The headline is the nut of this story, but here’s CircleCI CTO Rob Zuber with the announcement:

We wanted to make you aware that we are currently investigating a security incident, and that our investigation is ongoing. We will provide you updates about this incident, and our response, as they become available. At this point, we are confident that there are no unauthorized actors active in our systems; however, out of an abundance of caution, we want to ensure that all customers take certain preventative measures to protect your data as well.