Report it privately, through Report a vulnerability (the Security tab of the repository). Never in a public issue, a discussion or a pull request.

Include what you can: the version (/api/v1/info on the server, spawner --version), the part involved, how to reproduce it, and what an attacker gains. You get an answer within a week. The fix ships in a release, with a security advisory that credits you, unless you would rather not be named.

Only the latest release gets security fixes. Upgrade with the Update button of the dashboard, or install.sh --upgrade.

Spawner runs code from branches nobody has reviewed yet: its trust model says what it protects. Reports that matter most:

• the code of a branch, a compose file or an agent reaching the host, Spawner (its database, its secrets, the Docker socket), another environment, or traffic meant for them;
• access to the dashboard, the API, a protected preview or a terminal without the right session, token, role or share link;
• the installer or the release chain running or publishing something it should not.

Out of scope: what an admin can do (admins are trusted), what anyone with access to the server can do, and the limits listed in what Spawner does not do (yet).