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rtyler

It's not stealing when you're giving them away

My exact words were “I could have likely filled half a book with thoughts and practices on good security for Jenkins” and were I to write that book, my colleague Daniel would certainly be a guest author of a few chapters, such as one on the limitations of credentials masking. In that linked post Daniel highlights some of the ways in which credentials can be misused in Jenkins. As I mentioned on Twitter, this is not a unique problem to Jenkins, any system which allows user-defined build processes and allows the use of credentials in those build processes will allow exposure of those credentials.

Both Jenkins and Travis CI for example, can expose credentials or secrets as environment variables, which can then be used in the Jenkinsfile or .travis.yml respectively. Of course, once those credentials are exposed in any form to the user-defined build process, an incompetent or malicious user can leak those credentials.

Take this example from Daniel’s blog post:

withCredentials([usernamePassword(credentialsId: 'topSecretCredentials',
                               passwordVariable: 'PWD',
                               usernameVariable: 'USR')]) {
    echo 'Encrypting the password with base64, h4xx0r!'
    // will print e.g. dDBwczNjcjN0Cg=
    // which is trivially converted back to the top secret password
    sh 'echo $PWD | base64'
}

OH NOES HACKED! The only way to be certain credentials cannot be disclosed is to never expose them to the user-defined build process in the first place.

Mitigations

Of course, there are ways to reduce the surface area for possible credential leakage. One approach which Daniel discusses in the blog post, is segmenting where credentials are used:

Credentials can be defined in different scopes: Credentials defined on the root Jenkins store (the default) will be available to all jobs on the instance. The only exception are credentials with System scope, intended for the global configuration only, for example, to connect to agents. Credentials defined in a folder are only available within that folder (transitively, i.e. also in folders inside this folder).

This allows defining sensitive credentials, such as deployment credentials, on specific folders whose contents only users trusted with those credentials are allowed to configure: Directly in Jenkins using Matrix Authorization Plugin and by limiting write access to repositories defining pipelines as code.

There are a couple other approaches which can help:

  1. Utilize the built-in Jenkinsfile “Branch Source” configuration to allow only trusted contributors’ Pull Requests/Branches to be evaluated. This extends a modicum of trust, basically that if somebody can write to the repository, we trust them not to be foolish with credentials.
  2. Isolate the use of credentials to the master branch only. In many cases, I doubt that pull requests will need credentials for deployment. If the need for credentials is only once things land in master, for example, then we can use Jenkins Pipeline to further restrict where Pipelines might use credentials. Namely, only allowing the use once code has been merged, e.g.:
    stages {
        /* .. snip .. */
    
        stage('Deploy') {
            when { branch 'master' }
            environment {
                SECRET_KEY = credentials('deploy-key')
            }
            steps {
                /* .. */
            }
        }
    }
    

    This only reduces the possible disclosure surface area, but does not remove it entirely.

  3. Utilize a “deployment” environment, separate from the common “build” environment. This is more or less the approach we take in the Jenkins project. There is a non-public Jenkins instance which actually has deployment keys for a number of projects, and the Jenkinsfile for those projects is actually run in two different Jenkins environments. This also only reduces the possible disclosure surface area, since it only prevents problematic disclosures via the Console Output or archived artifacts. It would still be possible to run a script which POSTed to a third party service with those credentials.
  4. Utilize dynamic secrets with a tool like HashiCorp Vault. This reduces the effect of disclosure, since disclosed secrets would be invalid after their use in the Pipeline. This is not always possible, since many of the things we need secrets for are out of our control, such as GitHub, Slack, or other third-party service secrets.

Fundamentally, exposing credentials to user-defined code will always have the possibility for disclosure.

The Only Solution

Both Jenkins and Travis CI have multiple ways in which to use credentials. The simplest, the one which exposes those credentials to user-defined processes is fundamentally flawed. In Jenkins, an administrator can define credentials when configuring some plugins such as the Slack Notification plugin. In the case of Slack, the user cannot access the Slack credentials, but can invoke slackSend from within their Jenkins Pipeline. I strongly believe that variations on this pattern, wherein an administrator defines a credential and only allows administrator-governed code to utilize that credential, are the only ways in which credentials exposure can be avoided (barring bugs in the code, etc).

Unfortunately, in Jenkins this approach only applies to plugins. There is no way to expose credentials only to a Pipeline Shared Library, which would be administrator-approved code. This means there are some unfortunate limits to the reduction in surface area that administrators can provide.

As I have been thinking about this problem and some other design challenges in Jenkins, and as a result have been considering alternative system designs which would obviate many classes of security challenges Jenkins and other CI/CD systems face. Balancing user flexibility and security is among the harder problems in the space, and with continuous delivery becoming the standard, I believe we need to find safer ways to handle the deployment end of the pipeline.