My self-hosted CI runner was the least trusted machine I owned. Now every job runs in a disposable Firecracker microVM on Krova Cloud — here's the setup, the scripts, and the trade-offs.
Earlier this year a boring dependency-bump PR came in to one of my open-source projects. One hunk touched the CI config, just slightly. I almost merged it with one eye on the diff.
Then I actually read it: one extra step on every build. And my self-hosted runner — a cheap VPS I'd set up to save CI minutes — ran jobs from forks. It also had cloud credentials sitting in a config file some past version of me had left there "for convenience."
A malicious PR wouldn't have needed to escape anything. It could have curled my environment to a pastebin and the runner would have obliged.
The real lesson wasn't about that PR. It's that a self-hosted runner is a machine that executes strangers' code, and then keeps living. My cheapest machine was my most dangerous one.
