CVE-2026-103537: medium-severity vulnerability in Thelia

Thelia 2.6.1 contains an absolute path traversal vulnerability in its Smarty template handling that enables SSTI and remote code execution. The `checkTemplate()` validation restricts relative directory traversal sequences such as `../` but does not reject absolute paths, allowing an authorized user to supply an arbitrary filesystem path as a template. Smarty then loads and compiles the referenced file without an effective security policy, so depending on filesystem layout and deployment configuration, template code in the file can be executed with the web server's privileges.