[i18n] Routing fix (#70761)

This fixes a bug where an incorrectly sanitized query parameter would
cause an invalid routing condition resulting in the wrong route being
served to users. This currently requires some specific edge conditions
in order to trigger such as missing i18n configuration, self-hosted,
specifically crafted query parameter, and a path-based middleware for
authorization.

Author: wyattjoh

packages/next/src/server/base-server.ts

@@ -908,6 +908,14 @@ export default abstract class Server<ServerOptions extends Options = Options> {
       req.headers['x-forwarded-proto'] ??= isHttps ? 'https' : 'http'
       req.headers['x-forwarded-for'] ??= originalRequest.socket?.remoteAddress
 
+      // Validate that if i18n isn't configured or the passed parameters are not
+      // valid it should be removed from the query.
+      if (!this.i18nProvider?.validateQuery(parsedUrl.query)) {
+        delete parsedUrl.query.__nextLocale
+        delete parsedUrl.query.__nextDefaultLocale
+        delete parsedUrl.query.__nextInferredLocaleFromDefault
+      }
+
       // This should be done before any normalization of the pathname happens as
       // it captures the initial URL.
       this.attachRequestMeta(req, parsedUrl)

packages/next/src/server/future/helpers/i18n-provider.ts

@@ -134,6 +134,37 @@ export class I18NProvider {
     }
   }
 
+  /**
+   * Validates that the locale is valid.
+   *
+   * @param locale The locale to validate.
+   * @returns `true` if the locale is valid, `false` otherwise.
+   */
+  private validate(locale: string): boolean {
+    return this.lowerCaseLocales.includes(locale.toLowerCase())
+  }
+
+  /**
+   * Validates that the locales in the query object are valid.
+   *
+   * @param query The query object to validate.
+   * @returns `true` if the locale is valid, `false` otherwise.
+   */
+  public validateQuery(query: NextParsedUrlQuery) {
+    if (query.__nextLocale && !this.validate(query.__nextLocale)) {
+      return false
+    }
+
+    if (
+      query.__nextDefaultLocale &&
+      !this.validate(query.__nextDefaultLocale)
+    ) {
+      return false
+    }
+
+    return true
+  }
+
   /**
    * Analyzes the pathname for a locale and returns the pathname without it.
    *

packages/next/src/server/lib/router-utils/resolve-routes.ts

@@ -214,6 +214,11 @@ export function getResolveRoutes(
           parsedUrl.pathname = maybeAddTrailingSlash(parsedUrl.pathname)
         }
       }
+    } else {
+      // As i18n isn't configured we remove the locale related query params.
+      delete parsedUrl.query.__nextLocale
+      delete parsedUrl.query.__nextDefaultLocale
+      delete parsedUrl.query.__nextInferredLocaleFromDefault
     }
 
     const checkLocaleApi = (pathname: string) => {