GitHub Actions & Secret Inputs

Posted on Jul 2, 2024

I've been using GitHub Actions as my primary CI & CD tool for a while now, and I've been very happy with it. It's easy to use, reasonably flexible with the way actions are created, and has a massive marketplace which makes things all too easy to get what I need.

One of the features that I've been using a lot lately is manually triggered workflows, allowing me to create self-service workflows that can be triggered on demand, saving us from repetitive tasks; one automation at a time.

A simple example of these self-service workflow would be one that allows engineers to update their own environnment variables for a service. The team can simply go to our ops repositories Actions, and manually trigger the workflow with the service name, environment, key, and value as inputs:

name: Upsert Environment Variable

on:
  workflow_dispatch:
    inputs:
      service:
        description: 'Service Name'
        required: true
        type: choice
        options:
          - 'service-a'
          - 'service-b'
          - 'service-c'
      environment:
        description: 'Environment'
        required: true
        type: choice
        options:
          - 'staging'
          - 'production'
      key:
        description: 'Key'
        required: true
        type: string
      value:
        description: 'Value'
        required: true
        type: string

jobs:

  upsert:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      # .. omitted ..

And that's it! No more asking operations to update your environment variables.

ℹ️ Note: In an ideal world, we'd "shift left" and have our K8s manifests stored in application repositories, allowing engineers to maintain their own ConfigMap or other settings. Unfortunately, we're not quite there yet.

Depending on the service, updating an environment variable may be as simple as updating a secret in AWS Secrets Manager, or as complex as updating a file on an EC2 instance. This workflow allows us to abstract that complexity away from the engineers, and let them focus on what they do best: writing code.

Automation, done.

Except... not all environment variables are created equal.

Secrets

In order for this workflow to be successful, we had to make sure that secret values were not exposed in workflow logs, exposing the value to anyone with access to the repository.. and defeating the win of self-service.

Luckily, GitHub supports masking a value in a log, so let's just mask the input value and be done with it!

Failed Attempt

- name: Mask secret value securely
  run: |
    echo "SECRET_VALUE=${{ github.event.inputs.secret_value }}" >> $GITHUB_ENV
    echo "::add-mask::${{ github.event.inputs.secret_value }}"    
echo "SECRET_VALUE=DONT SHOW THE SECRET" >> $GITHUB_ENV
echo "::add-mask::DONT SHOW THE SECRET"

... and another

  env:
    SECRET_KEY: ${{ github.event.inputs.key }}
    SECRET_VALUE: ${{ github.event.inputs.secret_value }}
  steps:
    - run: echo "::add-mask::$SECRET_VALUE"
echo "::add-mask::$SECRET_VALUE"
shell: /usr/bin/bash -e {0}
env:
  SECRET_KEY: my-secret-key
  SECRET_VALUE: SHHH IM SECRET

...and on and on and on

All in all, I spent probably 2 hours trying to figure out how to mask the input value, but no matter what I did, the value would always be exposed in the logs.

1 Eternity Later

After a reasonable amount of trial and error, and so much searching, I came up with a fairly straightforward solution that allowed me to have a dynamic input without exposing the value until after it was masked. The solution?

A simple one-time link that would provide the secret value workflow.

To initially test my theory, I added a simple handler to my personal API that would behave like most secret sharing services, but without me having to bother picking (or paying for) one until I knew this could work:

  • POST https://api.syntaqx.com/secrets - Create a new secret
  • GET https://api.syntaqx.com/secrets/{id} - Retrieve a secret

⚠️ Warning: While this API is live, it currently does not provide any real security or reliability. The values are stored in memory, the encryption is weak, and you should not rely on these endpoints for real world use.

We ended up going with a self-hosted Yopass in our final workflow, which gave Engineers a UI to easily paste their secrets into and get a one-time link to retrieve them without the need for a script.

Choose your own adventure.

Creating a secret

To make things easy, I created a simple create-secret.sh script that would give me back a one-time link to retrieve the secret value:

Then, I could simply run the workflow, and provide the key and url as inputs:

name: Upsert Secret Environment Variable

on:
  workflow_dispatch:
    inputs:
      key:
        description: 'Key'
        required: true
        type: string
      url:
        description: 'One-click Link (https://api.syntaqx.com/secrets/{id})'
        required: true
        type: string

jobs:

  upsert:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      -
        name: Retrieve the secret value
        id: secret
        run: |
          RESPONSE=$(curl -sL "${{ github.event.inputs.url }}" | jq -r '.secret')
          echo "::add-mask::$RESPONSE"
          echo "value=$RESPONSE" >> "$GITHUB_OUTPUT"          
      -
        run: |
          echo "${{ github.event.inputs.key }}=${{ steps.secret.outputs.value }}"          

Which surprisingly worked! The value was masked and never exposed in the logs.

SECRET_KEY=***

Now it would just be a matter of cleaning up my bash to ensure that unexpected errors (or incorrectly entered URLs) would be handled gracefully. We have a path forward.

Conclusion

While this isn't perfect (anyone could easily just modify the workflow to echo $RESPONSE before the value is masked), assuming proper pull requests and code reviews, this should be secure enough for my use case.

Honestly, I'm pretty happy with the solution, but I'm not happy that I had to come up with it. I would love to see GitHub support dynamic secrets in workflows so that I don't have to jump through hoops to keep my secrets secret.

Until then, I'll keep using this, and hope that it helps someone else.

Open Letter to GitHub

Please, support dynamic secrets in workflows.

  workflow_dispatch:
    inputs:
      key:
        description: 'Key'
        required: true
        type: string
      value:
        description: 'Value'
        required: true
        type: secret

I know it's not just me asking.

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