compose-lint is a Docker Compose security linter that identifies dangerous misconfigurations before deployment. From 0.2.0 until 0.25.0, compose-lint follows a project-controlled symbolic link used as the Compose file outside the project. The same issue affects compose.override.yml and a COMPOSE_FILE entry from 0.22.0, and an env_file target from 0.23.0. If an external Compose-related target fails YAML parsing, the error quotes an attacker-uncontrolled line into standard error, JSON errors, and SARIF notifications, while an external env_file can expose credential-like key names through CL-0020 and CL-0021 findings. The attacker selects the target path but not the exact parser-failure line, values from env_file are not printed, and the pass or fail result is unchanged. This issue is fixed in version 0.25.0.

Affected: PyPI: compose-lint, GitHub Actions: tmatens/compose-lint

### Impact

Like Docker Compose, compose-lint follows symlinks when it reads a project's files. Unlike Compose, compose-lint up to 0.24.0 could then put the contents of a file from *outside* the project into its output, where the author of a pull request can read it.

A pull request can commit a symlink, under a name compose-lint reads, pointing at a file on the CI runner:

- **The Compose file itself (0.2.0–0.24.0).** A `compose.yml` that is a symlink, whether the pre-commit hook passes its name or the Action or a bare run finds it. - **`compose.override.yml` (0.22.0–0.24.0),** which compose-lint merges automatically. - **A `COMPOSE_FILE` entry (0.22.0–0.24.0)** in a `.env` beside the Compose file.

When the target is not valid YAML, the parse error quotes the line it failed on. Docker Compose, given the same file, reports only a line number. That line reaches standard error (the job log), JSON `errors[]`, and the SARIF `toolExecutionNotifications`, which the Action uploads to Code Scanning.

A fourth route: from 0.23.0, a symlinked `env_file:` target makes CL-0020 and CL-0021 report the credential-like **key names** in a file outside the project. Values are not printed.

What can be read is one line of a file chosen by path, at the point where the YAML parser fails, which the attacker cannot choose; or key names. A symlink to `/proc/self/environ` gave neither in testing. The pass/fail result is not affected. On a public repository the job log is visible to the pull request…