fast-jwt accepts unknown crit header extensions (RFC 7515 violation)
Summary
fast-jwt does not validate the crit (Critical) Header Parameter defined in RFC 7515 §4.1.11. When a JWS token contains a crit array listing extensions that fast-jwt does not understand, the library accepts the token instead of rejecting it. This violates the MUST requirement in the RFC.
RFC Requirement
RFC 7515 §4.1.11:
> If any of the listed extension Header Parameters are **not understood
> and supported by the recipient, then the JWS is invalid**.
Proof of Concept
```javascript
const { createSigner, createVerifier } = require("fast-jwt"); // v3.3.3
const signer = createSigner({ key: "secret", algorithm: "HS256" });
const token = signer({
sub: "attacker",
role: "admin",
header: { crit: ["x-custom-policy"], "x-custom-policy": "require-mfa" },
});
// Should REJECT — x-custom-policy is not understood
const verifier = createVerifier({ key: "secret", algorithms: ["HS256"] });
try {
const result = verifier(token);
console.log("ACCEPTED:", result);
// Output: ACCEPTED: { sub: 'attacker', role: 'admin' }
} catch (e) {
console.log("REJECTED:", e.message);
}
```
Expected: Error — unsupported critical extension
Actual: Token accepted.
Comparison
```javascript
// jose (panva) v4+ — correctly rejects
const jose = require("jose");
await jose.jwtVerify(token, new TextEncoder().encode("secret"));
// throws: Extension Header Parameter "x-custom-policy" is not recognized
```
Impact
- Split-brain verification in mixed-library environments
- Security policy bypass when
crit carries enforcement semantics - Token binding bypass (RFC 7800
cnf confirmation) - See CVE-2025-59420 for full impact analysis
Suggested Fix
In src/verifier.js, add crit validation after header decoding:
```javascript
const SUPPORTED_CRIT = new Set(["b64"]);
function validateCrit(header) {
if (!header.crit) return;
if (!Array.isArray(header.crit) || header.crit.length === 0)
throw new Error("crit must be a non-empty array");
for (const ext of header.crit) {
if (!SUPPORTED_CRIT.has(ext))
throw new Error(Unsupported critical extension: ${ext});
if (!(ext in header))
throw new Error(Critical extension ${ext} not present in header);
}
}
```
