Affected: npm: lodash, npm: lodash-es, npm: lodash-amd, npm: lodash.template

### Impact

The fix for [CVE-2021-23337](https://github.com/advisories/GHSA-35jh-r3h4-6jhm) added validation for the `variable` option in `_.template` but did not apply the same validation to `options.imports` key names. Both paths flow into the same `Function()` constructor sink.

When an application passes untrusted input as `options.imports` key names, an attacker can inject default-parameter expressions that execute arbitrary code at template compilation time.

Additionally, `_.template` uses `assignInWith` to merge imports, which enumerates inherited properties via `for..in`. If `Object.prototype` has been polluted by any other vector, the polluted keys are copied into the imports object and passed to `Function()`.

### Patches

Users should upgrade to version 4.18.0.

The fix applies two changes: 1. Validate `importsKeys` against the existing `reForbiddenIdentifierChars` regex (same check already used for the `variable` option) 2. Replace `assignInWith` with `assignWith` when merging imports, so only own properties are enumerated

### Workarounds

Do not pass untrusted input as key names in `options.imports`. Only use developer-controlled, static key names.