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| 关键词： nbsp style 函数 function var document return cmdTextbox panel closeButton |
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： <!--1.htm--> <script type="text/javascript"> <!-- var _alert = alert; window.alert = function(s) { if (confirm("是否要弹框框，内容是\"" + s + "\"？")) { _alert(s); } } //--> </script> <html> <body> <input type="button" onclick="javascript: alert('Hello World!')" value="test" /> </body> </html> 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： <script type="text/javascript"> <!-- var _eval = eval; eval = function(s) { if (confirm("eval被调用\n\n调用函数\n" + eval.caller + "\n\n调用参数\n" + s)) { _eval(s); } } //--> </script> <html> <head> </head> <body> <script type="text/javascript"> <!-- function test() { var a = "alert(1)"; eval(a); } function t() { test(); } t(); //--> </script> </body> </html> 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 <function name> - set a breakpoint on a function, e.g. "bp window.alert". bl - list all breakpoints. bc <breakpoint number> - 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： <script type="text/javascript"> <!-- function log(s) { var img = new Image(); img.style.width = img.style.height = 0; img.src = "http://yousite.com/log.php?caller=" + encodeURIComponent(s); } var _alert = alert; window.alert = function(s) { log(alert.caller); _alert(s); } //--> </script> 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，赫赫，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： <script type="text/javascript"> <!-- var _eval = eval; eval = window.execScript = window.document.write = window.document.writeln = function(s) { document.getElementById("output").value = s; } //--> </script> <html> <body> input: <textarea id="input" cols="80" rows="10"></textarea> <input type="button" onclick="javascript: _eval(document.getElementById('input').value);" value="decode" /> <br /> output: <textarea id="output" cols="80" rows="10"></textarea> </body> </html> 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： <textarea id="tb1" cols="80" rows="8"></textarea> <script type="text/javascript"> <!-- document.getElementById("tb1").value = eval + "\n"; var _eval = eval; eval = function(s) { alert(s); _eval(s); } document.getElementById("tb1").value += eval; //--> </script> 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("<script type=\"text/javascript\">window.parent.escape = escape;<\/script>"); d.close(); } 综合1、2节，整个测试代码如下： <!--antihook.htm--> <script type="text/javascript"> <!-- escape = function(s) { return s; } //--> </script> <html> <body> <input type="button" onclick="javascript: test();" value="test" /> <script type="text/javascript"> <!-- function test() { alert(escape("s y")); if (checkHook(escape)) { unHook(escape); } alert(escape("s y")); } function checkHook(proc) { if (proc.toString().indexOf("[native code]") > 0) { return false; } else { return true; } } 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("<script type=\"text/javascript\">window.parent.escape = escape;<\/script>"); d.close(); } //--> </script> </body> </html> 3. 不是上面两个问题都解决了么，为什么要有第3节？因为那不是个最好的解决办法，既然我们可以创建全新的iframe，何不把代码直接放到全新iframe里执行呢，这样做的话绿色环保，既不用考虑当前context里的hook问题，也不用改动当前context，不会影响本身的程序执行。给出两个比较通用点的函数： function createIframe(w) { var d = w.document; var newIframe = d.createElement("iframe"); newIframe.style.width = 0; newIframe.style.height = 0; d.body.appendChild(newIframe); newIframe.contentWindow.document.write("<html><body></body></html>"); return newIframe; } function injectScriptIntoIframe(f, proc) { var d = f.contentWindow.document; var s = "<script>\n(" + proc.toString() + ")();\n</script>"; 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。 还有就是一些不得不感谢的人，感谢剑心一直以来毫无保留的交流，感谢黑锅多次鼓励我把自己的心得体会写成文字，感谢幻影所有的朋友们、B.C.T的朋友们以及群里那帮经常一起扯淡的朋友们。 广告一下，没法幻影blog的朋友，可以添加hosts来突破： 72.14.219.190 pstgroup.blogspot.com 五、附录 [1] 简易的javascript inline debugger代码 <!--test.htm--> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head><title>Javascript Inline Debugger</title></head> <body> <script language="javascript" type="text/javascript" src="js_inline_debugger.js"></script> <input type="button" value="hahaha" onclick="javascript: alert(this.value);" style="margin-left: 300px;" /> </body> </html> /* FileName: js_inline_debugger.js Author: luoluo Contact: luoluonet_at_yahoo.cn Date: 2007-6-27 Version: 0.1 Usage: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> </head> <body> <script language="javascript" type="text/javascript" src="js_inline_debugger.js"></script> </body> </html> 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(/</g, "<"); 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 = "<div style=\"margin-top: -2px;margin-left: 3px;\">x</div>"; 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, "<br />"); 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 <function name> - set a breakpoint on a function, e.g. \"bp window.alert\".\n" + "bl - list all breakpoints.\n" + "bc <breakpoint number> - 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));}); |
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