Algorithm Confusion in LibJWT for RSA JWKs Without alg Parameter
CVE-2026-44699

9.1CRITICAL

Key Information:

Vendor

Benmcollins

Status
Libjwt
Vendor
CVE Published:
15 May 2026

What is CVE-2026-44699?

LibJWT, a C library for handling JSON Web Tokens, is vulnerable from versions 3.0.0 to 3.3.2. It accepts an RSA JSON Web Key (JWK) that lacks an 'alg' parameter, which can lead to an authentication bypass through algorithm confusion. This flaw allows attackers to forge valid JWTs without requiring access to a secret or RSA private key. The issue arises during HMAC verification when a zero-length key is unintentionally used, resulting in potential exploitation in applications that load RSA keys from JWKS with omitted 'alg' parameters. The vulnerability has been addressed in version 3.3.3.

Affected Version(s)

libjwt >= 3.0.0, < 3.3.3

References

https://github.com/benmcollins/libjwt/security/advisories...
x_refsource_CONFIRM

CVSS V4

Score:
9.1
Severity:
CRITICAL
Confidentiality:
High
Integrity:
High
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
Physical
Privileges Required:
Undefined
User Interaction:
None

Timeline

  • Vulnerability published

  • Vulnerability Reserved

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