### Summary act_runner appends workflow-controlled `jobs.<job>.container.options` directly to the Docker HostConfig for the job container. When runner privileged mode is disabled, only `Privileged` is forced false. Host namespace flags, capability expansion, and security profile overrides from workflow YAML are preserved in the final HostConfig. A workflow author can enter host PID/IPC namespaces and execute commands on the runner host as root. ### Details Source-to-sink path in act_runner: - `ContainerSpec.Options` accepts workflow YAML `container.options` - `RunContext.options()` appends workflow options to runner-level container options - Job container is created with `Privileged: rc.Config.Privileged` but also with `Options: rc.options(ctx)` - `mergeContainerConfigs()` parses Docker CLI-style options into HostConfig - When privileged mode is disabled, only `copts.privileged` is forced false - `sanitizeConfig()` only filters `Binds` and `Mounts` - Preserved dangerous HostConfig fields: ```text Privileged=false PidMode=host IpcMode=host CapAdd=["ALL"] SecurityOpt=["seccomp=unconfined","apparmor=unconfined"] ``` Attacker workflow YAML: ```yaml jobs: breakout: runs-on: ubuntu-latest container: image: ubuntu:22.04 options: >- --pid=host --ipc=host --cap-add=ALL --security-opt seccomp=unconfined --security-opt apparmor=unconfined steps: - name: host namespace marker run: | nsenter -t 1 -m -u -i -n -p -- sh -c "id > /tmp/marker" ``` ### Impact An attacker who can submit a workflow to a repository using a shared Docker-backed act_runner can: - Enter host PID, IPC, and mount namespaces - Execute arbitrary commands as root on the runner host - Access runner host secrets, deployment credentials, and environment variables - Pivot to adjacent jobs running on the same runner - Access internal build infrastructure reachable from the runner host Critical severity for shared runners where untrusted users can trigger workflows. High severity for single-tenant runners with privileged mode explicitly disabled as a security control. ### Fix Direction Treat `container.options` as untrusted input. Reject or strip when privileged mode is disabled: - Host namespaces: `--pid=host`, `--ipc=host`, `--uts=host`, `--network=host` - Capability expansion: `--cap-add ALL`, `--cap-add SYS_ADMIN` - Security overrides: `--security-opt seccomp=unconfined`, `--security-opt apparmor=unconfined` - Device access: `--device`, `--device-cgroup-rule` - Volume inheritance: `--volumes-from` - Runtime controls: `--runtime`, `--cgroup-parent`