Vulnerability in fast-jwt (CVE-2026-107723)
Summary
Vulnerability in fast-jwt (CVE-2026-107723). Confidential information can be exposed externally. Exploitable via createVerifier. Mitigation: upgrade to 6.3.0 or later.
Original description
The description as published by the source.
Summary
fast-jwt's createVerifier silently skips all configured claim validators (exp, nbf, iss, aud, sub, jti, nonce) when a validly-signed JWT carries a JSON array as its payload instead of an object. The verifier reports success while having enforced only the signature. This breaks the library's documented allowedIss / allowedAud / allowedSub / expiry / replay-protection guarantees and violates RFC 7519 §7.2 step 10, which requires the JWT Claims Set to be a JSON object.
Details
The decoder validates the header is a non-array object but the payload check is missing the Array.isArray guard:
// src/decoder.js:49 — header check (correct)
if (!header || typeof header !== 'object' || Array.isArray(header)) {
throw new TokenError(TokenError.codes.malformed, 'The token header is not a valid JSON object.')
}
// src/decoder.js:65 — payload check (vulnerable)
if (!payload || typeof payload !== 'object') { // typeof [] === 'object'
throw new TokenError(TokenError.codes.invalidPayload, 'The payload must be an object', { payload })
}
Because typeof [] === 'object' in JavaScript, an array payload passes the decoder.
In the verifier's validator loop, every check is short-circuited by an in-test that is always false for an array (arrays have only
numeric indices and length):
// src/verifier.js:304-323
for (const { type, claim, allowed, array, modifier, greater, errorCode, errorVerb } of…
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-107723 has been found in fast-jwt.
Attackers can target a specific entry point like createVerifier over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 8.1/10.
What to do: upgrade fast-jwt to 6.3.0 or later.
If unsure, ask your IT team or search "fast-jwt CVE-2026-107723" on the vendor's site.
CVE-2026-107723 (fast-jwt) — CWE-1287 / CVSS v3 8.1
Attack vector: remote (network-reachable) / no user interaction
Attack surface: createVerifier / exp / nbf / iss
Patched: 6.3.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-1287) exists in fast-jwt. Attackers reach the vulnerable code path via createVerifier.
Check the vendor advisory for the affected versions of fast-jwt. Attack surface: createVerifier / exp / nbf / iss.
Severity: High (CVSS 8.1/10). Confidential information can be exposed externally.
Update to 6.3.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'fast-jwt' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'fast-jwt' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
fast-jwtを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade fast-jwt to 6.3.0ステージング環境で 6.3.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。