This vulnerability is caused by a flaw in the expression execution mechanism of Thymeleaf prior to version 3.1.5.RELEASE. When templates are configured in sandboxed or restricted mode, the engine is supposed to neutralize dangerous expressions. Instead, it fails to recognize certain unauthorized syntax patterns, allowing those expressions to run. The result is Server‑Side Template Injection, which can compromise confidentiality, integrity, and availability by enabling arbitrary code execution on the server.

Affected products include the Thymeleaf template engine from the software vendor Thymeleaf. All releases prior to 3.1.5.RELEASE are vulnerable, including any web or standalone applications that embed Thymeleaf and utilize sandboxed contexts. The vulnerability applies to all Java applications that integrate Thymeleaf without applying the patch.

The CVSS score of 9 indicates high severity, and the lack of an EPSS score means current exploit probability data is not available, but high severity still warrants attention. The vulnerability is not listed in the CISA KEV catalog. The attack vector is inferred to be remote, as attackers can inject malicious expressions through unsanitized user input that is later processed by a sandboxed template. Successful exploitation can lead to execution of arbitrary code, data exfiltration, and denial‑of‑service.