CVE-2026-33308 mod_gnutls missing key purpose check in client certificate verification
Attackers can bypass TLS client authentication using certificates with incorrect key purposes, requiring specific server configurations.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-33308?
CVE-2026-33308 is a medium-severity authentication bypass vulnerability in airtower-luna mod_gnutls, published on 24 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Mod_gnutls is a TLS module for Apache HTTPD based on GnuTLS. Prior to version 0.13.0, code for client certificate verification did not check the key purpose as set in the Extended Key Usage extension. An attacker with access to the private key for a valid certificate issued by a CA trusted for TLS client authentication but designated for a different purpose could have used that certificate to improperly access resources requiring TLS client authentication. Server configurations that do not use client certificates (`GnuTLSClientVerify ignore`, the default) are not affected. The problem has been fixed in version 0.13.0 by rewriting certificate verification to use `gnutls_certificate_verify_peers()`, and requiring key purpose id-kp-clientAuth (also known as `tls_www_client` in GnuTLS) by default if the Extended Key Usage extension is present. The new `GnuTLSClientKeyPurpose` option allows overriding the expected key purpose if needed (please see the manual for details). Behavior for certificates without an Extended Key Usage extension is unchanged. If dedicated (sub-)CAs are used for issuing TLS client certificates only (not for any other purposes) the issue has no practical impact.
Weakness: CWE-295
Which versions are affected by CVE-2026-33308?
It affects 1 product.
Products include Mod Gnutls Project Mod Gnutls.
How do I fix CVE-2026-33308?
No fix has been published for CVE-2026-33308 yet. Limit exposure of the affected software until one is available.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-33308 being exploited?
There is no known exploitation of CVE-2026-33308 in the wild and no public exploit code.
Other vulnerabilities in airtower-luna mod_gnutls
Sources
Also known as GHSA-hm2g-m958-8qgh, UBUNTU-CVE-2026-33308.