CVE-2026-33308
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedMod_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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidencemod_gnutls prior to 0.13.0 did not validate the Extended Key Usage (EKU) extension during client certificate verification, allowing certificates issued for other purposes (e.g., server auth, email signing) to be improperly used for TLS client authentication when the CA is trusted for client auth.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 0.13.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- High
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Confirm mod_gnutls is installedCheck for the presence of mod_gnutls in the Apache configuration (typically loaded via 'LoadModule gnutls_module' in httpd.conf or mods-enabled/ directory) or query the server for mod_gnutls version if exposed.Affected if mod_gnutls is installed and used for TLS termination.
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Identify mod_gnutls versionRun 'gnutls-cli --version' if available, or check the package manager (e.g., 'dpkg -l | grep mod_gnutls', 'rpm -qa | grep mod_gnutls') to retrieve the installed version number.Affected if The installed version is lower than 0.13.0.
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Verify client certificate authentication is configuredInspect the Apache virtual host configuration for mod_gnutls directives: look for 'GnuTLSClientVerify' set to 'require' or 'GnuTLSCACertificateFile/Database' directives pointing to trusted CA certificates for client auth.Affected if Client certificate verification is enabled via GnuTLSClientVerify and CA certificates are configured.
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Inspect trusted CA certificate purposeReview the CA certificate file referenced by GnuTLSClientCAFile or GnuTLSCertificateDatabase. Check whether the CA issues certificates with mixed purposes (e.g., server auth, email signing, client auth) rather than being dedicated exclusively to client authentication.Affected if The trusted CA issues certificates for multiple purposes beyond client authentication.
You are affected if mod_gnutls version is below 0.13.0 and client certificate verification is enabled using a CA that issues certificates for purposes other than client authentication.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped0.13.0
Upgrade mod_gnutls to version 0.13.0 or later, which enforces key purpose validation using gnutls_certificate_verify_peers(). If upgrading is not feasible, ensure only dedicated CAs (issuing client-auth certificates exclusively) are trusted for TLS client authentication.
mod_gnutls version 0.13.0
- 1. Identify the current version of mod_gnutls installed on the system using the package manager or by checking the module binary.
- 2. Upgrade mod_gnutls to version 0.13.0 or later. For source compilation, download from the official repository and compile with appropriate configuration.
- 3. After upgrading, verify that client certificate verification now properly checks the Extended Key Usage extension by reviewing the server configuration.
- 4. If custom key purpose handling is required, configure the new GnuTLSClientKeyPurpose directive in the Apache configuration as documented in the mod_gnutls manual.
- 5. Test the configuration by restarting Apache and verifying that certificates with the correct key purpose (id-kp-clientAuth) are accepted while others are rejected.
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing3.0 h
- Review / QA1.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-33308 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-33308 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
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