Reflections on trusting Docker: Invisible Malware in continuous integration systems

Revisiting the famous compiler backdoor from Ken Thompson, we show that a container-based Continuous Integration system can be compromised without leaving any trace in the source code. Detecting such malware is challenging or even impossible with common practices such as peer review or static code analysis. We detail multiple ways to do the initial infection process such as malicious commit or dependencies confusion. Finally, we show that the malicious code is able to backdoor production images and to reinject itself on CI system updates to allow long-term compromise.