Handlebars.js has a Prototype Method Access Control Gap via Missing __lookupSetter__ Blocklist Entry

Summary

## Summary

The prototype method blocklist in `lib/handlebars/internal/proto-access.js` blocks `constructor`, `__defineGetter__`, `__defineSetter__`, and `__lookupGetter__`, but omits the symmetric `__lookupSetter__`. This omission is only exploitable when the non-default runtime option `allowProtoMethodsByDefault: true` is explicitly set — in that configuration `__lookupSetter__` becomes accessible while its counterparts remain blocked, creating an inconsistent security boundary.

`4.6.0` is the version that introduced `protoAccessControl` and the `allowProtoMethodsByDefault` runtime option.

## Description

In `lib/handlebars/internal/proto-access.js`:

```javascript const methodWhiteList = Object.create(null); methodWhiteList['constructor'] = false; methodWhiteList['__defineGetter__'] = false; methodWhiteList['__defineSetter__'] = false; methodWhiteList['__lookupGetter__'] = false; // __lookupSetter__ intentionally blocked in CVE-2021-23383, // but omitted here — creating an asymmetric blocklist ```

All four legacy accessor helpers (`__defineGetter__`, `__defineSetter__`, `__lookupGetter__`, `__lookupSetter__`) were involved in the exploit chain addressed by CVE-2021-23383. Three of the four were explicitly blocked; `__lookupSetter__` was left out.

When `allowProtoMethodsByDefault: true` is set, any prototype method **not present** in `methodWhiteList` is permitted by default. Because `__lookupSetter__` is absent from the list, it passes the `checkWhiteList` check and is accessible in templates, while `__lookupGetter__` (its sibling) is correctly denied.

## Workarounds

- Do **not** set `allowProtoMethodsByDefault: true`. The default configuration is not affected. - If `allowProtoMethodsByDefault` must be enabled, ensure templates do not reference `__lookupSetter__` through untrusted input.