Summary
The tj-actions/changed-files workflow allows for command injection in changed filenames, allowing an attacker to execute arbitrary code and potentially leak secrets.
Details
The changed-files action returns a list of files changed in a commit or pull request which provides an escape_json input enabled by default, only escapes " for JSON values.
This could potentially allow filenames that contain special characters such as ; and ` (backtick) which can be used by an attacker to take over the GitHub Runner if the output value is used in a raw fashion (thus being directly replaced before execution) inside a run block. By running custom commands an attacker may be able to steal secrets such as GITHUB_TOKEN if triggered on other events than pull_request. For example on push.
Proof of Concept
- Submit a pull request to a repository with a new file injecting a command. For example
$(whoami).txt which is a valid filename.
- Upon approval of the workflow (triggered by the pull request), the action will get executed and the malicious pull request filename will flow into the
List all changed files step below.
- name: List all changed files
run: |
for file in ${{ steps.changed-files.outputs.all_changed_files }}; do
echo "$file was changed"
doneExample output:
##[group]Run for file in $(whoami).txt; do
for file in $(whoami).txt; do
echo "$file was changed"
done
shell: /usr/bin/bash -e {0}
##[endgroup]
runner.txt was changedImpact
This issue may lead to arbitrary command execution in the GitHub Runner.
Resolution
-
A new safe_output input would be enabled by default and return filename paths escaping special characters like ;, ` (backtick), $, (), etc for bash environments.
-
A safe recommendation of using environment variables to store unsafe outputs.
- name: List all changed files
env:
ALL_CHANGED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
run: |
for file in "$ALL_CHANGED_FILES"; do
echo "$file was changed"
doneResources
References
Summary
The
tj-actions/changed-filesworkflow allows for command injection in changed filenames, allowing an attacker to execute arbitrary code and potentially leak secrets.Details
The
changed-filesaction returns a list of files changed in a commit or pull request which provides anescape_jsoninput enabled by default, only escapes"for JSON values.This could potentially allow filenames that contain special characters such as
;and ` (backtick) which can be used by an attacker to take over the GitHub Runner if the output value is used in a raw fashion (thus being directly replaced before execution) inside arunblock. By running custom commands an attacker may be able to steal secrets such asGITHUB_TOKENif triggered on other events thanpull_request. For example onpush.Proof of Concept
$(whoami).txtwhich is a valid filename.List all changed filesstep below.Example output:
Impact
This issue may lead to arbitrary command execution in the GitHub Runner.
Resolution
A new
safe_outputinput would be enabled by default and return filename paths escaping special characters like ;, ` (backtick), $, (), etc for bash environments.A safe recommendation of using environment variables to store unsafe outputs.
Resources
References