What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.4 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-08 · updated 2026-10-09
• Weakness: CWE-347

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 products

• nearform fast-jwt: from 6.2.4, < 6.3.0

References

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

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