Firecracker is supposed to keep untrusted code inside a microVM. We escaped it and achieved code execution as root on the host.
We found how it saved registers, changed where it would resume, and made it execute a call of our choosing when it returned from KVM. The guest then restored the temporary stack changes. Firecracker kept running, and the VM remained accessible over SSH.
PwnEscape Part 1 introduced the host memory primitive behind this work. Here we follow the next part of the chain: using that access to find Firecracker in memory and take over a live thread from inside the guest. This is a continuation to show CVE-2026-99999 on a Firecracker setup.
A Firecracker microVM has a process on the host that manages it. Each virtual CPU, or vCPU, has a thread in that process. The thread enters KVM to run the guest and returns to Firecracker when an event needs handling in userspace. Firecracker documents this arrangement in its architecture overview.
