diff --git a/.changeset/jwt-fail-closed-exp-nbf.md b/.changeset/jwt-fail-closed-exp-nbf.md
new file mode 100644
index 00000000000..3af70096f68
--- /dev/null
+++ b/.changeset/jwt-fail-closed-exp-nbf.md
@@ -0,0 +1,5 @@
+---
+"thirdweb": patch
+---
+
+Reject JWTs whose nbf or exp is missing or not a finite number instead of skipping the time-bound checks.
diff --git a/packages/thirdweb/src/auth/core/verify-jwt.test.ts b/packages/thirdweb/src/auth/core/verify-jwt.test.ts
index 4c1f2cd7adb..1a41334a8e8 100644
--- a/packages/thirdweb/src/auth/core/verify-jwt.test.ts
+++ b/packages/thirdweb/src/auth/core/verify-jwt.test.ts
@@ -3,6 +3,11 @@ import {
   TEST_ACCOUNT_A,
   TEST_ACCOUNT_B,
 } from "../../../test/src/test-wallets.js";
+import { stringToBytes } from "../../utils/encoding/to-bytes.js";
+import { stringify } from "../../utils/json.js";
+import { PRECOMPILED_B64_ENCODED_JWT_HEADER } from "../../utils/jwt/jwt-header.js";
+import { decodeJWT } from "../../utils/jwt/decode-jwt.js";
+import { uint8ArrayToBase64 } from "../../utils/uint8-array.js";
 import { generateJWT } from "./generate-jwt.js";
 import { generateLoginPayload } from "./generate-login-payload.js";
 import { signLoginPayload } from "./sign-login-payload.js";
@@ -10,6 +15,18 @@ import type { AuthOptions } from "./types.js";
 import { verifyJWT } from "./verify-jwt.js";
 import { verifyLoginPayload } from "./verify-login-payload.js";
 
+async function signJwtPayload(payload: Record<string, unknown>) {
+  const message = stringify(payload);
+  const signature = await TEST_ACCOUNT_A.signMessage({ message });
+  const encodedData = uint8ArrayToBase64(stringToBytes(message), {
+    urlSafe: true,
+  });
+  const encodedSignature = uint8ArrayToBase64(stringToBytes(signature), {
+    urlSafe: true,
+  });
+  return `${PRECOMPILED_B64_ENCODED_JWT_HEADER}.${encodedData}.${encodedSignature}`;
+}
+
 const options: AuthOptions = {
   adminAccount: TEST_ACCOUNT_A,
   domain: "example.com",
@@ -117,4 +134,58 @@ describe("verifyJWT", () => {
       );
     }
   });
+
+  test("should fail closed on a non-numeric expiration", async () => {
+    const jwt = await validJwt();
+    const { payload } = decodeJWT(jwt);
+    const forged = await signJwtPayload({ ...payload, exp: "never" });
+    const result = await verifyJWT(options)({ jwt: forged });
+    expect(result.valid).toBe(false);
+    if (!result.valid) {
+      expect(result.error).toBe("Payload has invalid Expiration Time");
+    }
+  });
+
+  test("should fail closed when expiration is missing", async () => {
+    const jwt = await validJwt();
+    const { payload } = decodeJWT(jwt);
+    const { exp: _exp, ...rest } = payload;
+    const forged = await signJwtPayload(rest);
+    const result = await verifyJWT(options)({ jwt: forged });
+    expect(result.valid).toBe(false);
+    if (!result.valid) {
+      expect(result.error).toBe("Payload has invalid Expiration Time");
+    }
+  });
+
+  test("should fail closed on a non-numeric not-before", async () => {
+    const jwt = await validJwt();
+    const { payload } = decodeJWT(jwt);
+    const forged = await signJwtPayload({ ...payload, nbf: null });
+    const result = await verifyJWT(options)({ jwt: forged });
+    expect(result.valid).toBe(false);
+    if (!result.valid) {
+      expect(result.error).toBe("Payload has invalid Not Before time");
+    }
+  });
 });
+
+async function validJwt() {
+  const generatedPayload = await generateLoginPayload(options)({
+    address: TEST_ACCOUNT_B.address,
+  });
+  const signedPayload = await signLoginPayload({
+    account: TEST_ACCOUNT_B,
+    payload: generatedPayload,
+  });
+  const verifiedPayload = await verifyLoginPayload(options)({
+    payload: signedPayload.payload,
+    signature: signedPayload.signature,
+  });
+  if (!verifiedPayload.valid) {
+    throw new Error("Invalid payload");
+  }
+  return generateJWT(options)({
+    payload: verifiedPayload.payload,
+  });
+}
diff --git a/packages/thirdweb/src/auth/core/verify-jwt.ts b/packages/thirdweb/src/auth/core/verify-jwt.ts
index b6d7c165007..c85398d5284 100644
--- a/packages/thirdweb/src/auth/core/verify-jwt.ts
+++ b/packages/thirdweb/src/auth/core/verify-jwt.ts
@@ -57,19 +57,36 @@ export function verifyJWT(options: AuthOptions) {
       };
     }
 
+    // Invalid Date / missing NumericDate comparisons are always false in JS,
+    // so a non-finite nbf or exp previously skipped the time bounds.
+    const nbf = finiteEpoch(payload.nbf);
+    if (nbf === undefined) {
+      return {
+        error: "Payload has invalid Not Before time",
+        valid: false,
+      };
+    }
+    const exp = finiteEpoch(payload.exp);
+    if (exp === undefined) {
+      return {
+        error: "Payload has invalid Expiration Time",
+        valid: false,
+      };
+    }
+
     // Check that the token is past the invalid before time
     const currentTime = Math.floor(Date.now() / 1000);
-    if (currentTime < payload.nbf) {
+    if (currentTime < nbf) {
       return {
-        error: `This token is invalid before epoch time '${payload.nbf}', current epoch time is '${currentTime}'`,
+        error: `This token is invalid before epoch time '${nbf}', current epoch time is '${currentTime}'`,
         valid: false,
       };
     }
 
     // Check that the token hasn't expired
-    if (currentTime > payload.exp) {
+    if (currentTime > exp) {
       return {
-        error: `This token expired at epoch time '${payload.exp}', current epoch time is '${currentTime}'`,
+        error: `This token expired at epoch time '${exp}', current epoch time is '${currentTime}'`,
         valid: false,
       };
     }
@@ -101,3 +118,10 @@ export function verifyJWT(options: AuthOptions) {
     };
   };
 }
+
+function finiteEpoch(value: unknown): number | undefined {
+  if (typeof value !== "number" || !Number.isFinite(value)) {
+    return undefined;
+  }
+  return value;
+}