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浅谈Javascript函数劫持

　　本文标签：Javascript 函数 劫持 JS

一、概述

Javascript函数劫持，也就是老外提到的javascript hijacking技术。最早还是和剑心同学讨论问题时偶然看到的一段代码，大概这样写的：

window.alert = function(s) {}; 

觉得这种用法很巧妙新颖，和API Hook异曲同工，索性称之为javascript function hook，也就是函数劫持。通过替换js函数的实现来达到劫持这个函数调用的目的，一个完整的hook alert函数例子如下：







 

搞过API Hook的同学们看到这个代码一定会心的一笑，先保存原函数实现，然后替换为我们自己的函数实现，添加我们自己的处理逻辑后最终再调用原来的函数实现，这样这个alert函数就被我们劫持了。原理非常简单，下面举些典型的应用来看看我们能利用它来做些什么。

二、应用举例

1、实现一个简易的javascript debugger，这里说是debugger比较标题党，其实只是有点类似于debugger的功能，主要利用js函数劫持来实现函数的break point，来看看个简单的例子：








 

通过js函数劫持中断函数执行，并显示参数和函数调用者代码，来看一个完整例子的效果：

>help
debug commands:
bp  - set a breakpoint on a function, e.g. "bp window.alert".
bl - list all breakpoints.
bc  - remove a breakpoint by specified number, e.g. "bc 0".
help - help information.
>bp window.alert
* breakpoint on function "window.alert" added successfully.
>bl
* 1 breakpoints:
0 - window.alert
>bc 0
* breakpoint on function "window.alert" deleted successfully.

这里演示设置断点，察看断点和删除断点，完整代码在本文附录[1]给出。

2、设置陷阱实时捕捉跨站测试者，搞跨站的人总习惯用alert来确认是否存在跨站，如果你要监控是否有人在测试你的网站xss的话，可以在你要监控的页面里hook alert函数，记录alert调用情况：

当然，你这个函数要加到页面的最开始，而且还要比较隐蔽一些，呵呵！你甚至可以使alert不弹框或者弹个警告框，让测试者抓狂一把。

3、实现DOM XSS自动化扫描，目前一般的XSS自动化扫描的方法是从http返回结果中搜索特征来确定是否存在漏洞，但是这种方法不适用于扫描DOM XSS，因为DOM XSS是由客户端脚本造成的，比如前段时间剑心发现的google的跨站(见附录[2])原理如下：

document.write(document.location.hash); 

这样的跨站无法反映在http response里，所以传统扫描方法没法扫描出来。但是如果你从上个例子里受到启发的话，一定会想到设置陷阱的办法，DOM XSS最终导致alert被执行，所以我们hook alert函数设置陷阱，如果XSS成功则会去调用alert函数，触发我们的陷阱记录结果，这样就可以实现DOM XSS的自动化扫描，陷阱代码类似于上面。

4、灵活的使用js劫持辅助你的页面代码分析工作，比如分析网页木马时，经常会有通过eval或者document.write来进行加密的情况，于是我们编写段hook eval和document.write的小工具，辅助解密：




input:

onclick="javascript: _eval(document.getElementById('input').value);" value="decode" />


output:


 

在input框里输入加密的代码：

eval(unescape("%61%6C%65%72%74%28%31%29%3B"));

在output框里输出解码后的代码：

alert(1); 

当然你还能想到更多的灵活应用：）

三、反劫持

谈到劫持也就必然要谈谈反劫持的话题，假设你要写一个健壮的xss playload，就需要考虑反劫持，有两个问题要解决：

◆如何判断是否被劫持了？
◆如果发现被劫持了，如何反劫持？

1、判断某个函数是否被劫持，这个好办，写个小程序对比一下一个函数被hook前后，有什么差别：


结果：

function eval() {
    [native code]
}
function(s) {
    alert(s);
    _eval(s);
}

我们发现那些内置函数是[native code]，而自定义的则是具体的函数定义，用这个特征就可以简单的检测函数是否被劫持：

function checkHook(proc) {
    if (proc.toString().indexOf("[native code]") > 0) {
        return false;
    } else {
        return true;
    }
} 

2、如何反劫持，第一个想法就是恢复被劫持的函数，如果劫持的人把原函数保存在某个变量里那还好办，直接调用原函数就可以了，但是劫持者自己也没保存副本怎么办，只能自己创建个新的环境，然后用新环境里的干净的函数来恢复我们这里被hook了的，怎么创建新环境？整个新的iframe好了，里面就是个全新的环境。ok，动手吧！

function unHook(proc) {
var f = document.createElement("iframe");
f.style.border = "0";
f.style.width = "0";
f.style.height = "0";
document.body.appendChild(f);
var d = f.contentWindow.document;
d.write("";
    d.write(s);
}

把你的payload封装进一个函数，然后调用这两个方法来在iframe里执行：

function payload() {
    // your code goes here
}
var f = createIframe(top);
injectScriptIntoIframe(f, payload);

四、最后

由于国内很少有见文章提及这个东西，所以才草成这篇，希望能够抛砖引玉。由于本人水平有限，难免有错误或者疏漏之处请谅解，没有说清楚的地方，欢迎和我交流。

五、附录

[1]简易的javascript inline debugger代码


 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">



style="margin-left: 300px;" />

 
/*
FileName:        js_inline_debugger.js
Author:            luoluo
Contact:        luoluonet_at_yahoo.cn
Date:            2007-6-27
Version:        0.1
Usage:    
 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">






Instruction:    
It is a simple javascript inline debugger. You must
add xhtml1-transitional dtd to your
html document if you wanna to use the script.
*/
//--------------------------------------------------------------------------//
// 公用的函数
//--------------------------------------------------------------------------//
// 判断是否是IE
function isIE() {
return document.all && window.external;
}
// 去除字符串两边的空格
String.prototype.trim = function() {
var re = /(^\s*)|(\s*)$/g;
return this.replace(re, "");
}
// 删除数组中某个元素
Array.prototype.remove = function(i) {
var o = this[i];
for (var j = i; j < this.length - 1; j ++) {
this[j] = this[j + 1];
}
-- this.length; 
return o;
}
// 判断一个数组中是否存在相同的元素
Array.prototype.search = function(o) {
for (var i = 0; i < this.length; i ++) {
if (this[i] == o) {
return i;
}
}
return -1;
}
// html编码
function htmlEncode(s) {
s = s.replace(/&/g, "&");
s = s.replace(/s = s.replace(/>/g, ">");
s = s.replace(/\"/g, """);
s = s.replace(/\'/g, """);
return s;
}
// js编码
function jsEncode(s) {
s = s.replace(/\\/g, "\\\\");
s = s.replace(/\n/g, "\\n");
s = s.replace(/\"/g, "\\\"");
s = s.replace(/\'/g, "\\\'");
return s;
}
//--------------------------------------------------------------//
// 命令行窗口工具
//--------------------------------------------------------------//
function Console(parentNode, processInputProc) {
// 窗口
var panel = document.createElement("div");
panel.style.width = "250px";
panel.style.height = "250px";
panel.style.borderColor = "#666666";
panel.style.borderWidth = "1px";
panel.style.backgroundColor = "#ffffff";
panel.style.borderStyle = "solid";
panel.style.position = "absolute";
panel.style.zIndex = 100;
parentNode.appendChild(panel);
// 标题栏
var title = document.createElement("div");
title.style.width = "250px";
title.style.height = "15px";
title.style.backgroundColor = "#dddddd";
title.style.fontSize = "12px";
title.style.fontFamily = "verdana,tahoma";
panel.appendChild(title);
// 标题栏拖动窗口功能
var isDragging = false;
var startPos = new Position(panel.offsetLeft, panel.offsetTop);
var startMousePos = new Position(0, 0);
title.onmouseover = function(evt) {
this.style.cursor = "pointer";
}
title.onmousedown = function(evt) {
if (isDragging == true) {
return;
}
var event = evt || window.event;
startMousePos.x = event.clientX;
startMousePos.y = event.clientY;
isDragging = true;
}
title.onmousemove = function(evt) {
if (isDragging == false) {
return;
}

activateWindow();
var event = evt || window.event;
panel.style.left = (event.clientX - startMousePos.x) + startPos.x + "px";
panel.style.top = (event.clientY - startMousePos.y) + startPos.y + "px";
}
title.onmouseup = function(evt) {
if (isDragging == false) {
return;
}
var event = evt || window.event;
startPos.x =  (event.clientX - startMousePos.x) + startPos.x;
panel.style.left = startPos.x + "px";
startPos.y = (event.clientY - startMousePos.y) + startPos.y;
panel.style.top = startPos.y + "px";
isDragging = false;
}
// 关闭窗口功能
var closeButton = document.createElement("div");
closeButton.style.width = "13px";
closeButton.style.height = "13px";
closeButton.style.backgroundColor = "#ffffff";
closeButton.style.styleFloat = closeButton.style.cssFloat = "left";
closeButton.style.fontSize = "12px";
closeButton.style.borderWidth = "1px";
closeButton.style.borderColor = "#999999";
closeButton.style.borderStyle = "solid";
closeButton.style.paddingButton = "2px";
closeButton.innerHTML = "
";
title.appendChild(closeButton);
var isVisible = true;
// 窗口关闭
closeButton.onclick = function(evt) {
isVisible = false;
panel.style.display = "none";
}
// 标题栏文字
var titleLabel = document.createElement("div");
titleLabel.style.height = "14px";
titleLabel.style.width = "200px";
titleLabel.style.fontSize = "11px";
titleLabel.style.color = "#ffffff";
titleLabel.style.styleFloat = titleLabel.style.cssFloat = "left";
titleLabel.style.fontFamily = "verdana,tahoma";
titleLabel.innerHTML = "Javascript Inline Debugger";
//titleLabel.style.marginTop = isIE() ? -14 : "-14px";
titleLabel.style.marginLeft = isIE() ? 18 : "18px";
title.appendChild(titleLabel);
// 中间的显示部分
var showPanel = document.createElement("div");
showPanel.style.width = "250px";
showPanel.style.height = isIE() ? "221px" : "219px";
showPanel.style.fontSize = "11px";
showPanel.style.padding = "0";
showPanel.style.margin = "0";
showPanel.style.overflow = "auto";
showPanel.style.marginTop = isIE() ? -1 : "0";
panel.appendChild(showPanel);
// 命令输入框
var cmdArea = document.createElement("div");
panel.appendChild(cmdArea);
var cmdTextbox = document.createElement("input");
cmdTextbox.type = "text";
cmdTextbox.style.width = "250px";
cmdTextbox.style.height = "12px";
cmdTextbox.style.fontSize = "12px";
cmdTextbox.style.padding = "0";
cmdTextbox.style.margin = "0";
cmdTextbox.style.marginTop = isIE() ? -2 : "0";
cmdTextbox.style.borderWidth = "0";
cmdTextbox.style.borderTopWidth = "1px";
cmdTextbox.style.paddingTop = "2px";
cmdArea.appendChild(cmdTextbox);
// 窗口激活或者不激活
var isActive = false;
// 激活窗口
var activateWindow = function() {
if (! isVisible) {
return;
}
if (isActive) {
return;
}
cmdTextbox.focus();
title.style.backgroundColor = "#015DF4";
isActive = true;
}
panel.onclick = activateWindow;
// 不激活窗口
var deactivateWindow = function() {
if (! isVisible) {
return;
}
if (! isActive) {
return;
}
title.style.backgroundColor = "#cccccc";
isActive = false;
}
cmdTextbox.onfocus = activateWindow;
cmdTextbox.onblur = deactivateWindow;
// 输出函数
var dbgPrint = function(s) {
showPanel.innerHTML += htmlEncode(s).replace(/\n|(\r\n)/g, "
");

if (parseInt(showPanel.scrollHeight) > parseInt(showPanel.style.height)) {
showPanel.scrollTop += parseInt(showPanel.scrollHeight) -
parseInt(showPanel.style.height);
}
}
// 调用输入处理回调函数
cmdTextbox.onkeydown = function(evt) {
var event = evt || window.event;
if (event.keyCode == 0x0d) {
processInputProc(this.value, dbgPrint);
this.value = "";
}
}
}
// 坐标类
function Position(x, y) {
this.x = x;
this.y = y;
}
//--------------------------------------------------------------------------//
// 内联调试器类
//--------------------------------------------------------------------------//
function InlineDebugger() {
var bpList = new Array();
var id_eval;
// 设置断点
var bp = function(funcName) {
// 检查eval是否被hook
if ((new String(eval)).indexOf("[native code]") < 0) {
return "error: eval function was hooked by other codes in the front.\n";    
}
// 保存未被hooked的eval
id_eval = eval;
var re = /^[a-zA-Z0-9_\.]+$/i;
if (! re.test(funcName)) {
return "error: bad argument of command bp \"" + funcName + "\".\n";
}
try {
id_eval("if (typeof(" + funcName + ") == \"object\" ||
typeof(" + funcName + ") == \"function\") {var obj = " + funcName + ";}");
} catch (e) {
return "error: " + e + ", invalid function name \"" + funcName + "\".\n";
}
if (obj == undefined) {
return "error: the argument of command bp \"" + funcName +
"\" is not a function object.\n";
}
if ((new String(obj)).indexOf("function") < 0) {
return "error: the argument of command bp \"" + funcName +
"\" is a property, a function is required.\n";
}
if (bpList.search(funcName) >= 0) {
return "error: there is a breakpoint on function \"" + funcName + "\"\n";
}
try {
var sc = "window." + funcName.replace(/\./g, "_") + "_bak = " + funcName + ";\n" + 
funcName + " = " + 
"function() {\n" +
"    var args = \"\";\n" +
"    for (var i = 0; i < arguments.length; i ++) {\n" +
"        args += arguments[i] + (i == (arguments.length - 1) ?
\"\" : \",\");\n" +
"    }\n" +
"    if (confirm(\"function \\\"" + funcName +
"\\\" was called, execute it?\\n\\narguments:\\n\" + args +
\"\\n\\ncaller:\\n\" + " + funcName + ".caller)) {\n" +
"        id_eval(\"" + funcName.replace(/\./g, "_") +
"_bak(\\\"\" + jsEncode(args) + \"\\\")\");\n" +
"    }\n" +
"};" +
"\n";
id_eval(sc);
bpList.push(funcName);
return "* breakpoint on function \"" + funcName + "\" added successfully.\n";
} catch (e) {
return "unkown error: " + e + ".\n";
}
}
// 枚举所有的断点
var bl = function() {
if (bpList.length == 0) {
return "* no breakpoint.\n";
}
var bps = "* " + bpList.length + " breakpoints: \n";
for (var i = 0; i < bpList.length; i ++) {
bps += i + " - " + bpList[i] + "\n";
}
return bps;
}
// 清除某个断点
var bc = function(n) {
try {
n = parseInt(n);
} catch (e) {
return "error: bc command requires a numeric argument.\n";
}
if (bpList.length == 0) {
return "error: no breakpoint.\n";
}
var funcName = bpList.remove(n);

try {
eval(funcName + " = window." + funcName.replace(/\./g, "_") + "_bak;");
return "* breakpoint on function \"" + funcName + "\" deleted successfully.\n";
} catch (e) {
return "error: " + e + ".\n";
}
}
// 帮助
var help = function() {
var s = "debug commands:\n\n" +
"bp  - set a breakpoint on a function,
e.g. \"bp window.alert\".\n" +
"bl - list all breakpoints.\n" +
"bc  - remove a breakpoint by
specified number, e.g. \"bc 0\".\n" +
"help - help information.\n"
"\n";
return s;
}
// 处理命令
this.exeCmd = function(cmd) {
cmd = cmd.trim();
var cmdParts = cmd.split(/\s+/g);
var cmdName;
var cmdArg;
if (cmdParts.length == 1) {
cmdName = cmd;
} else {
cmdName = cmdParts[0];
cmdArg = cmdParts[1];
}
switch (cmdName) {
case "bp":
if (cmdArg == undefined) {
return "error: bp command requires an argument.\n";
} else {
return bp(cmdArg);
}
break;
case "bl":
return bl();
break;
case "bc":
if (cmdArg == undefined) {
return "error: bc command requires an argument \"number of breakpoint\".\n";
} else {
return bc(cmdArg);
}
break;
case "help":
return help();
break;
default: return "error: command \"" + cmdName + "\" not found,
you can get information by \"help\" command.\n";
break;
}
}
}
//-----------------------------------------------------------------------------//
// 主过程
//-----------------------------------------------------------------------------//
/*try {
debugger;
} catch (e) {}*/
var id = new InlineDebugger();
var console = new Console(document.body, function(s, printProc){printProc(id.exeCmd(s));});