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February 3, 2021

GHSL-2020-242: Command injection in telegramdesktop/tdesktop workflow

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

The ‘user_agent_updater.yml’ GitHub workflow is vulnerable to arbitrary command injection.

Product

telegramdesktop/tdesktop GitHub repository

Tested Version

The latest changeset 1a2afda to the date.

Details

Issue: The forked branch name is used to format a shell command

When a PR is closed it automatically starts the GitHub workflow. The forked branch name is used to format a bash script.

on:
...
  pull_request_target:
    types: [closed]
...
      - name: Delete branch.
        if: |
          env.isPull == '1'
            && github.event.action == 'closed'
            && startsWith(github.head_ref, env.headBranchPrefix)
        run: |
          git push origin --delete ${{ github.head_ref }}
...

An attacker can fork the repository, create a specially crafted branch name, make any commit, open a pull request and immediately close it to trigger the workflow. This vulnerability allows for arbitrary command injection into the bash script. For a Proof of Concept create a PR from branch named chrome_`echo${IFS}"abc"`.

Impact

The injection allows for exfiltration of secrets and the temporary GitHub repository authorization token to the attacker controlled server. Although the token is not valid after the workflow finishes, since the attacker controls the execution of the workflow he or she can delay it to give the malicious server time to modify the repository.

Credit

This issue was discovered and reported by GHSL team member @JarLob (Jaroslav Lobačevski).

Contact

You can contact the GHSL team at securitylab@github.com, please include a reference to GHSL-2020-242 in any communication regarding this issue.