Java MethodHandles can work like reflection

I'm helping test a Java execution sandbox. We want to allow untrusted code to use streams, so we can't deny the suppressAccessChecks and accessDeclaredMembers permissions, so instead we made the classloader for untrusted code reject anything in the java.lang.reflect package. Today I found that the MethodHandles API allows essentially the same capabilities as reflection. A lookup object obtained through privateLookupIn on a class belonging to a classloader that is an ancestor of the sandbox classloader can bypass the class lookup restrictions. Handles can then be obtained to normal reflection API methods, which can be used to override access modifiers.

Blocking the entire java.lang.invoke package isn't viable because streams/lambdas need parts of it, but blocking MethodHandles and MethodHandles.Lookup should make it impossible to dynamically invoke arbitrary methods.