GitHub Actions workflows support special workflow commands that allow workflows to interact with the runner. These commands can be invoked by
printing specially crafted messages to the standard output. For example, ::set-mask can be used to mask sensitive values in the workflow
logs and ::error to terminate execution with a useful error message.
Two of such commands, ::set-env and ::add-path, allow setting arbitrary environment variables and modifying the system
PATH for the subsequent workflow steps. In 2020, these commands were deprecated and disabled by default due to security concerns. However,
ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable can still be used to re-enable them.
If this environment variable is set, the runner will process ::set-env and ::add-path commands printed to the standard
output. If an adversary can influence the content printed to the standard output (e.g., by injecting something into the build logs), they could use
these commands to manipulate the runtime environment.
What is the potential impact?
When user-supplied values are used to manipulate environment variables, an attacker can supply carefully chosen values that cause the system to
behave unexpectedly. This can be used to execute arbitrary commands as well as affect the behavior of other tools and scripts executed after the
injection point. In most cases, this leads to arbitrary code execution on the system.
It is important to note, that these commands are triggered by printing to standard output. Therefore, this issue can be exploited if an attacker
can influence the output of any command executed during the workflow.
The consequences of a successful attack can be severe and far-reaching:
Information disclosure
Attackers can extract sensitive information from the runner environment, including:
- Repository secrets and environment variables
- Authentication tokens and API keys
- Build artifacts and deployment credentials
System compromise
Successful command injection allows attackers to:
- Execute arbitrary commands on the runner
- Install malware or backdoors
- Access other resources available to the runner
Supply chain attacks
Compromised workflows can be used to:
- Inject malicious code into build artifacts
- Modify dependencies or packages being published
- Compromise downstream systems that consume the artifacts
Repository manipulation
Attackers may be able to:
- Modify repository contents or history
- Create unauthorized releases or tags
- Access or modify other repositories the workflow has access to
The impact is particularly severe because GitHub Actions runners often have elevated privileges and access to sensitive resources, making them
attractive targets for attackers.
