Summary 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. Details "Dockerfile:263" sets "ENV APP_SECRET=change_this_to_something_unique". This value is consumed by "config/packages/framework.yaml:7" as "kernel.secret", which Symfony uses to HMAC-sign: - The "KIMAI_REMEMBER" remember-me cookie - LoginLink signatures - Password reset URLs - CSRF tokens The ".docker/entrypoint.sh" does not check for or replace the default sentinel value. The bare-metal ".env.dist:38" ships the same default. No startup-time guard exists anywhere in the codebase that refuses to start when "APP_SECRET" equals the sentinel. User IDs are sequential integers starting from 1. The first super_admin account is almost always "id=1". User IDs are visible in some URLs and API responses. A PoC was provided, but removed for security reasons. Impact Any Kimai instance deployed via the official Docker image without overriding "APP_SECRET" can be compromised from the internet. An unauthenticated attacker who can reach the Kimai URL can forge authentication tokens and log in as any user if: - a username is known AND - the correct account ID for this username is guessed AND - the account has no active 2FA (two factor) authentication Solution - The entrypoint.sh file is updated and now contains a script that generates a random "APP_SECRET" via "bin2hex(random_bytes(32))" which will be stored in "/opt/kimai/var/data/.appsecret" - The entrypoint.sh will create the file "/opt/kimai/.env.local" containing the "APP_SECRET", either fetched from the Docker Environment or from the newly created secret file - The documentation was updated to highlight the importance of using a random secret for "APP_SECRET" - The Dockerfile removed default "APP_SECRET=change_this_to_something_unique" - Login links now contain more entropy (see GHSA-m492-gv72-xvxj) - so even without all previous changes, attackers won't be able to generate Login links even for installations that have a hard-coded "APP_SECRET=change_this_to_something_unique" See "https://www.kimai.org/en/security/ghsa-jr9p-4h4j-6c58" (https://www.kimai.org/en/security/ghsa-jr9p-4h4j-6c58) for more information.