GHSA-h9w6-f932-gq62

### Impact

Web pages and web extensions using `ses` and the `Compartment` API to evaluate third-party code in an isolated execution environment that have also elsewhere used `const`, `let`, and `class` bindings in the top-level scope of a `<script>` tag will have inadvertently revealed these bindings in the lexical scope of third-party code.

### Patches

This compromise is addressed in `ses` version `1.12.0`. The mechanism for confining third-party code involves a `with` block and a semi-opaque scope `Proxy`. The proxy previously revealed any named property to the surrounding lexical scope if it were absent on `globalThis`, so that the third-party code would receive an informative `ReferenceError`, relying on the invalid assumption that only properties of `globalThis` are in the top-level lexical scope. The solution makes the scope proxy fully opaque. Consequently, accessing an unbound free lexical name will produce `undefined` instead of throwing `ReferenceError`. Assigning to an unbound free lexical name will continue to throw a `ReferenceError`.

### Workarounds

This problem can be mitigated either by avoiding top-level `let`, `const`, or `class` bindings in `<script>` tags, which is an existing industry best-practice, or change these to `var` bindings to be reflected on `globalThis`, or upgrade `ses` to version `1.12.0` or greater.

Some bundlers by default transform top-level `let`, `const`, and `class` bindings to `var`.

### Disclosure

This vulnerability was disclosed by @mingijunggrape in the course of their studies at UNIST (Ulsan National Institute of Science and Technology) as a member of the Web Security Lab (https://websec-lab.github.io/).