CVE-2026-52824: Kimai: Default APP_SECRET in Docker Image Enables Cookie Forgery and Account Takeover

The official Kimai Docker image ships with APP_SECRET=change_this_to_something_unique as the default environment variable. The Docker entrypoint does not override or validate this value. Any Kimai instance deployed using the Docker image without explicitly setting APP_SECRET runs with a publicly-known Symfony kernel.secret, enabling an unauthenticated attacker to forge HMAC-signed cookies and login links to take over any account including super_admin.