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Gh Actions - Context Script Injections

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Basic Information

Note that there are certain github contexts whose values are controlled by the user creating the PR. If the github action is using that data to execute anything, it could lead to arbitrary code execution. These contexts typically end with body, default_branch, email, head_ref, label, message, name, page_name,ref, and title. For example (list from this writeup):
  • github.event.comment.body
  • github.event.issue.body
  • github.event.issue.title
  • github.head_ref
  • github.pull_request.*
  • github.*.*.authors.name
  • github.*.*.authors.email
Note that here are less obvious sources of potentially untrusted input, such as branch names and email addresses, which can be quite flexible in terms of their permitted content. For example, zzz";echo${IFS}"hello";# would be a valid branch name and would be a possible attack vector for a target repository.

Example of a script injection attack

A script injection attack can occur directly within a workflow's inline script. In the following example, an action uses an expression to test the validity of a pull request title, but also adds the risk of script injection:
- name: Check PR title
run: |
title="${{ github.event.pull_request.title }}"
if [[ $title =~ ^octocat ]]; then
echo "PR title starts with 'octocat'"
exit 0
else
echo "PR title did not start with 'octocat'"
exit 1
fi
Before the shell script is run, the expressions inside ${{ }} are evaluated and then substituted with the resulting values, which can make it vulnerable to shell command injection.
To inject commands into this workflow, the attacker could create a pull request with a title of a"; ls $GITHUB_WORKSPACE"
In this example, the " character is used to interrupt the title="${{ github.event.pull_request.title }}" statement, allowing the ls command to be executed on the runner.
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