CVE-2026-107724

Deferred Deferred - Pending Action

HMAC Key Confusion in fast-jwt

Publication date: 2026-10-08

Last updated on: 2026-10-08

Assigner: GitHub, Inc.

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.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-10-08

Last Modified

2026-10-08

Generated

2026-10-09

AI Q&A

2026-10-09

EPSS Evaluated

N/A

NVD

CVE-2026-107724

Affected Vendors & Products

Vendor	Product	Version / Range
nearform	fast-jwt	>= 6.2.4, < 6.3.0

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-347	The product does not verify, or incorrectly verifies, the cryptographic signature for data.

Attack-Flow Graph

Executive Summary

fast-jwt versions up to 6.2.4 incorrectly treat raw serialized public JWK or JWKS JSON as HMAC secrets. The library's public key detection function assumes non-PEM strings are symmetric keys, allowing attackers to use public key bytes as HMAC keys if HS256 is allowed. This enables token forgery with arbitrary claims.

Impact Analysis

If you use fast-jwt 6.2.4 or earlier with HS256 or mixed algorithm allowlists, attackers could forge JWT tokens with elevated privileges or false claims. This may lead to unauthorized access, data tampering, or privilege escalation in affected applications.

Compliance Impact

This vulnerability could violate integrity requirements in GDPR and HIPAA by allowing unauthorized data modification. Forged tokens may grant improper access to sensitive data, leading to compliance breaches and potential legal consequences.

Mitigation Strategies

Upgrade fast-jwt to version 6.3.0 or later to address the issue where raw serialized public JWK or JWKS JSON is incorrectly treated as an HMAC secret.

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