It's an unfortunately common problem with GitHub Actions, it's easy to set things up to where any PR that's opened against your repo runs the workflows as defined in the branch. So you fork, make a malicious change to an existing workflow, and open a PR, and your code gets executed automatically.

Frankly at this point PRs from non-contributors should never run workflows, but I don't think that's the default yet.

“ At 5:40PM on November 18th, now-deleted user brwjbowkevj opened a pull request against our posthog repository, including this commit. This PR changed the code of a script executed by a workflow we were running against external contributions, modifying it to send the secrets available during that script's execution to a webhook controlled by the attacker. These secrets included the Github Personal Access Token of one of our bots, which had broad repo write permissions across our organization.”