⚠ 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-2026-42498: Information Disclosure 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, 9.0.2 <= 9.0.117, 8.5.24 <= 8.5.100, 7.0.83 <= 7.0.109
Fixed inSee vendor advisory
Type (CWE)CWE-200: Exposure of HTTP Authentication Header to unexpected hosts

What is CVE-2026-42498?

CVE-2026-42498 is an information disclosure flaw in Apache Tomcat. The product returns sensitive data to a caller who should not have access, including credentials, session tokens, or configuration. Disclosure often feeds a follow-up attack chain. Vendor description: Exposure of HTTP Authentication Header to unexpected hosts during WebSocket authentication vulnerability 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 9.0.2 through 9.0.117, from 8.5.24 through 8.5.100, from 7.0.83 through 7.0.109.

Why this CVE matters

Information disclosure flaws are dangerous because they make the next attack easier. Sensitive configuration, session material, or credentials leaked from one endpoint frequently power the follow-on attack that actually takes over the system.

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-2026-42498

  1. Read the vendor advisory in full: https://lists.apache.org/thread/n61zwf75jrv09rz90j4jssncm244bwdb
  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-2026-42498 affects Apache Tomcat 11.0.0-M1 <= 11.0.21.
# Fixed in 11.0.21. Vendor advisory: https://lists.apache.org/thread/n61zwf75jrv09rz90j4jssncm244bwdb

# 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/n61zwf75jrv09rz90j4jssncm244bwdb
# 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-2026-42498 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-2026-42498.

# 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/n61zwf75jrv09rz90j4jssncm244bwdb

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 unusually long URI paths containing traversal sequences, unexpectedly large responses from the affected endpoint, and outbound requests from the application to internal addresses or cloud-metadata endpoints. Treat any sensitive file the bug could disclose as exposed.

Frequently asked questions

Is CVE-2026-42498 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-2026-42498?

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.*