⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Critical · CVSS 9

How to Fix CVE-2026-41901: Server-Side Template Injection in thymeleaf

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 9 - Critical
Actively exploited?Not currently listed in CISA KEV
Affected< 3.1.5.RELEASE
Fixed in3.1.5.RELEASE.
Type (CWE)CWE-917: Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

What is CVE-2026-41901?

CVE-2026-41901 is a server-side template injection flaw in thymeleaf. User input is rendered as part of a template expression, which the engine then evaluates, giving the attacker code execution. Vendor description: Thymeleaf is a server-side Java template engine for web and standalone environments. Prior to 3.1.5.RELEASE, a security bypass vulnerability exists in the expression execution mechanisms of Thymeleaf.

Why this CVE matters

Server-side template injection is one of the cleanest paths from web request to code execution. The vulnerable parameter is usually attacker-controlled by design, which means proof-of-concept payloads are short and reliable.

For deployments of thymeleaf 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 thymeleaf'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-2026-41901

  1. Read the vendor advisory in full: https://github.com/thymeleaf/thymeleaf/security/advisories/GHSA-c9ph-gxww-7744
  2. Upgrade thymeleaf 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).

Update the maven / gradle dependency


# CVE-2026-41901 affects thymeleaf < 3.1.5.RELEASE. Fixed in 3.1.5.RELEASE.
# Vendor advisory: https://github.com/thymeleaf/thymeleaf/security/advisories/GHSA-c9ph-gxww-7744

# Maven: bump the dependency in pom.xml then re-resolve.
mvn versions:use-dep-version -Dincludes=thymeleaf -DdepVersion=3.1.5.RELEASE -DforceVersion=true
mvn clean install

# Gradle: bump in build.gradle then refresh.
./gradlew --refresh-dependencies build

# Restart the affected JVM service so the patched JAR loads.
sudo systemctl restart <service-name>

# Verify the runtime is on the patched build.
java -version


# Vendor advisory: https://github.com/thymeleaf/thymeleaf/security/advisories/GHSA-c9ph-gxww-7744
# Restart the equivalent Windows service after upgrading the JAR.
Restart-Service -Name "<service-name>"
Get-Service -Name "<service-name>"

Verify the fix landed


# CVE-2026-41901 verification checklist.

# 1. Confirm the running version matches 3.1.5.RELEASE (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-2026-41901.

# 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://github.com/thymeleaf/thymeleaf/security/advisories/GHSA-c9ph-gxww-7744

If you cannot patch immediately

Restrict access to the management interface to trusted internal IP addresses only. Block public access at the firewall and require VPN for any remote administration. Apply the patch as soon as a maintenance window allows.

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 unexpected administrator accounts in thymeleaf, scheduled tasks or cron jobs you did not create, new files in web-accessible directories, and outbound connections to addresses not in your baseline. Suspicious requests to the vulnerable endpoint immediately followed by successful 200-class responses with unusually large bodies are a strong indicator of exploitation.

Frequently asked questions

Is CVE-2026-41901 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-2026-41901?

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 thymeleaf 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.*