⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 7.5

How to Fix CVE-2026-33307: Stack Buffer Overflow in mod_gnutls

Other vulnerabilities in the same area that are worth patching alongside this one:

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 7.5 - High
Actively exploited?Not currently listed in CISA KEV
Affected< 0.12.3
Fixed inversion
Type (CWE)CWE-121: Stack-based Buffer Overflow

What is CVE-2026-33307?

CVE-2026-33307 is a stack-based buffer overflow in mod_gnutls. A remote attacker can send a crafted message that overflows a fixed-size stack buffer, corrupting the return address and, on un-mitigated builds, achieving code execution. Vendor description: Mod_gnutls is a TLS module for Apache HTTPD based on GnuTLS. In versions prior to 0.12.3 and 0.13.0, code for client certificate verification imported the certificate chain sent by the client into a fixed size gnutls_x509_crt_t x509[] array without checking the number of certificates is less than or equal to the array size.

Why this CVE matters

Stack-based buffer overflows in network-reachable services have driven some of the highest-impact incidents of the past two years. Modern compiler protections raise the bar, but real-world exploits for unpatched appliances continue to appear quickly after disclosure.

For deployments of mod_gnutls that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

Open mod_gnutls's About dialog or run the vendor-documented version-check command. Compare the result against the affected ranges in the advisory.

How to fix CVE-2026-33307

  1. Read the vendor advisory in full: https://github.com/airtower-luna/mod_gnutls/security/advisories/GHSA-gjpm-55p4-c76r
  2. Upgrade mod_gnutls to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  5. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

<!-- enrich-agent-8 -->

Patch mod_gnutls (mod_gnutls) to 0.12.3

Vendor advisory: https://github.com/airtower-luna/mod_gnutls/security/advisories/GHSA-gjpm-55p4-c76r


# Apache (httpd) on Debian / Ubuntu.
sudo apt update
sudo apt install --only-upgrade mod_gnutls
sudo systemctl restart mod_gnutls

# Apache on RHEL / Rocky / AlmaLinux.
sudo dnf upgrade mod_gnutls
sudo systemctl restart mod_gnutls

# Verify the running version matches 0.12.3.
mod_gnutls -v


# Vendor advisory: https://github.com/airtower-luna/mod_gnutls/security/advisories/GHSA-gjpm-55p4-c76r
# IIS (Windows): apply cumulative updates and recycle the affected app pool.
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Get-WindowsUpdate -AcceptAll -Install -AutoReboot
Restart-WebAppPool -Name "<AppPoolName>"

Verify the fix landed


# Vendor advisory: https://github.com/airtower-luna/mod_gnutls/security/advisories/GHSA-gjpm-55p4-c76r
# 1. Confirm the running version matches the fixed-in version listed above.

# 2. Re-scan with your vulnerability scanner (Nessus, Qualys, Tenable, OpenVAS).
#    The scanner should no longer flag this CVE on the patched target.

# 3. Inspect recent service / kernel logs for crash-loops or rollback events.
journalctl -u <service> --since "10 minutes ago"
dmesg --since "10 minutes ago"

<!-- enrich-agent-8 -->

If you cannot patch immediately

Block network reachability to the vulnerable service from untrusted networks and apply the patched build. Memory-corruption bugs cannot be reliably mitigated at the network layer; the patch is the fix.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for repeated service restarts, crash logs from the affected daemon, and core files generated around the time of any anomalous traffic. A memory-corruption flaw used for exploitation often leaves a trail of failed attempts before the successful one.

Frequently asked questions

Is CVE-2026-33307 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-33307?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

How long should I plan for the upgrade?

Typical vendor-documented upgrade windows for mod_gnutls run from a few minutes to under an hour depending on cluster size. Test in a staging environment first and follow the vendor's documented HA upgrade order.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*