fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion

{
  "id": "097aa1a498bd675023cd7b933d7376c85279f3c807113f661dbb47f887c78928",
  "url": "https://osv.dev/vulnerability/GHSA-ww5h-9m49-7xx4",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion",
    "summary": "### Summary The fix for CVE-2026-34950 (CVSS 9.1, released in v6.2.0) is **incomplete**. It adds `key.trim()` to the PEM-detection path in `src/crypto.js`, but `String.prototype.trim()` only strips characters classified as whitespace by the ECMAScript specification. The subsequent `^`-anchored regex (`/^-----BEGIN(?: (RSA))? PUBLIC KEY-----/`) still requires the PEM header at position 0 — so **any non-whitespace leading byte** (control chars, zero-width unicode, `#` comments, HTTP-style headers, PGP wrappers) bypasses detection and falls through to the HMAC verification path, using the RSA public key as the HMAC shared secret. Net result: the **exact same RSA→HS256 algorithm-confusion attack** the original CVE addressed is fully re-enabled with a slightly different leading byte. **Attack prerequisites are identical to CVE-2026-34950**: attacker knows the target's public RSA key (which is public by definition), and the target loads that key from a source whose content may have a non-whitespace prefix (DB column with corrupted encoding, YAML config with inline comment, copy-paste from formatted document, etc.). **Verified on fast-jwt@6.2.2 (latest as of 2026-04-23) with a 10-line PoC.** ### Details Post-patch code (`src/crypto.js`, lines ~74-171 in `performDetectPublicKeyAlgorithms`): ```js function performDetectPublicKeyAlgorithms(key) { const trimmedKey = key.trim() // <-- CVE-2026-34950 patch added this if (publicKeyPemMatcher.test(trimmedKey)) { // treat as RSA/EC public ke",
    "evidence": "osv:GHSA-ww5h-9m49-7xx4",
    "products": [
      {
        "name": {
          "value": "fast-jwt",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.3.0",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "critical",
    "structured": {
      "id": "GHSA-ww5h-9m49-7xx4",
      "aliases": [
        "CVE-2026-107722",
        "GHSA-ww5h-9m49-7xx4"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "6.2.0"
                },
                {
                  "fixed": "6.3.0"
                }
              ]
            }
          ],
          "package": {
            "name": "fast-jwt",
            "purl": "pkg:npm/fast-jwt",
            "ecosystem": "npm"
          },
          "database_specific": {
            "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-ww5h-9m49-7xx4/GHSA-ww5h-9m49-7xx4.json",
            "last_known_affected_version_range": "<= 6.2.4"
          }
        }
      ],
      "modified": "2026-10-08T22:15:05.895384054Z",
      "provider": "osv",
      "references": [
        "https://github.com/nearform/fast-jwt/security/advisories/GHSA-ww5h-9m49-7xx4",
        "https://github.com/nearform/fast-jwt/pull/632",
        "https://github.com/nearform/fast-jwt/commit/d96bbc6c5336055a6dbfa318fdbab01197264cfa",
        "https://github.com/nearform/fast-jwt",
        "https://github.com/nearform/fast-jwt/releases/tag/v6.3.0"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-347"
        ],
        "severity": "CRITICAL",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-08T22:02:07Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-107722",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 104810,
  "published_at": "2026-10-08T22:02:07+00:00"
}