LibJWT: Algorithm confusion allows JWT forgery with RSA JWK as empty-key HMAC

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Summary

CVECVE-2026-44699
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-15 17:16:47 UTC
Updated2026-05-15 19:17:00 UTC
DescriptionLibJWT is a C JSON Web Token Library. From 3.0.0 to 3.3.2, libjwt accepts an RSA JWK that does not contain an alg parameter as the verification key for an HS256/HS384/HS512 token. In the OpenSSL backend, this causes HMAC verification to run with a zero-length key, so an attacker can forge a valid JWT without knowing any secret or RSA private key. This is an algorithm-confusion authentication bypass. It affects applications that load RSA keys from JWKS where alg is omitted, which is valid JWK syntax and common in real deployments, and then choose the verification algorithm from the JWT header, for example in a kid lookup callback. This vulnerability is fixed in 3.3.3.

Risk And Classification

Primary CVSS: v4.0 9.1 CRITICAL from [email protected]

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Problem Types: CWE-327 | CWE-347 | CWE-327 CWE-327: Use of a Broken or Risky Cryptographic Algorithm | CWE-347 CWE-347: Improper Verification of Cryptographic Signature

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary9.1CRITICALCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/C...
4.0CNADECLARED9.1CRITICALCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
Present
Privileges Required
None
User Interaction
None
Confidentiality
High
Integrity
High
Availability
None
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Benmcollins Libjwt affected >= 3.0.0, < 3.3.3 Not specified

References

ReferenceSourceLinkTags
github.com/benmcollins/libjwt/security/advisories/GHSA-q843-6q5f-w55g 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
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