GHSA-hm7r-c7qw-ghp6

## 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.

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## 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**.

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## 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 ```

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## 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

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## 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`); } } ```