A bug exists in the pooling instance allocator in Wasmtime's runtime. When a failure to instantiate an instance for a module that defines an `externref` global occurs, it results in an invalid drop of a `VMExternRef` via an uninitialized pointer. This can happen under specific conditions, including when support for the reference types proposal is enabled and a pooling allocation strategy is configured. The module must define at least one table or memory and at least one `externref` global. During instantiation, a call to `mprotect` or `VirtualAlloc` must fail, or a resource limiter must be configured in the associated `Store` and return `false` from the initial call to `memory growing` or `table growing`. The effective impact of this bug is believed to be relatively small due to the uncommon usage of `externref` and the specific conditions required to trigger the bug.