.1 defense was theatre against the realistic threat —
silent-allow when the attacker abandons the header they don't want
checked.

Fix: flipped to strict-when-stored. When the pre-login row carries a
binding value (storedUA != "" or storedIP != ""), the request MUST
present a matching value. An empty request side with a non-empty
stored side now rejects with two new sentinels:

  ErrPreLoginUAMissing  — request omitted User-Agent header
  ErrPreLoginIPMissing  — request had no resolvable client IP

Distinguished from the existing *Mismatch sentinels so the audit
row can tell apart "binding violation" (operator mis-configured the
proxy) from "missing-header bypass attempt" (active exploit indicator).
The handler-side classifyOIDCFailure adds typed errors.Is dispatch:

  ErrPreLoginUAMissing → "prelogin_ua_missing"
  ErrPreLoginIPMissing → "prelogin_ip_missing"

SIEM rules can now alert specifically on the bypass-attempt category
distinctly from operator config drift.

Legacy-row compat preserved: pre-migration rows where storedUA == ""
/ storedIP == "" still pass through unchecked. That window is
bounded by the 10-minute pre-login TTL — within 10 minutes of the
MED-16 deploy every legacy row has expired and the strict path is
universal.

Operator escape hatches preserved: CERTCTL_OIDC_PRELOGIN_REQUIRE_UA=false
(symmetric for IP) bypasses both the *Mismatch AND the new *Missing
reject paths. Required for environments where a proxy strips the
User-Agent header in transit (rare but documented in the operator
advisory).

Regression coverage:

  service_test.go (5 new tests under
  `Audit 2026-05-11 A-6 — strict-when-stored` block):
    TestService_HandleCallback_MED16_A6_UAStoredButRequestEmpty_Rejects
      — the load-bearing bypass-closure leg
    TestService_HandleCallback_MED16_A6_IPStoredButRequestEmpty_Rejects
      — symmetric for IP
    TestService_HandleCallback_MED16_A6_LegacyRowEmptyStoredStillPasses
      — legacy-row compat preserved
    TestService_HandleCallback_MED16_A6_ToggleOff_AllowsBypass
      — UA toggle off allows the bypass (operator escape hatch)
    TestService_HandleCallback_MED16_A6_ToggleOff_IP_AllowsBypass
      — IP toggle off allows the bypass

  auth_session_oidc_test.go::TestClassifyOIDCFailure extended:
    ErrPreLoginUAMismatch → prelogin_ua_mismatch (new explicit pin)
    ErrPreLoginIPMismatch → prelogin_ip_mismatch (new explicit pin)
    ErrPreLoginUAMissing → prelogin_ua_missing
    ErrPreLoginIPMissing → prelogin_ip_missing
    fmt.Errorf wrapped variants of the *Missing sentinels round-trip
    through errors.Is (defense against future context-wrapping in
    the service layer)

Verify gate green: gofmt clean, go vet clean, all 10 MED-16 tests
+ extended TestClassifyOIDCFailure pass; full short-mode test run
across internal/auth/oidc + internal/api/handler also green.

Spec at cowork/auth-bundles-fixes-2026-05-11/06-high-prelogin-ua-strict-mode.md.
Audit doc: MED-16 row in cowork/auth-bundles-audit-2026-05-10.md
appended with the A-6 follow-up closure annotation; status table
row updated to "CLOSED + A-6 follow-up CLOSED 2026-05-11".
Operator advisory in CHANGELOG.md v2.1.0 release notes covers the
two operator-visible behaviour changes: (1) callback requests
without User-Agent now reject when a binding was stored, and (2)
the CERTCTL_OIDC_PRELOGIN_REQUIRE_UA=false escape hatch is the
documented path for environments where the proxy strips the header.

internal/auth/oidc/service.go

@@ -240,6 +240,26 @@ var (
 	// CERTCTL_OIDC_PRELOGIN_REQUIRE_IP=false to disable.
 	ErrPreLoginIPMismatch = errors.New("oidc: pre-login row client IP does not match callback request")
 
+	// ErrPreLoginUAMissing: the pre-login row carries a User-Agent
+	// binding (MED-16) but the /auth/oidc/callback request omitted
+	// the User-Agent header. Audit 2026-05-11 A-6 closure — the
+	// original MED-16 logic short-circuited the compare when the
+	// request side was empty, which let an attacker bypass the
+	// binding by sending a callback with no User-Agent (trivial:
+	// curl, many programmatic clients omit the header by default).
+	// Distinguished from ErrPreLoginUAMismatch so the audit row
+	// can tell a binding violation apart from a missing-header
+	// bypass attempt. HTTP 400. Operators on enterprise proxies
+	// that strip the User-Agent header in transit can disable the
+	// check with CERTCTL_OIDC_PRELOGIN_REQUIRE_UA=false.
+	ErrPreLoginUAMissing = errors.New("oidc: pre-login row has User-Agent binding but callback omitted User-Agent header")
+
+	// ErrPreLoginIPMissing: symmetric to ErrPreLoginUAMissing for
+	// the source IP binding. Reachable when XFF-trust gating zeros
+	// the resolved client IP for a request whose pre-login row
+	// captured one. Audit 2026-05-11 A-6 closure.
+	ErrPreLoginIPMissing = errors.New("oidc: pre-login row has client-IP binding but callback request had no resolvable client IP")
+
 	// ErrAudienceMismatch: ID token `aud` doesn't include the
 	// configured client_id. HTTP 400.
 	ErrAudienceMismatch = errors.New("oidc: audience mismatch")
@@ -508,12 +528,33 @@ func (s *Service) HandleCallback(
 	// 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 != "" {
+	// Audit 2026-05-11 A-6 — strict-when-stored. The original MED-16
+	// closure short-circuited the compare when the request side was
+	// empty (`userAgent != ""` / `ip != ""`), which was an
+	// attacker-controllable bypass: an attacker forging a callback
+	// request can simply omit the User-Agent header (curl does this by
+	// default; many programmatic HTTP clients omit it) and the binding
+	// check skips silently. Now: when the pre-login row carries a
+	// binding, the request MUST present a matching value; an empty
+	// request value with a non-empty stored value rejects with
+	// ErrPreLoginUA{IP}Missing (distinct from the mismatch leg so the
+	// audit row can tell them apart). Legacy-row compat — pre-migration
+	// rows with `storedUA == ""` / `storedIP == ""` — still passes
+	// unchecked; that window is bounded by the 10-minute pre-login TTL,
+	// so within 10 minutes of the MED-16 deploy the strict path is
+	// universal.
+	if s.preLoginRequireUA && storedUA != "" {
+		if userAgent == "" {
+			return nil, ErrPreLoginUAMissing
+		}
 		if subtle.ConstantTimeCompare([]byte(userAgent), []byte(storedUA)) != 1 {
 			return nil, ErrPreLoginUAMismatch
 		}
 	}
-	if s.preLoginRequireIP && storedIP != "" && ip != "" {
+	if s.preLoginRequireIP && storedIP != "" {
+		if ip == "" {
+			return nil, ErrPreLoginIPMissing
+		}
 		if subtle.ConstantTimeCompare([]byte(ip), []byte(storedIP)) != 1 {
 			return nil, ErrPreLoginIPMismatch
 		}