这是Facebook出于安全考虑而设计的。下面是Facebook团队成员埃里克·奥斯古德的解释:

This has been marked as 'by design' because it prevents a potential security vulnerability. Some browsers will append the hash fragment from a URL to the end of a new URL to which they have been redirected (if that new URL does not itself have a hash fragment). For example if example1.com returns a redirect to example2.com, then a browser going to example1.com#abc will go to example2.com#abc, and the hash fragment content from example1.com would be accessible to a script on example2.com. Since it is possible to have one auth flow redirect to another, it would be possible to have sensitive auth data from one app accessible to another. This is mitigated by appending a new hash fragment to the redirect URL to prevent this browser behavior. If the aesthetics, or client-side behavior, of the resulting URL are of concern, it would be possible to use window.location.hash (or even a server-side redirect of your own) to remove the offending characters.

来源:https://developers.facebook.com/bugs/318390728250352/

这是Facebook出于安全考虑而设计的。下面是Facebook团队成员埃里克·奥斯古德的解释:

This has been marked as 'by design' because it prevents a potential security vulnerability. Some browsers will append the hash fragment from a URL to the end of a new URL to which they have been redirected (if that new URL does not itself have a hash fragment). For example if example1.com returns a redirect to example2.com, then a browser going to example1.com#abc will go to example2.com#abc, and the hash fragment content from example1.com would be accessible to a script on example2.com. Since it is possible to have one auth flow redirect to another, it would be possible to have sensitive auth data from one app accessible to another. This is mitigated by appending a new hash fragment to the redirect URL to prevent this browser behavior. If the aesthetics, or client-side behavior, of the resulting URL are of concern, it would be possible to use window.location.hash (or even a server-side redirect of your own) to remove the offending characters.

来源:https://developers.facebook.com/bugs/318390728250352/

使用angular和angular ui router，你可以修复这个问题

    app.config(function ($stateProvider, $urlRouterProvider, $locationProvider) {

      // Make a trailing slash optional for all routes
      // - Note: You'll need to specify all urls with a trailing slash if you use this method.
      $urlRouterProvider.rule(function ($injector, $location) {
        /***
        Angular misbehaves when the URL contains a "#_=_" hash.

        From Facebook:
          Change in Session Redirect Behavior
          This week, we started adding a fragment #_=_ to the redirect_uri when this field is left blank.
          Please ensure that your app can handle this behavior.

        Fix:
          http://stackoverflow.com/questions/7131909/facebook-callback-appends-to-return-url#answer-7297873
        ***/
        if ($location.hash() === '_=_'){
          $location.hash(null);
        }

        var path = $location.url();

        // check to see if the path already has a slash where it should be
        if (path[path.length - 1] === '/' || path.indexOf('/?') > -1) {
          return;
        }
        else if (path.indexOf('?') > -1) {
          $location.replace().path(path.replace('?', '/?'));
        }
        else {
          $location.replace().path(path + '/');
        }
      });

      // etc ...
    });
});

Facebook最近在处理会话重定向的方式上做出了改变。有关公告，请参阅本周Operation Developer Love博客文章中的“会话重定向行为的更改”。

将此添加到我的重定向页面解决了我的问题…

if (window.location.href.indexOf('#_=_') > 0) {
    window.location = window.location.href.replace(/#.*/, '');
}

我看不出这个问题与facebook AJAX有什么关系。事实上，禁用JavaScript和完全基于重定向的登录也会出现这个问题。

一个与facebook交换的例子:

1. GET <https://www.facebook.com/dialog/oauth?client_id=MY_APP_ID&scope=email&redirect_uri=MY_REDIRECT_URL> RESPONSE 302 Found Location: <https://www.facebook.com/connect/uiserver.php?[...]>  
2. GET <https://www.facebook.com/connect/uiserver.php?[...]> RESPONSE 302 Found MY_REDIRECT_URL?code=FB_CODE#_  
3. GET MY_REDIRECT_URL?code=FB_CODE#_  

我也只在火狐浏览器上遇到过这种情况。