CVE-2026-103535: high-severity vulnerability in Thelia

Thelia v2.6.1 contains path traversal and PHP code injection vulnerabilities in its translation editor that can be chained to achieve remote code execution. The `item_name` parameter, used to build the translation file's write path, does not sanitize directory traversal sequences such as `../`. This enables an authenticated user with `UPDATE`-level permissions on `admin.configuration.language` to write files outside the expected `templates/` directory, including to web‑accessible paths. Separately, the translation‑writing logic does not escape `\` in user‑supplied values, allowing inputs to break out of the PHP string literal and inject executable code into the translation file. Chained together, these flaws allow a malicious PHP file to be written to a web-accessible path and executed with the web‑server’s privileges.