Vulnerability · Published Oct 8, 2026 · Assigned by GitHub_M

CVE-2026-107724fast-jwt treats raw public JWK JSON as an HMAC secret, enabling HS256 token forgery

High 7.4

In plain English

A high-severity security flaw in fast-jwt from nearform. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 21% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.4High

Exploit odds (30 days)0.3%top 79.2% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.2.4, fast-jwt can classify raw serialized public JWK or JWKS JSON as an HMAC secret because src/crypto.js performDetectPublicKeyAlgorithms treats non-PEM strings as symmetric key material. If HS256 is explicitly allowed or inferred, an attacker who knows the exact serialized public-key bytes can use those bytes as an HMAC key and create a token containing arbitrary claims that createVerifier accepts. Serialization ordering or whitespace differences can prevent exploitation, and applications using supported PEM keys with an asymmetric-only algorithm allowlist are not affected. This issue is fixed in version 6.3.0.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	fast-jwt	6.3.0	GHSA-g3jj-5cmm-3hxx

Affected products (as reported)

Vendor	Product	Versions
nearform	fast-jwt	>= 6.2.4, < 6.3.0

Weakness: CWE-347 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

References

https://github.com/nearform/fast-jwt/security/advisories/GHSA-g3jj-5cmm-3hxx x_refsource_CONFIRM
https://github.com/nearform/fast-jwt/pull/636 x_refsource_MISC
https://github.com/nearform/fast-jwt/commit/10f9591349199ed2ab9fa1748ce92cdca697f6cf x_refsource_MISC
https://github.com/nearform/fast-jwt/releases/tag/v6.3.0 x_refsource_MISC