Craft CMS versions >= 5.0.0-RC1 before 5.10.7 and >= 4.0.0-RC1 before 4.18.3 contain a remote code execution vulnerability in the Twig sandbox mechanism. Because Craft marks the ElementInterface as safe (via the AllowedInSandbox attribute) and the sandbox allowlisting extends to the entire class hierarchy (craft\base\Component up to yii\base\Component), an authenticated attacker with permission to access the control panel can render a malicious Twig template that abuses the yii\base\Component arbitrary function-call gadget to execute arbitrary code, even when the Twig sandbox is enabled via enableTwigSandbox().

Affected: Packagist: craftcms/cms

The Twig sandbox mechanism in Craft CMS is configured to allow dangerous functionality from the Yii framework, leading to authenticated RCE in a manner similar to previously disclosed vulnerabilities.

The Twig sandbox in Craft CMS works by implementing Twig's `SecurityPolicyInterface`. The resulting `SecurityPolicy` class implements the `checkMethodAllowed` and `checkPropertyAllowed` methods of the interface. The implementations compare whether the called method or property is in a configured allowlist or is marked with the `AllowedInSandbox` attribute.

Additionally, `SecurityPolicy` allows the allowlisting of whole classes, which is shorthand for allowing all methods and properties of that class to be called. Both the allowlists for tags, filters, variables, methods, properties, and classes, as well as the `AllowedInSandbox` attribute mechanism, are preconfigured by Craft CMS to allow user-defined templates to use core functionality unimpeded. Additionally, Craft CMS project developers can add more to the allowlists or mark their custom methods and so on as safe with `AllowedInSandbox`.

The `SecurityPolicy` class's allowlisting mechanism is potentially dangerous, as it allows all methods and properties on an object of the allowed class. This means that the checked object can be a subclass and be allowed. And it means that the allowed methods and properties can be defined on any class of the class hierarchy of the object, which does not have to be the allowed class itself…