fix(security): reject WebAuthn signature-counter rollback

Sign counters were stored but never checked, so a cloned authenticator or a
replayed assertion with an equal/lower counter was accepted. Reject the
authentication when the presented counter does not exceed the stored one,
while allowing counter-less/synced passkeys that always report 0.

Refs: porchlight-3cr

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/porchlight/authn/routes.py

@@ -23,6 +23,19 @@ def _login_redirect_target(request: Request) -> str:
     return "/manage/credentials"
 
 
+def _is_sign_count_rollback(stored: int, presented: int) -> bool:
+    """Detect a WebAuthn signature-counter rollback (cloned authenticator/replay).
+
+    Authenticators that don't maintain a counter (incl. synced passkeys) report
+    0; when both stored and presented are 0 the counter is meaningless and no
+    rollback can be inferred. Otherwise the presented counter must strictly
+    exceed the stored one.
+    """
+    if stored == 0 and presented == 0:
+        return False
+    return presented <= stored
+
+
 def _establish_authenticated_session(request: Request, user: User) -> None:
     """Reset the session before recording the authenticated identity.
 
@@ -156,7 +169,7 @@ async def login_webauthn_begin(request: Request) -> Response:
 
 @router.post("/login/webauthn/complete")
 @limiter.limit("10/minute")
-async def login_webauthn_complete(request: Request) -> Response:
+async def login_webauthn_complete(request: Request) -> Response:  # noqa: PLR0911
     webauthn_service = request.app.state.webauthn_service
     user_repo = request.app.state.user_repo
     cred_repo = request.app.state.credential_repo
@@ -194,6 +207,9 @@ async def login_webauthn_complete(request: Request) -> Response:
 
     stored = await cred_repo.get_webauthn_by_credential_id(matched_credential_id)
     if stored is not None:
+        if _is_sign_count_rollback(stored.sign_count, new_counter):
+            # Counter went backwards — likely a cloned authenticator or replay.
+            return JSONResponse({"error": "Authentication failed"}, status_code=400)
         stored.sign_count = new_counter
         await cred_repo.update_webauthn(stored)