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CVE-2026-107723 · GHSA-5hjw-83fp-phq9

fast-jwt : Silent claim-validator bypass when JWT payload is a JSON array

Risk score
59/100
CVSS
8.1 v3.1
Exploit chance (30 days)
0.4%
Published
8 Oct 2026

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What to know

What it is
fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.0, fast-jwt createVerifier accepts a validly signed JWT whose payload is a JSON array because src/decoder.js checks that the payload is an object but does not reject arrays. The claim validator loop then finds no named exp, nbf, iss, aud, sub, jt…
Affected
fast-jwt (npm) <= 6.2.4
Fix
Upgrade fast-jwt to 6.3.0 or later.
Exploitation
No known exploitation

Story map

Story map: fast-jwtTHE FLAWSTHE FIXCVE-2026-107723Fast-jwt : Silentclaim-validator bypass whenJWT payload is a JSON arrayCVSS 8.1 · EPSS 0%CVE-2026-107722Incomplete patch ofCVE-2026-34950:Non-whitespace key-prefix…CVSS 9.8 · EPSS 0%CVE-2026-107720CreateVerifier acceptsunsigned JWTs when key is '' ornull and algorithms is…CVSS 7.4 · EPSS 0%CVE-2026-107724Fast-jwt treats raw publicJWK JSON as an HMAC secret,enabling HS256 token forgeryCVSS 7.4 · EPSS 0%Upgrade fast-jwt6.3.0 · 6.3.1or later on your release lineWho is exposedfast-jwt (npm) >= 6.2.0, <=6.2.4AMBER · PATCH THIS WEEKfast-jwtfast-jwt provides fast JSONWeb Token (JWT)implementation. From 6.2.0 until6.3.0, fast-jwt can misclassifyRSA public-key text as an HMACsecret when the key hasnon-whitespace content before…Where it standsNo known exploitationCONNECTS TOSame weaknessCWE-1287 (CWE-1287)6 other warnings in 30 days →
Story map: the flaws, the fix, how it unfolded and what it connects to. Boxes link to the record or article.

At a glance

fast-jwt: Amber, patch this week. fast-jwt provides fast JSON Web Token (JWT) implementation. From 6.2.0 until 6.3.0, fast-jwt can misclassify RSA public-key text as an HMAC secret when the key has non-whitespace content before its PEM header. In src/crypto.js, performDetectPublicKeyAlgorithms trims whitespace but publicKeyPemMatcher remains start-anc…

The flaws

  • CVE-2026-107723: Fast-jwt : Silent claim-validator bypass when JWT payload is a JSON array (CVSS 8.1 · EPSS 0%)
  • CVE-2026-107722: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion (CVSS 9.8 · EPSS 0%)
  • CVE-2026-107720: CreateVerifier accepts unsigned JWTs when key is '' or null and algorithms is explicitly set (CVSS 7.4 · EPSS 0%)
  • CVE-2026-107724: Fast-jwt treats raw public JWK JSON as an HMAC secret, enabling HS256 token forgery (CVSS 7.4 · EPSS 0%)

The fix

Upgrade fast-jwt: 6.3.0 · 6.3.1 or later on your release line

Who is exposed: fast-jwt (npm) >= 6.2.0, <= 6.2.4

Where it stands

  • No known exploitation

Connects to

Why it is Yellow

  • CVSS 8.1 high
  • On your watchlist: npm

Affected packages

PackageEcosystemVulnerableFixed in
fast-jwtnpm<= 6.2.46.3.0

References

Original description

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.0, fast-jwt createVerifier accepts a validly signed JWT whose payload is a JSON array because src/decoder.js checks that the payload is an object but does not reject arrays. The claim validator loop then finds no named exp, nbf, iss, aud, sub, jti, or nonce properties and silently skips those configured checks, returning the array as a successfully verified payload. An attacker who can produce or influence a validly signed token may bypass expiry, issuer, audience, subject, revocation, and replay protections. The opt-in requiredClaims option can block missing claims, and signature verification itself is not bypassed. This issue is fixed in version 6.3.0.

Last activity 8 Oct 2026. Patch Forecast updates every hour.