Inclusion of Functionality from Untrusted Control Sphere
A 3rd party development library included with Drupal 8 development dependencies is vulnerable to remote code execution. This is mitigated by the default .htaccess protection against PHP execution, and the fact that Composer development dependencies aren't normally installed. You might be vulnerable to this if you are running a version of Drupal before 8.2.7 or 8.3.x before 8.3.0-rc2. To be sure you aren't vulnerable, you can remove the <siteroot>/vendor/phpunit directory from your production deployments
CWE-829 - Inclusion of Functionality from Untrusted Control Sphere
The software imports, requires, or includes executable functionality (such as a library) from a source that is outside of the intended control sphere.