diff --git a/src/porchlight/app.py b/src/porchlight/app.py
index d399923..e07d51f 100644
--- a/src/porchlight/app.py
+++ b/src/porchlight/app.py
@@ -8,6 +8,7 @@ from urllib.parse import urlparse
 from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
+from fido2.webauthn import UserVerificationRequirement
 from slowapi.errors import RateLimitExceeded
 from starlette.middleware.sessions import SessionMiddleware
 from starlette.requests import Request
@@ -55,6 +56,7 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
                 rp_id=rp_id,
                 rp_name=app.title,
                 origin=settings.issuer,
+                user_verification=UserVerificationRequirement(settings.webauthn_user_verification),
             )
 
             app.state.magic_link_service = MagicLinkService(
@@ -80,8 +82,8 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
                 }
                 oidc_server.keyjar.add_symmetric(client_id, client_cfg.client_secret)
 
-            # Register management client
-            manage_secret = settings.session_secret or secrets.token_hex(32)
+            # Register management client (stable secret tied to the session secret)
+            manage_secret = app.state.session_secret
             oidc_server.context.cdb[settings.manage_client_id] = {
                 "client_id": settings.manage_client_id,
                 "client_secret": manage_secret,
@@ -99,6 +101,21 @@ async def lifespan(app: FastAPI) -> AsyncIterator[None]:
         raise NotImplementedError("MongoDB backend not yet implemented")
 
 
+def _resolve_session_secret(settings: Settings) -> str:
+    """Return the session signing secret, requiring one in production.
+
+    A random per-process secret silently invalidates sessions on restart and
+    rotates the management client secret, so it is only acceptable for local
+    development (debug or a localhost issuer).
+    """
+    if settings.session_secret:
+        return settings.session_secret
+    host = urlparse(settings.issuer).hostname or ""
+    if settings.debug or host in ("localhost", "127.0.0.1", "::1"):
+        return secrets.token_hex(32)
+    raise RuntimeError("OIDC_OP_SESSION_SECRET must be set in production (non-debug, non-localhost issuer).")
+
+
 def create_app(settings: Settings | None = None) -> FastAPI:
     if settings is None:
         settings = Settings()
@@ -114,7 +131,8 @@ def create_app(settings: Settings | None = None) -> FastAPI:
     app.state.settings = settings
 
     # Session middleware
-    session_secret = settings.session_secret or secrets.token_hex(32)
+    session_secret = _resolve_session_secret(settings)
+    app.state.session_secret = session_secret
     app.add_middleware(
         CSRFMiddleware,  # ty: ignore[invalid-argument-type]
         exempt_paths={"/token", "/userinfo"},
diff --git a/src/porchlight/authn/routes.py b/src/porchlight/authn/routes.py
index 606b315..465ebde 100644
--- a/src/porchlight/authn/routes.py
+++ b/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.
 
@@ -144,7 +157,7 @@ async def register_magic_link(request: Request, token: str) -> Response:
     return RedirectResponse("/manage/credentials?setup=1", status_code=303)
 
 
-@router.get("/login/webauthn/begin")
+@router.post("/login/webauthn/begin")
 async def login_webauthn_begin(request: Request) -> Response:
     webauthn_service = request.app.state.webauthn_service
 
@@ -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)
 
diff --git a/src/porchlight/authn/webauthn.py b/src/porchlight/authn/webauthn.py
index 1c9d8ce..5c4c4e2 100644
--- a/src/porchlight/authn/webauthn.py
+++ b/src/porchlight/authn/webauthn.py
@@ -18,9 +18,16 @@ from fido2.webauthn import (
 class WebAuthnService:
     """FIDO2 WebAuthn registration and authentication."""
 
-    def __init__(self, rp_id: str, rp_name: str, origin: str) -> None:
+    def __init__(
+        self,
+        rp_id: str,
+        rp_name: str,
+        origin: str,
+        user_verification: UserVerificationRequirement = UserVerificationRequirement.PREFERRED,
+    ) -> None:
         rp = PublicKeyCredentialRpEntity(name=rp_name, id=rp_id)
         self._server = Fido2Server(rp, verify_origin=lambda o: o == origin)
+        self._user_verification = user_verification
 
     def begin_registration(
         self,
@@ -39,7 +46,7 @@ class WebAuthnService:
             user=user,
             credentials=existing_credentials,
             resident_key_requirement=ResidentKeyRequirement.REQUIRED,
-            user_verification=UserVerificationRequirement.PREFERRED,
+            user_verification=self._user_verification,
         )
         return dict(options), state
 
@@ -64,7 +71,7 @@ class WebAuthnService:
         """
         options, state = self._server.authenticate_begin(
             credentials=credentials,
-            user_verification=UserVerificationRequirement.PREFERRED,
+            user_verification=self._user_verification,
         )
         return dict(options), state
 
diff --git a/src/porchlight/config.py b/src/porchlight/config.py
index dff55a5..a8225d2 100644
--- a/src/porchlight/config.py
+++ b/src/porchlight/config.py
@@ -49,6 +49,10 @@ class Settings(BaseSettings):
     session_secret: str | None = None  # If None, a random secret is generated per process
     session_https_only: bool = True
 
+    # WebAuthn user verification requirement: "preferred" (default), "required",
+    # or "discouraged". Identity providers may want "required".
+    webauthn_user_verification: str = "preferred"
+
     # Magic links
     invite_ttl: int = 86400  # seconds
 
diff --git a/src/porchlight/static/webauthn.js b/src/porchlight/static/webauthn.js
index 0ed2a50..9052d0f 100644
--- a/src/porchlight/static/webauthn.js
+++ b/src/porchlight/static/webauthn.js
@@ -19,6 +19,16 @@ function getCsrfToken() {
   return meta ? meta.getAttribute('content') : '';
 }
 
+// Render an alert with the message as text (never as HTML) to avoid XSS.
+function showAlert(el, message) {
+  if (!el) return;
+  el.replaceChildren();
+  const div = document.createElement('div');
+  div.setAttribute('role', 'alert');
+  div.textContent = message;
+  el.appendChild(div);
+}
+
 async function beginRegistration() {
   const statusEl = document.getElementById('webauthn-status');
 
@@ -29,7 +39,7 @@ async function beginRegistration() {
       headers: { 'Content-Type': 'application/json', 'X-CSRF-Token': getCsrfToken() },
     });
     if (!beginRes.ok) {
-      if (statusEl) statusEl.innerHTML = '<div role="alert">Failed to start registration</div>';
+      showAlert(statusEl, 'Failed to start registration');
       return;
     }
     const options = await beginRes.json();
@@ -74,7 +84,7 @@ async function beginRegistration() {
       if (statusEl) statusEl.innerHTML = text;
     }
   } catch (err) {
-    if (statusEl) statusEl.innerHTML = '<div role="alert">Registration failed: ' + err.message + '</div>';
+    showAlert(statusEl, 'Registration failed: ' + err.message);
   }
 }
 
@@ -83,10 +93,13 @@ async function beginAuthentication() {
 
   try {
     // Step 1: Get options from server (no username needed)
-    const beginRes = await fetch('/login/webauthn/begin');
+    const beginRes = await fetch('/login/webauthn/begin', {
+      method: 'POST',
+      headers: { 'X-CSRF-Token': getCsrfToken() },
+    });
     if (!beginRes.ok) {
       const data = await beginRes.json();
-      if (statusEl) statusEl.innerHTML = '<div role="alert">' + (data.error || 'Failed to start authentication') + '</div>';
+      showAlert(statusEl, data.error || 'Failed to start authentication');
       return;
     }
     const options = await beginRes.json();
@@ -129,10 +142,10 @@ async function beginAuthentication() {
       window.location.href = data.redirect || '/manage/credentials';
     } else {
       const data = await completeRes.json().catch(function () { return {}; });
-      if (statusEl) statusEl.innerHTML = '<div role="alert">' + (data.error || 'Authentication failed') + '</div>';
+      showAlert(statusEl, data.error || 'Authentication failed');
     }
   } catch (err) {
-    if (statusEl) statusEl.innerHTML = '<div role="alert">Authentication failed: ' + err.message + '</div>';
+    showAlert(statusEl, 'Authentication failed: ' + err.message);
   }
 }
 
diff --git a/src/porchlight/store/sqlite/migrations/003_webauthn_credential_id_unique.sql b/src/porchlight/store/sqlite/migrations/003_webauthn_credential_id_unique.sql
new file mode 100644
index 0000000..23eaa20
--- /dev/null
+++ b/src/porchlight/store/sqlite/migrations/003_webauthn_credential_id_unique.sql
@@ -0,0 +1,5 @@
+-- A WebAuthn credential_id must be globally unique: lookups by credential_id
+-- alone (during usernameless authentication) must never resolve to the wrong
+-- user. The table PK is (user_id, credential_id), which does not prevent the
+-- same credential_id under two users, so add an explicit unique index.
+CREATE UNIQUE INDEX ix_webauthn_credential_id ON webauthn_credentials (credential_id);
diff --git a/tests/test_app.py b/tests/test_app.py
index 82f1358..14c3aca 100644
--- a/tests/test_app.py
+++ b/tests/test_app.py
@@ -1,7 +1,10 @@
 from unittest.mock import MagicMock
 
+import pytest
 from httpx import AsyncClient
 
+from porchlight.app import create_app
+from porchlight.config import Settings
 from porchlight.dependencies import (
     get_credential_repo,
     get_magic_link_repo,
@@ -56,3 +59,19 @@ async def test_dependency_functions() -> None:
     assert get_user_repo(request) == "user_repo_sentinel"
     assert get_credential_repo(request) == "credential_repo_sentinel"
     assert get_magic_link_repo(request) == "magic_link_repo_sentinel"
+
+
+def test_create_app_requires_session_secret_in_production() -> None:
+    settings = Settings(issuer="https://op.example.com", sqlite_path=":memory:", debug=False)
+    with pytest.raises(RuntimeError, match="SESSION_SECRET"):
+        create_app(settings)
+
+
+def test_create_app_allows_missing_secret_on_localhost() -> None:
+    settings = Settings(issuer="http://localhost:8000", sqlite_path=":memory:")
+    assert create_app(settings) is not None
+
+
+def test_create_app_allows_missing_secret_in_debug() -> None:
+    settings = Settings(issuer="https://op.example.com", sqlite_path=":memory:", debug=True)
+    assert create_app(settings) is not None
diff --git a/tests/test_auth_routes/test_webauthn_login.py b/tests/test_auth_routes/test_webauthn_login.py
index 5952633..4959045 100644
--- a/tests/test_auth_routes/test_webauthn_login.py
+++ b/tests/test_auth_routes/test_webauthn_login.py
@@ -46,10 +46,12 @@ async def _setup_user_with_webauthn(
 
 
 async def test_webauthn_login_begin_returns_options(client: AsyncClient) -> None:
-    """Begin is now GET with no username — returns options with empty allowCredentials."""
+    """Begin is a POST (mutates session) with no username — returns options
+    with empty allowCredentials."""
     await _setup_user_with_webauthn(client)
 
-    res = await client.get("/login/webauthn/begin")
+    token = await get_csrf_token(client)
+    res = await client.post("/login/webauthn/begin", headers={"X-CSRF-Token": token})
     assert res.status_code == 200
     data = res.json()
     assert "publicKey" in data
@@ -59,7 +61,8 @@ async def test_webauthn_login_begin_returns_options(client: AsyncClient) -> None
 
 
 async def test_webauthn_login_begin_has_user_verification_preferred(client: AsyncClient) -> None:
-    res = await client.get("/login/webauthn/begin")
+    token = await get_csrf_token(client)
+    res = await client.post("/login/webauthn/begin", headers={"X-CSRF-Token": token})
     assert res.status_code == 200
     data = res.json()
     assert data["publicKey"]["userVerification"] == "preferred"
@@ -81,10 +84,9 @@ async def test_webauthn_login_complete_returns_json_redirect(client: AsyncClient
     _userid, _pk, _credential_id, _att = await _setup_user_with_webauthn(client)
 
     # Begin to get state into session
-    res1 = await client.get("/login/webauthn/begin")
-    assert res1.status_code == 200
-
     token = await get_csrf_token(client)
+    res1 = await client.post("/login/webauthn/begin", headers={"X-CSRF-Token": token})
+    assert res1.status_code == 200
     # We can't easily complete the full assertion without browser interaction,
     # but we verify the endpoint returns 400 JSON (not HTML) for bad assertions
     res2 = await client.post(
diff --git a/tests/test_authn/test_sign_count.py b/tests/test_authn/test_sign_count.py
new file mode 100644
index 0000000..5c8a65d
--- /dev/null
+++ b/tests/test_authn/test_sign_count.py
@@ -0,0 +1,22 @@
+from porchlight.authn.routes import _is_sign_count_rollback
+
+
+def test_increasing_counter_is_not_rollback() -> None:
+    assert _is_sign_count_rollback(stored=5, presented=6) is False
+
+
+def test_equal_counter_is_rollback() -> None:
+    assert _is_sign_count_rollback(stored=5, presented=5) is True
+
+
+def test_lower_counter_is_rollback() -> None:
+    assert _is_sign_count_rollback(stored=5, presented=3) is True
+
+
+def test_both_zero_sync_passkey_is_allowed() -> None:
+    # Sync passkeys (and counter-less authenticators) always report 0.
+    assert _is_sign_count_rollback(stored=0, presented=0) is False
+
+
+def test_first_increment_from_zero_is_allowed() -> None:
+    assert _is_sign_count_rollback(stored=0, presented=1) is False
diff --git a/tests/test_authn/test_webauthn.py b/tests/test_authn/test_webauthn.py
index 9c3da22..339d5a5 100644
--- a/tests/test_authn/test_webauthn.py
+++ b/tests/test_authn/test_webauthn.py
@@ -17,6 +17,7 @@ from fido2.webauthn import (
     PublicKeyCredentialDescriptor,
     PublicKeyCredentialType,
     RegistrationResponse,
+    UserVerificationRequirement,
 )
 
 from porchlight.authn.webauthn import WebAuthnService
@@ -194,6 +195,19 @@ def test_begin_authentication_prefers_user_verification() -> None:
     assert pub_key["userVerification"] == "preferred"
 
 
+def test_user_verification_is_configurable() -> None:
+    service = WebAuthnService(
+        rp_id=RP_ID,
+        rp_name=RP_NAME,
+        origin=ORIGIN,
+        user_verification=UserVerificationRequirement.REQUIRED,
+    )
+    reg, _ = service.begin_registration(user_id=b"user-123", username="alice")
+    assert reg["publicKey"]["authenticatorSelection"]["userVerification"] == "required"
+    auth, _ = service.begin_authentication()
+    assert auth["publicKey"]["userVerification"] == "required"
+
+
 def test_begin_authentication_returns_options_and_state() -> None:
     service = _make_service()
     _, cred_id, _attested = _generate_credential()
diff --git a/tests/test_client_registration.py b/tests/test_client_registration.py
index d1d7e2b..ea6ba37 100644
--- a/tests/test_client_registration.py
+++ b/tests/test_client_registration.py
@@ -19,7 +19,7 @@ scope = ["openid", "profile"]
     toml_file = tmp_path / "test.toml"
     toml_file.write_text(toml_content)
 
-    settings = Settings(_toml_file=str(toml_file))
+    settings = Settings(_toml_file=str(toml_file), session_secret="x" * 32)
     app = create_app(settings)
 
     async with (
@@ -41,7 +41,7 @@ scope = ["openid", "profile"]
 
 async def test_manage_app_always_registered() -> None:
     """The internal manage-app client is always registered, even without config file clients."""
-    settings = Settings(issuer="https://test.example.com")
+    settings = Settings(issuer="https://test.example.com", session_secret="x" * 32)
     app = create_app(settings)
 
     async with (
diff --git a/tests/test_store/test_migrations.py b/tests/test_store/test_migrations.py
index 386aa5a..4466d76 100644
--- a/tests/test_store/test_migrations.py
+++ b/tests/test_store/test_migrations.py
@@ -13,7 +13,7 @@ async def test_run_migrations_applies_initial() -> None:
     async with aiosqlite.connect(":memory:") as db:
         await db.execute("PRAGMA foreign_keys=ON")
         count = await run_migrations(db, MIGRATIONS_DIR)
-        assert count == 2
+        assert count == 3
         async with db.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='users'") as cursor:
             row = await cursor.fetchone()
         assert row is not None
@@ -24,7 +24,7 @@ async def test_run_migrations_skips_already_applied() -> None:
         await db.execute("PRAGMA foreign_keys=ON")
         first_count = await run_migrations(db, MIGRATIONS_DIR)
         second_count = await run_migrations(db, MIGRATIONS_DIR)
-        assert first_count == 2
+        assert first_count == 3
         assert second_count == 0
 
 
diff --git a/tests/test_store/test_sqlite_credential_repo.py b/tests/test_store/test_sqlite_credential_repo.py
index 1c3396a..3b54b61 100644
--- a/tests/test_store/test_sqlite_credential_repo.py
+++ b/tests/test_store/test_sqlite_credential_repo.py
@@ -227,3 +227,21 @@ async def test_delete_webauthn_if_not_last_allows_when_others_exist(
 
     assert await credential_repo.delete_webauthn_if_not_last(alice.userid, b"\x01") is True
     assert len(await credential_repo.get_webauthn_by_user(alice.userid)) == 0
+
+
+async def test_webauthn_credential_id_is_globally_unique(
+    credential_repo: SQLiteCredentialRepository, user_repo: SQLiteUserRepository
+) -> None:
+    # The same credential_id must not be registrable for two different users,
+    # otherwise lookup-by-credential-id could resolve to the wrong account.
+    await user_repo.create(User(userid="u-alice", username="alice2"))
+    await user_repo.create(User(userid="u-bob", username="bob2"))
+    cred_id = b"\xde\xad\xbe\xef"
+
+    await credential_repo.create_webauthn(
+        WebAuthnCredential(user_id="u-alice", credential_id=cred_id, public_key=b"\x01")
+    )
+    with pytest.raises(DuplicateError):
+        await credential_repo.create_webauthn(
+            WebAuthnCredential(user_id="u-bob", credential_id=cred_id, public_key=b"\x02")
+        )