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codeowners.md

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Overview

Github allows you to define for individuals or teams that are responsible for code in a repository or code owners. To do so, you should add the CODEOWNERS file to one of the following locations:

  • .github/
  • /
  • docs/

After which you can set up rules for protected branches and require mandatory approval from code owners.

References:

Code ownership takeover

The documentation for code owners said that the CODEOWNERS file can be defined in /, docs/, or .github/ directory:

To use a CODEOWNERS file, create a new file called CODEOWNERS in the root, docs/, or .github/ directory of the repository, in the branch where you'd like to add the code owners.

However, what happens if a repository contains multiple CODEOWNERS files? Actually, among the allowed paths there is the following priority:

  • .github/
  • /
  • docs/

So, if Github finds CODEOWNERS file in .github/, it will ignore CODEOWNERS files in / and docs/. In other words, if CODEOWNERS file has been created in / or docs/, an attacker with write permissions is able to add CODEOWNERS file to .github/, takeover code ownership, and bypass branch protection rules.Now the attacker is the owner of the code of the entire repository and can approve any changes.

Suppose there is a repository where .github/ has separate owners who are responsible for changes to that directory and CODEOWNERS file is stored in /. In such case, the CODEOWNERS file may look like this:

* @owner-team
.github/ @dev-team

A member of the @dev-team team, or an attacker who gains access to the account of this member, can elevate their privileges in this repository using the next steps:

  1. Using a personal Github account or other compromised account fork the repository.

  2. Add .github/CODEOWNERS file with the following content:

    * @dev-team

  3. Create a PR to the target repo.

  4. Approve the PR (since an attacker has access to the account that is an code owner of the .github/, they can approve any changes within .github/).

  5. Merge changes.

  6. Now an attacker is a code owner for the whole repository and they are able to approve any changes, including those outside .github/.