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

How to Fix CVE-2026-43512: Critical Vulnerability in Apache Tomcat

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityNot verified - see advisory
Actively exploited?Not currently listed in CISA KEV
Affected11.0.0-M1 <= 11.0.21, 10.1.0-M1 <= 10.1.54, 9.0.0.M1 <= 9.0.117, 8.5.0 <= 8.5.100, 7.0.0 <= 7.0.109, 0 < 7.0.0
Fixed inSee vendor advisory
Type (CWE)CWE-592: DEPRECATED: Authentication Bypass Issues

What is CVE-2026-43512?

CVE-2026-43512 is a security flaw in Apache Tomcat. DEPRECATED: Authentication Bypass Issues vulnerability in digest authentication in Apache Tomcat. This issue affects Apache Tomcat: from 11.0.0-M1 through 11.0.21, from 10.1.0-M1 through 10.1.54, from 9.0.0.M1 through 9.0.117, from 8.5.0 through 8.5.100, from before 7.0.0.

Why this CVE matters

Unpatched network-facing software is the leading initial-access vector in public breach reporting. Treat any CVSS-9 class flaw on an internet-reachable system as urgent, regardless of whether public exploit code has been observed yet.

For deployments of Apache Tomcat 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.

Run the project-specific version command (for example httpd -v, tomcat version, or check pom.xml / package metadata) and compare against the advisory.

How to fix CVE-2026-43512

  1. Read the vendor advisory in full: https://lists.apache.org/thread/7x09x7o12solvclslw3sz0288xc8wx73
  2. Upgrade Apache Tomcat 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).

Patch the web server package


# CVE-2026-43512 affects Apache Tomcat 11.0.0-M1 <= 11.0.21.
# Fixed in 11.0.21. Vendor advisory: https://lists.apache.org/thread/7x09x7o12solvclslw3sz0288xc8wx73

# Debian / Ubuntu
sudo apt update
sudo apt install --only-upgrade tomcat9
sudo systemctl restart tomcat9

# RHEL / Rocky / AlmaLinux / Fedora
sudo dnf upgrade tomcat
sudo systemctl restart tomcat

# Verify the running version.
tomcat9 -v


# Vendor advisory: https://lists.apache.org/thread/7x09x7o12solvclslw3sz0288xc8wx73
# IIS (Windows) — apply via Windows Update.
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Get-WindowsUpdate -AcceptAll -Install -AutoReboot
Restart-WebAppPool -Name "<AppPoolName>"

Verify the fix landed


# CVE-2026-43512 verification checklist.

# 1. Confirm the running version matches 11.0.21 (replace the version probe with
#    the platform-specific command shown above).

# 2. Re-scan the host with your vulnerability scanner (Nessus, Qualys, Tenable,
#    OpenVAS, Wazuh). The scanner must no longer flag CVE-2026-43512.

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

# 4. Cross-check the running build against the vendor advisory:
#    https://lists.apache.org/thread/7x09x7o12solvclslw3sz0288xc8wx73

If you cannot patch immediately

No official workaround exists beyond restricting network exposure to the affected component. Apply the vendor patch as the primary remediation.

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 log entries that do not match your normal request patterns, especially repeated requests to the same uncommon endpoint, and any administrative changes you cannot tie back to a known operator.

Frequently asked questions

Is CVE-2026-43512 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-2026-43512?

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 Apache Tomcat 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.*