Regex audience matching without anchors is a security footgun

Start with the audience check in verify.js and the README audience section. Review how string and RegExp audiences are currently described, then confirm which proposed behavior maintainers want. Done should make the security implications of unanchored regex audiences explicit, with any related verification coverage updated if required.

Summary

When options.audience contains a RegExp, verify() calls audience.test(targetAudience) without enforcing that the regex is anchored. This means a developer who writes:

jwt.verify(token, secret, { audience: /api\.myapp\.com/ });

will inadvertently accept tokens with audiences like evil-api.myapp.com.attacker.com — the . matches any character and there are no ^/$ anchors.

The string comparison path (audience === targetAudience) is strict. The regex path silently shifts the security burden to the caller.

Reproduction

const jwt = require('jsonwebtoken');

const secret = 'test-secret';
const token = jwt.sign({ aud: 'evil-api.myapp.com.attacker.com' }, secret);

// Developer intends to only accept "api.myapp.com"
jwt.verify(token, secret, { audience: /api\.myapp\.com/ }, (err, decoded) => {
  console.log(err);     // null — no error!
  console.log(decoded); // token accepted despite malicious audience
});

Impact

This is not a vulnerability in the library itself — the regex works as designed. But it's a footgun: developers who mix string and regex audience checks may not realize the security model differs between the two paths. The string path is exact-match. The regex path accepts partial matches unless the developer manually adds ^ and $.

Given that audience validation is a security-critical check, the gap between "looks like it works" and "actually secure" is a concern.

Suggestions (pick any)

1. Document it prominently — Add a note to the README's audience section warning that regex audiences must be anchored to avoid partial matches.
2. Warn on unanchored regexes — If the regex source doesn't start with ^ or end with $, emit a console warning or throw.
3. Auto-anchor — Wrap unanchored regexes in ^(?:...)$ before testing. This would be a breaking change for anyone relying on partial matches intentionally.

Option 1 is the lowest-friction fix. Option 2 provides defense-in-depth without breaking existing behavior.

Relevant code

verify.js audience check:

return audiences.some(function (audience) {
  return audience instanceof RegExp ? audience.test(targetAudience) : audience === targetAudience;
});