CVE-2026-40478: Improper neutralization of specific syntax patterns for unauthorized expressions in Thymeleaf

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).