The vulnerability lies in the addcrypted function in src/pyload/webui/app/blueprints/cnl_blueprint.py. The function takes a package parameter from a POST request and uses it to construct a file path for a .dlc file. The initial implementation had insufficient input sanitization, only removing slashes and colons, but failing to prevent path traversal sequences like ../. This allowed an unauthenticated attacker to craft a package parameter that, when combined with os.path.join, results in a path outside of the intended download directory. The attacker could then write arbitrary content to this path. The provided PoC demonstrates this by writing a cron job to /etc/cron.d/, leading to remote code execution. The patch introduces a check to verify that the final, normalized path is within the designated storage folder, thus preventing the path traversal.