diff --git a/src/auth/security.ts b/src/auth/security.ts
index f27c4bb74..3ee0f3a0a 100644
--- a/src/auth/security.ts
+++ b/src/auth/security.ts
@@ -116,7 +116,10 @@ export async function authenticateSessionToken(env: Env, token: string | undefin
   if (!token) return null;
   const session = await getAuthSessionByTokenHash(env, await hashToken(token));
   if (!session) return null;
-  if (session.revokedAt || Date.parse(session.expiresAt) <= Date.now()) return null;
+  // Fail closed on an unparseable expiry: Date.parse → NaN makes `NaN <= Date.now()` false, which would
+  // otherwise authenticate a session whose stored expires_at is malformed/empty as if it never expired.
+  const expiresAtMs = Date.parse(session.expiresAt);
+  if (session.revokedAt || !Number.isFinite(expiresAtMs) || expiresAtMs <= Date.now()) return null;
   await touchAuthSession(env, session.id);
   return { kind: "session", actor: session.login, session };
 }
diff --git a/test/unit/auth.test.ts b/test/unit/auth.test.ts
index 98833e887..fa1f94619 100644
--- a/test/unit/auth.test.ts
+++ b/test/unit/auth.test.ts
@@ -26,6 +26,11 @@ describe("private-beta auth and rate limiting", () => {
     const expired = await createSessionForGitHubUser(env, { login: "expired-user" });
     await env.DB.prepare("update auth_sessions set expires_at = ? where login = ?").bind("2020-01-01T00:00:00.000Z", "expired-user").run();
     await expect(authenticatePrivateToken(env, expired.token)).resolves.toBeNull();
+
+    // Fail closed when the stored expiry is unparseable (NaN), not authenticate it as a never-expiring session.
+    const malformed = await createSessionForGitHubUser(env, { login: "malformed-expiry-user" });
+    await env.DB.prepare("update auth_sessions set expires_at = ? where login = ?").bind("not-a-date", "malformed-expiry-user").run();
+    await expect(authenticatePrivateToken(env, malformed.token)).resolves.toBeNull();
   });
 
   it("handles auth helper fallbacks for cookies, login lists, and token comparison", async () => {