secure-action-inputs

A GitHub Action that scans the full event payload for common attack vectors — hidden Unicode characters, bidirectional text (Trojan Source), shell injection, path traversal, script injection, template expression injection, and AI prompt injection. Use it in any workflow to detect whether an actor (human or bot) is attempting to inject malicious content through PR titles, branch names, issue bodies, comments, or any other event field.

When a threat is found the step fails with a non-zero exit code and writes a detailed report to the Job Summary.

Attack vectors detected

Category	What is flagged
Homoglyphs	Cyrillic, Greek, and fullwidth Latin letters that look identical to ASCII (e.g. Cyrillic а→a, о→o, р→p; Greek Ο→O; fullwidth Ａ→A). Used in IDN homograph and supply-chain spoofing attacks
Hidden Unicode	Zero-width spaces (U+200B/C/D), BOM (U+FEFF), soft hyphen, null byte, LTR/RTL marks, word joiner, line/paragraph separators
Bidirectional / Trojan Source	All BIDI control characters (U+202A–202E, U+2066–2069, U+061C) that make malicious content appear benign to reviewers
Shell injection	Backtick substitution `cmd`, dollar-paren $(cmd), eval()/exec() code execution, semicolon/pipe chaining to bash, curl, python, etc.
Path traversal	../ and ..\ sequences that can escape intended directories
Script injection	<script>, javascript:, <iframe>, onerror= and other HTML event handler attributes
Template/expression injection	${{ (GitHub Actions context leakage) and {{...}} template expressions
Prompt injection	AI/LLM override phrases such as ignore previous instructions, pretend you are, jailbreak, and other directives designed to manipulate AI assistants that process event data

Usage

Add the action as an early step in any workflow that handles untrusted input. No inputs are required — the action reads the event payload automatically from GITHUB_EVENT_PATH.

name: Security scan

on:
  pull_request:
  issues:
  issue_comment:
  pull_request_review_comment:

jobs:
  scan:
    runs-on: ubuntu-latest
    steps:
      - name: Scan event payload for attack vectors
        uses: devops-actions/secure-action-inputs@v1

Recommended: scan before using event data in shell

Place this step before any step that interpolates event fields into shell commands or scripts.

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Scan event payload for attack vectors
        uses: devops-actions/secure-action-inputs@v1

      - name: Checkout
        uses: actions/checkout@v4

      # Safe: the scan above would have already failed the job
      # if the branch name contained shell injection characters.
      - name: Build
        run: |
          echo "Building branch: ${{ github.head_ref }}"

Example output

Clean payload

When no threats are detected the step passes and the Job Summary shows:

✅ No security threats detected in the GitHub Actions event payload.

Malicious payload

When threats are detected the step fails and the Job Summary shows a table like:

Field	Attack Type	Details
pull_request.title	hidden_unicode	Hidden Unicode character: Zero Width Space (U+200B) (×1)
pull_request.head.ref	homoglyph	Homoglyph attack: Cyrillic letters visually similar to Latin ASCII (e.g. а→a, е→e, о→o, р→p, с→c, х→x) (×2)
pull_request.body	bidi_attack	Bidirectional text (Trojan Source) attack: Right-to-Left Override (U+202E) (×1)
pull_request.head.ref	shell_injection	Potential shell injection: Backtick command substitution
issue.body	script_injection	Potential script injection: HTML script tag
comment.body	template_injection	Potential template/expression injection: GitHub Actions expression injection ${{

And each finding is also emitted as a workflow error annotation:

::error::[hidden_unicode] pull_request.title: Hidden Unicode character: Zero Width Space (U+200B) (×1)
::error::[shell_injection] pull_request.head.ref: Potential shell injection: Backtick command substitution
::error::Security scan failed: 5 potential attack vector(s) found in 4 field(s). See the step summary for details.

How it works

1. The action reads the JSON event payload from $GITHUB_EVENT_PATH.
2. It recursively walks every field in the payload (objects, arrays, strings).
3. Each string value is checked against all detector patterns.
4. Results are aggregated and written to $GITHUB_STEP_SUMMARY.
5. If any findings exist, process.exitCode is set to 1 to fail the step.

The action has zero runtime dependencies — all logic is bundled into dist/index.js with @vercel/ncc, and GitHub Actions workflow commands are issued directly over stdout to avoid supply-chain risk from transitive dependencies.

License

MIT