The `mergechanges` task uses `debian.deb822.Changes` to parse the supplied `.changes` file. This parser happily accepts paths containing slashes including absolute paths in the `Files` and `Checksums-*` sections. When `_symlink_files` is called, `merged_file_path` is computed as `merged_dir / file["name"]` where the latter is unverified input. Therefore, the `merged_file_path` is fully user controlled (by prepending sufficiently many `../` or using an absolute path). Then, this location is created as a symbolic link unless it already exists. Therefore a malicious user can create possibly arbitrary symbolic links with the permission of the debusine-worker without being able to independently control the symlink target. This poses a minor containment breach.