A prototype pollution vulnerability exists in next-intl versions prior to 4.9.2 when using the experimental.messages.precompile feature. An attacker can craft a JSON translation catalog containing a top-level __proto__ key, which causes the build process to assign properties onto Object.prototype. This pollution affects all objects during the Next.js build, potentially enabling tampering with build tool internals and generated bundles. The vulnerability arises from improper handling of nested keys in setNestedProperty, which does not block reserved prototype keys. The issue is fixed in version 4.9.2.