When determining the installer type that is being used, the variable was not properly validated to ensure that it was ony one of a specific set of installer types.
CIviCRM 5.13.0 and earlier
CiviCRM version 5.13.4 and 5.7.6
Upgrade to the latest version of CiviCRM
Patrick Figel of Greenpeace for reporting the issue
Seamus Lee of Australian Greens for fixin the issue