Vulnerability in fast-jwt (CVE-2026-107720)
Summary
Vulnerability in fast-jwt (CVE-2026-107720). Confidential information can be exposed externally. Exploitable via createVerifier. Mitigation: upgrade to 6.3.1 or later.
Original description
The description as published by the source.
Summary
createVerifier in fast-jwt ≤ 6.3.0 skips signature verification entirely when the key option is a falsy synchronous value ('' or null) and the algorithms option is set to a non-empty allowlist. An attacker who can present a JWT to the application — regardless of algorithm — can forge arbitrary claims without possessing any signing key.
Details
Root cause — four cooperating code paths:
A. Falsy sync keys bypass prepareKeyOrSecret
createVerifier branches on typeof key:
const keyType = typeof key
if (keyType !== 'string' && keyType !== 'object' && keyType !== 'function') {
throw new TokenError(/* ... */)
}
if (key && keyType !== 'function') {
key = prepareKeyOrSecret(key, hsAlgorithms.includes(availableAlgorithms[0]))
}
When key is '' (string, falsy) or null (object, falsy) the outer type check passes but the if (key && ...) guard never calls prepareKeyOrSecret. The empty-secret rejection added in that function is therefore never reached for sync keys:
function prepareKeyOrSecret(key, isSecret) {
if (isSecret && key.length === 0) {
throw new TokenError(TokenError.codes.invalidKey, 'The key cannot be an empty string or buffer.')
}
return isSecret ? createSecretKey(key) : createPublicKey(key)
}
B. Explicit algorithms keeps an allowlist active with no key
When key is falsy, autodetection is skipped and the caller-supplied algorithms (e.g. ['HS256']) is retained in `allowedA…
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-2026-107720 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: 7.4/10.
What to do: upgrade fast-jwt to 6.3.1 or later.
If unsure, ask your IT team or search "fast-jwt CVE-2026-107720" on the vendor's site.
CVE-2026-107720 (fast-jwt) — CWE-20 / CVSS v3 7.4
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: createVerifier / key / null / algorithms
Patched: 6.3.1 — 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-20) exists in fast-jwt. Attackers reach the vulnerable code path via createVerifier without authentication.
Check the vendor advisory for the affected versions of fast-jwt. Attack surface: createVerifier / key / null / algorithms.
Severity: High (CVSS 7.4/10). Confidential information can be exposed externally.
Update to 6.3.1.
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 6.3.1ステージング環境で 6.3.1 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。