Improper Control of Generation of Code ('Code Injection')

Summary

Grav is a file-based Web platform. In getgrav/grav versions prior to 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 the 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 the 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`. 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.