OPA Compiler: Bypass of WithUnsafeBuiltins using `with` keyword to mock functions

Open Policy Agent (OPA) is an open source, general-purpose policy engine. The Rego compiler provides a (deprecated) WithUnsafeBuiltins function, which allows users to provide a set of built-in functions that should be deemed unsafe — and as such rejected — by the compiler if encountered in the policy compilation stage. A bypass of this protection has been found, where the use of the with keyword to mock such a built-in function (a feature introduced in OPA v0.40.0), isn’t taken into account by WithUnsafeBuiltins. Multiple conditions need to be met in order to create an adverse effect. Version 0.43.1 contains a patch for this issue. As a workaround, avoid using the WithUnsafeBuiltins function and use the capabilities feature instead.