fast-jwt: Verifier cache accepts expired JWTs without iat.

{
  "id": "238b132349a6072283c85fe23e5d18e18266861843433dd3c30260a2f86d8132",
  "url": "https://osv.dev/vulnerability/GHSA-x937-hj6v-793p",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "fast-jwt: Verifier cache accepts expired JWTs without iat.",
    "summary": "### Summary `cacheSet` only derives its `exp` cache deadline inside `hasIat` (`src/verifier.js:127-140`). JWT `iat` is optional. For a valid token with `exp` but no `iat`, `cacheSet` substitutes `clockTimestamp + clockTolerance + cacheTTL` (`src/verifier.js:142-146`). Later, the cache path returns the saved payload before decoding or invoking `verifyToken` (`src/verifier.js:372-388`). Since expiration validation is in `verifyToken`, the expired token remains accepted until the cache deadline. ### Details The bug is an expiration-check bypass caused by caching. Normally, verification works like this: 1. Verify signature. 2. Check exp against the current time. 3. Return the JWT claims. With caching enabled, `fast-jwt` saves successful verification results so it can avoid repeating crypto work for the same token. The intended invariant is: > A cached token must stop being accepted at the same time as a non-cached token. But the cache computes its expiration incorrectly. In `fast-jwt/src/verifier.js:127`, the code first checks whether the payload has an iat (“issued at”) claim: ```js const hasIat = payload && typeof payload.iat === 'number' if (hasIat) { if (!ignoreExpiration && typeof payload.exp === 'number') { cacheValue[2] = payload.exp * 1000 + clockTolerance } } ``` That means `exp` is considered only when `iat` exists. However, `iat` is optional in JWT. A perfectly valid JWT may contain: ```js { \"sub\": \"alice\", \"exp\": 1700000001 } ``` with no iat. For that token, the expir",
    "evidence": "osv:GHSA-x937-hj6v-793p",
    "products": [
      {
        "name": {
          "value": "fast-jwt",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.3.4",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "medium",
    "structured": {
      "id": "GHSA-x937-hj6v-793p",
      "aliases": [
        "CVE-2026-107719",
        "GHSA-x937-hj6v-793p"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "6.3.4"
                }
              ]
            }
          ],
          "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-x937-hj6v-793p/GHSA-x937-hj6v-793p.json",
            "last_known_affected_version_range": "<= 6.3.3"
          }
        }
      ],
      "modified": "2026-10-08T22:30:41.090413721Z",
      "provider": "osv",
      "references": [
        "https://github.com/nearform/fast-jwt/security/advisories/GHSA-x937-hj6v-793p",
        "https://github.com/nearform/fast-jwt/commit/fc1ddbbe5ce38066ba2cea0b0ce0233932757167",
        "https://github.com/nearform/fast-jwt",
        "https://github.com/nearform/fast-jwt/releases/tag/v6.3.4"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-613"
        ],
        "severity": "MODERATE",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-08T22:10:35Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-107719",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 105239,
  "published_at": "2026-10-08T22:10:35+00:00"
}