CVE-2026-35042: Fast jwt Fast-jwt Accepts Unknown `crit` Header Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hm7r-c7qw-ghp6",
  "modified" : "2026-04-06T23:41:50Z",
  "published" : "2026-04-03T22:01:25Z",
  "aliases" : [ "CVE-2026-35042" ],
  "summary" : "fast-jwt accepts unknown `crit` header extensions (RFC 7515 violation)",
  "details" : "## Summary\n\n`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.\n\n---\n\n## RFC Requirement\n\nRFC 7515 §4.1.11:\n\n> If any of the listed extension Header Parameters are **not understood\n> and supported** by the recipient, then the **JWS is invalid**.\n\n---\n\n## Proof of Concept\n\n```javascript\nconst { createSigner, createVerifier } = require(\"fast-jwt\"); // v3.3.3\n\nconst signer = createSigner({ key: \"secret\", algorithm: \"HS256\" });\nconst token = signer({\n  sub: \"attacker\",\n  role: \"admin\",\n  header: { crit: [\"x-custom-policy\"], \"x-custom-policy\": \"require-mfa\" },\n});\n\n// Should REJECT — x-custom-policy is not understood\nconst verifier = createVerifier({ key: \"secret\", algorithms: [\"HS256\"] });\ntry {\n  const result = verifier(token);\n  console.log(\"ACCEPTED:\", result);\n  // Output: ACCEPTED: { sub: 'attacker', role: 'admin' }\n} catch (e) {\n  console.log(\"REJECTED:\", e.message);\n}\n```\n\n**Expected:** Error — unsupported critical extension\n**Actual:** Token accepted.\n\n### Comparison\n\n```javascript\n// jose (panva) v4+ — correctly rejects\nconst jose = require(\"jose\");\nawait jose.jwtVerify(token, new TextEncoder().encode(\"secret\"));\n// throws: Extension Header Parameter \"x-custom-policy\" is not recognized\n```\n\n---\n\n## Impact\n\n- **Split-brain verification** in mixed-library environments\n- **Security policy bypass** when `crit` carries enforcement semantics\n- **Token binding bypass** (RFC 7800 `cnf` confirmation)\n- See CVE-2025-59420 for full impact analysis\n\n---\n\n## Suggested Fix\n\nIn `src/verifier.js`, add crit validation after header decoding:\n\n```javascript\nconst SUPPORTED_CRIT = new Set([\"b64\"]);\n\nfunction validateCrit(header) {\n  if (!header.crit) return;\n  if (!Array.isArray(header.crit) || header.crit.length === 0)\n    throw new Error(\"crit must be a non-empty array\");\n  for (const ext of header.crit) {\n    if (!SUPPORTED_CRIT.has(ext))\n      throw new Error(`Unsupported critical extension: ${ext}`);\n    if (!(ext in header))\n      throw new Error(`Critical extension ${ext} not present in header`);\n  }\n}\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "fast-jwt"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "6.1.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nearform/fast-jwt/security/advisories/GHSA-hm7r-c7qw-ghp6"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-35042"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-9ggr-2464-2j32"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nearform/fast-jwt"
  }, {
    "type" : "WEB",
    "url" : "https://www.rfc-editor.org/rfc/rfc7515.html#section-4.1.11"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-345", "CWE-636" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-03T22:01:25Z",
    "nvd_published_at" : "2026-04-06T17:17:13Z"
  }
}