CVE-2026-40478

Workarounds

No workaround is available beyond ensuring applications do not pass unvalidated user input directly to the template engine. Upgrading to 3.1.4.RELEASE is strongly recommended in any case.

Credits

Thanks to Dawid Bakaj (VIPentest.com) for responsible disclosure.

A security bypass vulnerability exists in the expression execution mechanisms of Thymeleaf up to and including 3.1.3.RELEASE. Although the library provides mechanisms to prevent expression injection, it fails to properly neutralize specific syntax patterns that allow for the execution of unauthorized expressions. If an application developer passes unvalidated user input directly to the template engine, an unauthenticated remote attacker can bypass the library's protections to achieve Server-Side Template Injection (SSTI).

CVE-2026-40478 has a CVSS score of 9.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.1.4.RELEASE); upgrading removes the vulnerable code path.

Kodem intelligence

This has been fixed in Thymeleaf 3.1.4.RELEASE.

1. What is CVE-2026-40478? CVE-2026-40478 is a critical-severity security vulnerability in org.thymeleaf:thymeleaf (maven), affecting versions <= 3.1.3.RELEASE. It is fixed in 3.1.4.RELEASE.
2. How severe is CVE-2026-40478? CVE-2026-40478 has a CVSS score of 9.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-40478?
   • org.thymeleaf:thymeleaf (maven) (versions <= 3.1.3.RELEASE)
   • org.thymeleaf:thymeleaf-spring5 (maven) (versions <= 3.1.3.RELEASE)
   • org.thymeleaf:thymeleaf-spring6 (maven) (versions <= 3.1.3.RELEASE)
4. Is there a fix for CVE-2026-40478? Yes. CVE-2026-40478 is fixed in 3.1.4.RELEASE. Upgrade to this version or later.
5. Is CVE-2026-40478 exploitable, and should I be worried? Whether CVE-2026-40478 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40478 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-40478?
   • Upgrade org.thymeleaf:thymeleaf to 3.1.4.RELEASE or later
   • Upgrade org.thymeleaf:thymeleaf-spring5 to 3.1.4.RELEASE or later
   • Upgrade org.thymeleaf:thymeleaf-spring6 to 3.1.4.RELEASE or later