Grav Server-side Template Injection via Insufficient Validation in filterFilter

Grav is a flat-file content management system. Prior to version 1.7.42, there is a logic flaw in the GravExtension.filterFilter() function whereby validation against a denylist of unsafe functions is only performed when the argument passed to filter is a string. However, passing an array as a callable argument allows the validation check to be skipped. Consequently, a low privileged attacker with login access to Grav Admin panel and page creation/update permissions is able to inject malicious templates to obtain remote code execution. The vulnerability can be found in the GravExtension.filterFilter() function declared in /system/src/Grav/Common/Twig/Extension/GravExtension.php. Version 1.7.42 contains a patch for this issue. End users should also ensure that twig.undefined_functions and twig.undefined_filters properties in /path/to/webroot/system/config/system.yaml configuration file are set to false to disallow Twig from treating undefined filters/functions as PHP functions and executing them.