Rebuild-bot workflow may allow unauthorised repository modifications

Impact

projen is a project generation tool that synthesizes project configuration files such as package.json, tsconfig.json, .gitignore, GitHub Workflows, eslint, jest, and more, from a well-typed definition written in JavaScript. Users of projen's NodeProject project type (including any project type derived from it) include a .github/workflows/rebuild-bot.yml workflow that may allow any GitHub user to trigger execution of un-trusted code in the context of the "main" repository (as opposed to that of a fork). In some situations, such untrusted code may potentially be able to commit to the "main" repository.

The rebuild-bot workflow is triggered by comments including @projen rebuild on pull-request to trigger a re-build of the projen project, and updating the pull request with the updated files. This workflow is triggered by an issue_comment event, and thus always executes with a GITHUB_TOKEN belonging to the repository into which the pull-request is made (this is in contrast with workflows triggered by pull_request events, which always execute with a GITHUB_TOKEN belonging to the repository from which the pull-request is made).

Repositories that do not have branch protection configured on their default branch (typically main or master) could possibly allow an untrusted user to gain access to secrets configured on the repository (such as NPM tokens, etc). Branch protection prohibits this escalation, as the managed GITHUB_TOKEN would not be able to modify the contents of a protected branch and affected workflows must be defined on the default branch.

Patches

The issue was mitigated in version 0.16.41 of the projen tool, which removes the issue_comment trigger from this workflow. Version 0.17.0 of projen completely removes the rebuild-bot.yml workflow.

Workarounds

The recommended way to address the vulnerability is to upgrade projen. Users who cannot upgrade projen may also remove the .github/workflows/rebuild-bot.yml file and add it to their .gitignore file (via projenrc.js) to mitigate the issue.

References

The rebuild-bot.yml workflow managed by projen is only one occurrence of a GitHub Workflows mis-configuration, but it may also be present in other workflows not managed by projen (either hand-written, or managed by other tools). For more information on this class of issues, the Keeping your GitHub Actions and workflows secure: Preventing pwn requests article provides a great overview of the problem.