GHSA-hm7r-c7qw-ghp6 fast-jwt accepts unknown `crit` header extensions (RFC 7515 violation)

• npm fast-jwt: <= 6.1.0 (no fix yet)

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`); } } ```