Thymeleaf’s expression execution mechanism fails to properly restrict the scope of accessible objects, allowing an attacker to reach sensitive objects from within a template. When an application developer passes unvalidated user input directly to the template engine, an unauthenticated remote attacker can bypass the library’s protections and achieve Server‑Side Template Injection, potentially exposing confidential data or executing arbitrary code. The weakness corresponds to CWE‑1336 and CWE‑917.

Versions 3.1.3.RELEASE and earlier of Thymeleaf, including the thymeleaf‑spring5, thymeleaf‑spring6, and core thymeleaf components, are affected. The fix began in 3.1.4.RELEASE.

The CVSS score of 9.1 indicates high severity, and the vulnerability is not listed in CISA KEV. Although no EPSS score is available, the description shows that the flaw can be exploited by any unauthenticated user who can inject input into a template. The likely attack vector involves inserting a malicious expression into user‑controlled data that is then rendered by the template engine.