CVE-2026-35042

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

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

// 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

• 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

CVE-2026-35042 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

In src/verifier.js, add crit validation after header decoding:

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`);
  }
}

1. What is CVE-2026-35042? CVE-2026-35042 is a high-severity security vulnerability in fast-jwt (npm), affecting versions <= 6.1.0. No fixed version is listed yet.
2. How severe is CVE-2026-35042? CVE-2026-35042 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fast-jwt are affected by CVE-2026-35042? fast-jwt (npm) versions <= 6.1.0 is affected.
4. Is there a fix for CVE-2026-35042? No fixed version is listed for CVE-2026-35042 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-35042 exploitable, and should I be worried? Whether CVE-2026-35042 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-35042 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.