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.

Affected: Go: github.com/open-policy-agent/opa

### Impact

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`.

The same method is exposed via `rego.UnsafeBuiltins` in the `github.com/open-policy-agent/opa/rego` package.

When provided e.g. the `http.send` built-in function to `WithUnsafeBuiltins`, the following policy would still compile, and call the `http.send` function with the arguments provided to the `is_object` function when evaluated:

```rego package policy

foo := is_object({ "method": "get", "url": "https://www.openpolicyagent.org" })

allow := r { r := foo with is_object as http.send } ```

Both built-in functions and user provided (i.e. custom) functions are mockable using this construct.

In addition to `http.send`, the `opa.runtime` built-in function is commonly considered unsafe in integrations where policy provided by untrusted parties is evaluated, as it risks exposing configuration, or environment variables, potentially carrying sensitive information.

#### Affected Users

**All of these conditions have to be met** to create an adverse effect:

* Use the Go API for policy evaluation (not the OPA s…