fix: Prototype safety guards for precompile: true (#2307)

Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Author: amannn

packages/icu-minify/.size-limit.ts

@@ -11,7 +11,7 @@ const config: SizeLimitConfig = [
     name: "import compile from 'icu-minify/compile' (production)",
     import: 'compile',
     path: 'dist/esm/production/compile.js',
-    limit: '7.055 kB'
+    limit: '7.1 kB'
   }
 ];
 

packages/icu-minify/src/compile.tsx

@@ -189,7 +189,7 @@ function compileDateTimeStyle(
 }
 
 function compileSelect(node: SelectElement): CompiledNode {
-  const options: SelectOptions = {};
+  const options = Object.create(null) as SelectOptions;
 
   for (const [key, option] of Object.entries(node.options)) {
     options[key] = compileNodesToNode(option.value);
@@ -203,7 +203,7 @@ function compilePlural(node: PluralElementBase): CompiledNode {
   if (process.env.NODE_ENV !== 'production' && node.offset) {
     throw new Error('Plural offsets are not supported');
   }
-  const options: PluralOptions = {};
+  const options = Object.create(null) as PluralOptions;
 
   for (const [key, option] of Object.entries(node.options)) {
     options[key] = compileNodesToNode(option.value);

packages/icu-minify/test/roundtrip.test.ts

@@ -553,6 +553,21 @@ describe('select', () => {
     ).toMatchInlineSnapshot(`"They"`);
   });
 
+  it('falls back to other when the value matches an Object.prototype key', () => {
+    const compiled = compile(
+      '{role, select, admin {Admin} user {User} other {Guest}}'
+    );
+    for (const key of [
+      'constructor',
+      'hasOwnProperty',
+      'toString',
+      'valueOf',
+      '__proto__'
+    ]) {
+      expect(formatMessage(compiled, 'en', {role: key})).toBe('Guest');
+    }
+  });
+
   it('formats arguments in branches', () => {
     const compiled = compile(
       '{gender, select, female {{name} is a woman} other {{name} is a person}}'

packages/next-intl/src/extractor/format/codecs/JSONCodec.tsx

@@ -1,4 +1,8 @@
-import {getSortedMessages, setNestedProperty} from '../../utils.js';
+import {
+  getSortedMessages,
+  isForbiddenObjectKey,
+  setNestedProperty
+} from '../../utils.js';
 import {defineCodec} from '../ExtractorCodec.js';
 
 interface StoredFormat {
@@ -38,6 +42,9 @@ function traverseMessages(
   const NAMESPACE_SEPARATOR = '.';
 
   for (const key of Object.keys(obj)) {
+    if (isForbiddenObjectKey(key)) {
+      throw new Error(`Invalid message catalog key: \`${key}\`.`);
+    }
     const newPath = path ? path + NAMESPACE_SEPARATOR + key : key;
     const value = obj[key];
     if (typeof value === 'string') {

packages/next-intl/src/extractor/utils.test.tsx

@@ -1,5 +1,5 @@
 import {describe, expect, it} from 'vitest';
-import {getSortedMessages} from './utils.js';
+import {getSortedMessages, setNestedProperty} from './utils.js';
 
 describe('getSortedMessages', () => {
   it('sorts by reference path', () => {
@@ -63,3 +63,21 @@ describe('getSortedMessages', () => {
     ).toEqual(['c', 'a', 'b']);
   });
 });
+
+describe('setNestedProperty', () => {
+  it('rejects __proto__ segments (prototype pollution)', () => {
+    expect(() => setNestedProperty({}, '__proto__.polluted', 'x')).toThrow(
+      'Invalid message id segment: __proto__'
+    );
+    expect(
+      (Object.prototype as unknown as {polluted?: string}).polluted
+    ).toBeUndefined();
+  });
+
+  it('creates plain data properties for nested paths', () => {
+    const root = Object.create(null) as Record<string, unknown>;
+    setNestedProperty(root, 'a.b', 1);
+    expect(Object.hasOwn(root, 'a')).toBe(true);
+    expect(({} as Record<string, unknown>).b).toBeUndefined();
+  });
+});

packages/next-intl/src/extractor/utils.tsx

@@ -9,23 +9,38 @@ export function normalizePathToPosix(filePath: string): string {
   );
 }
 
+const FORBIDDEN_OBJECT_KEYS = new Set([
+  '__proto__',
+  'constructor',
+  'prototype'
+]);
+
+export function isForbiddenObjectKey(key: string): boolean {
+  return FORBIDDEN_OBJECT_KEYS.has(key);
+}
+
 // Essentialls lodash/set, but we avoid this dependency
 export function setNestedProperty(
   obj: Record<string, any>,
   keyPath: string,
   value: any
 ): void {
   const keys = keyPath.split('.');
-  let current = obj;
+  for (const key of keys) {
+    if (isForbiddenObjectKey(key)) {
+      throw new Error(`Invalid message id segment: ${key}`);
+    }
+  }
 
+  let current = obj;
   for (let i = 0; i < keys.length - 1; i++) {
     const key = keys[i];
     if (
-      !(key in current) ||
+      !Object.prototype.hasOwnProperty.call(current, key) ||
       typeof current[key] !== 'object' ||
       current[key] === null
     ) {
-      current[key] = {};
+      current[key] = Object.create(null);
     }
     current = current[key];
   }

packages/next-intl/src/plugin/catalog/catalogLoader.tsx

@@ -90,7 +90,7 @@ function precompileMessages(
   messages: Array<ExtractorMessage>,
   cache: Map<string, CompiledMessageCacheEntry>
 ): Record<string, unknown> {
-  const result: Record<string, unknown> = {};
+  const result = Object.create(null) as Record<string, unknown>;
   const cacheKeysToEvict = new Set(cache.keys());
 
   for (const message of messages) {

packages/swc-plugin-extractor/.gitignore

@@ -1,3 +1,2 @@
 /target/*
-!/target/wasm32-wasip1/release/swc_plugin_extractor.wasm
-Cargo.lock
+!/target/wasm32-wasip1/release/swc_plugin_extractor.wasm