Due to a misconfiguration in Github action workflows, a security researcher was able to discover and exploit a ‘pwn request’ vulnerability in a public Stripe repository. This allowed the researcher to merge unauthorized commits into the main branch and exfiltrate the GitHub token used in the workflow to an endpoint they controlled.
Core Insights:
GitHub Actions Workflow Vulnerability:
- GitHub Actions workflows are automated scripts that run various tasks in a development pipeline.
- A misconfigured workflow likely allowed unauthorized actions, potentially leading to the exploitation of the “pwn request” vulnerability.
“Pwn Request” Vulnerability:
- This vulnerability may relate to manipulating or injecting malicious requests into the pipeline, exploiting the workflows to gain unauthorized access or execute malicious code.
CI/CD Pipeline Security:
- The incident highlights the importance of securing CI/CD pipelines. Such pipelines are a target for attackers due to their critical role in building, testing, and deploying code.
Best Practices for Security:
- Strict Workflow Configurations: Ensuring only authorized entities can trigger workflows.
- Token Security: Limiting the scope and lifespan of tokens used in workflows to reduce exposure.
- Branch Protection Policies: Enforcing policies to prevent unauthorized changes to critical branches.
Classic Pwn Request Vulnerability in the Workflow
Multiple workflows in the repository were vulnerable to a Pwn Request attack. Below is the breakdown of one such workflow’s misconfiguration:
Risky Trigger –
pull_request_target:- The workflow was configured to run on the
pull_request_targetevent. - This trigger executes workflows in the context of the base repository, rather than the fork, and grants elevated privileges, including access to sensitive secrets such as the GitHub token.
- While useful for certain use cases, it introduces significant risks when used improperly.
- The workflow was configured to run on the
Explicit Ref Checkout:
- The vulnerable workflow included a job that checked out the code using an explicit ref of the pull request.
- If the pull request originates from an untrusted fork, malicious actors can craft a pull request containing malicious code. This code gets executed with the elevated privileges provided by the
pull_request_targettrigger.
Structured Breakdown of The Attack
1. Initial Pull Request and Exploit Execution
- The security researcher submitted a pull request to the repository:
PR #2719. - The pull request exploited the vulnerable
pull_request_targettrigger in the GitHub Actions workflow. - Exploit Code:
- The exploit retrieved the
GITHUB_TOKENfrom the Git configuration. - Using the token, the attacker was able to merge the pull request into the main branch, bypassing authorization checks.
- The exploit retrieved the
2. Exfiltration of GitHub Token
- After the initial exploit, the researcher created another pull request to exfiltrate the GitHub token:
PR #2723. - How the Exfiltration Worked:
- The malicious code used
wgetto send theGITHUB_TOKENto a researcher-controlled endpoint. - This exfiltration demonstrated the potential misuse of sensitive secrets exposed by the vulnerable workflow.
- The malicious code used
Key Attack Mechanisms:
- Privilege Escalation:
- By exploiting the
pull_request_targettrigger, the attacker gained elevated privileges within the workflow environment.
- By exploiting the
- Token Abuse:
- The GitHub token was used for unauthorized actions, including merging a pull request and leaking sensitive credentials.
- Untrusted Code Execution:
- The workflow executed untrusted pull request code, enabling the attacker to manipulate the repository.
Lessons Learned:
This attack underscores the critical need for:
- Restricting GitHub Token Permissions: Tokens should have the least privileges required for workflows.
- Careful Use of
pull_request_target: This trigger should be used sparingly, with strict security controls. - Branch Protection Policies: Enforcing these policies can prevent unauthorized merges into protected branches.
- Audit and Review Workflows: Regularly review and secure workflows to eliminate exploitable configurations.
This is a clear example of how misconfigured CI/CD workflows can lead to severe repository security breaches.
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References
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