CVE-2026-33308

mod_gnutls missing key purpose check in client certificate verification

Published Mar 24, 2026 CVSS 3.1 · 6.8 Medium EPSS < 1% KEV · Not listed

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.

Analysis

Technical impact

Prior to version 0.13.0, the mod_gnutls module for Apache HTTPD failed to validate the key purpose specified in the Extended Key Usage extension during client certificate verification. An attacker possessing the private key of a certificate issued by a CA trusted for TLS client authentication but intended for another purpose could present that certificate during a TLS handshake, causing the server to accept it and grant access to resources protected by client‑certificate verification. This results in an unauthorized bypass of authentication, potentially compromising confidentiality and integrity of data accessed through such resources.

Affected Systems

The vulnerability impacts all releases of the Airtower‑Luna mod_gnutls module prior to 0.13.0. Server configurations that do not enforce client certificate verification (e.g., GnuTLSClientVerify set to ignore, which is the default) are not affected. If a deployment uses dedicated CAs that issue only client authentication certificates, the flaw poses no practical risk.

Risk and Exploitability

The CVSS score of 6.8 indicates a medium severity vulnerability with potential for significant impact if client certificate verification is in use. The EPSS score is below 1%, suggesting limited exploitation activity to date, and the vulnerability is not listed in the CISA KEV catalog. Exploitation requires the attacker to control a certificate issued for an alternative purpose by a trusted CA, and the server to require client authentication. For organizations that rely on client certificates for access control, prompt remediation is advisable to mitigate the risk of unauthorized access.

Affected Products

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

airtower-luna

mod_gnutls

affected

Version	Status	Constraints
`< 0.13.0`	affected	—

Configuration 1 [-]

cpe:2.3:a:mod_gnutls_project:mod_gnutls:*:*:*:*:*:*:*:*

No data.

Vendor Product Versions

Airtower-luna

Mod Gnutls

Version	Status	Scheme	Platform
`[0,0.13.0)`	affected	semver	—

Remediation

No solution or workaround provided in the CVE record.

OpenCVE Recommended Actions

Upgrade mod_gnutls to version 0.13.0 or later to restore proper key‑purpose checks and use gnutls_certificate_verify_peers() instead of custom code.
If an upgrade cannot be performed immediately, configure the GnuTLSClientKeyPurpose option to explicitly require the clientAuth (id‑kp‑clientAuth) key purpose and verify that the CA chain used for client authentication does not include certificates issued for other purposes.
Review server configurations to ensure client certificate verification is only enabled when necessary; disable GnuTLSClientVerify when client authentication is not required.
Stay informed of updates from the mod_gnutls project and apply security patches as soon as they become available.

Generated by OpenCVE AI on March 24, 2026 at 20:53 UTC.

Tracking

Advisories

No advisories available

Metrics

No CVSS v4.0

Attack Vector Network

Attack Complexity High

Privileges Required None

Scope Changed

Confidentiality Impact High

Integrity Impact None

Availability Impact None

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploit prediction score: < 1% (0.209% probability within 30 days)

Exploitation none

Automatable no

Technical Impact partial

References

Link	Providers
https://github.com/airtower-luna/mod_gnutls/security/advisories/GHSA-hm2g-m958-8qgh

History

Mar 24, 2026 7:30 PM First appearance in the feed. Affected versions updated. Affected vendors updated View changes

Type	Values Removed	Values Added
First Time appeared		Mod Gnutls Project Mod Gnutls Project mod Gnutls
CPEs		cpe:2.3:a:mod_gnutls_project:mod_gnutls::::::::
Vendors & Products		Mod Gnutls Project Mod Gnutls Project mod Gnutls

Mar 24, 2026 2:15 PM Scoring metrics updated View changes

Type	Values Removed	Values Added
Metrics		ssvc `{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}`

Mar 24, 2026 10:45 AM First appearance in the feed. Affected vendors updated View changes

Type	Values Removed	Values Added
First Time appeared		Airtower-luna Airtower-luna mod Gnutls
Vendors & Products		Airtower-luna Airtower-luna mod Gnutls

Mar 24, 2026 2:30 AM Description updated. Title updated. Weaknesses updated. Added references. Scoring metrics updated View changes

Type	Values Removed	Values Added
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.
Title		mod_gnutls missing key purpose check in client certificate verification
Weaknesses		CWE-295
References		https://github.com/airtower-luna/mod_gnutls/security/advisories/GHSA-hm2g-m958-8qgh
Metrics		cvssV3_1 `{'score': 6.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:N/A:N'}`