Vulnerability · Published May 15, 2026 · Assigned by GitHub_M
CVE-2026-44699LibJWT: Algorithm confusion allows JWT forgery with RSA JWK as empty-key HMAC
Critical 9.1
In plain English
A critical-severity broken cryptography in libjwt from benmcollins. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 18% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
LibJWT 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:14 | libjwt3 | 3.3.3-1 | DEBIAN-CVE-2026-44699 |
| Ubuntu:22.04:LTS | libjwt | no fix listed | UBUNTU-CVE-2026-44699 |
| Ubuntu:24.04:LTS | libjwt | no fix listed | UBUNTU-CVE-2026-44699 |
| Ubuntu:25.10 | libjwt | no fix listed | UBUNTU-CVE-2026-44699 |
| Ubuntu:26.04:LTS | libjwt | no fix listed | UBUNTU-CVE-2026-44699 |
| Ubuntu:26.10 | libjwt | no fix listed | UBUNTU-CVE-2026-44699 |
| Ubuntu:26.04:LTS | libjwt3 | no fix listed | UBUNTU-CVE-2026-44699 |
| Ubuntu:26.10 | libjwt3 | no fix listed | UBUNTU-CVE-2026-44699 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| benmcollins | libjwt | >= 3.0.0, < 3.3.3 |
Weakness: CWE-327, CWE-347 · Vector: 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
References
- https://github.com/benmcollins/libjwt/security/advisories/GHSA-q843-6q5f-w55g x_refsource_CONFIRM