[轉]淺談javascript函數劫持

時間 2019-12-02

原文原文鏈接

轉自：Ph4nt0m Security Teamjavascript

這麼多年了，如今學習依然仍是有不少收貨，向前輩致敬。轉載一方面是本身存檔一份，另外一方面是讓更多喜歡安全的人一同窗習。php

==================================================css

by luoluo
on 2007-11-30
luoluonet_at_yahoo.cn
http://www.ph4nt0m.orghtml

1、概述

javascript函數劫持，也就是老外提到的javascript hijacking技術。最先仍是和劍心同窗討論問題時偶然看到的一段代碼，大概這樣寫的：java

window.alert = function(s) {};

以爲這種用法很巧妙新穎，和API Hook殊途同歸，索性稱之爲javascript function hook，也就是函數劫持。經過替換js函數的實現來達到劫持這個函數調用的目的，一個完整的hook alert函數例子以下：小程序

<!--1.htm-->
<script type="text/javascript">
<!--
var _alert = alert;
window.alert = function(s) {
    if (confirm("是否要彈框框，內容是\"" + s + "\"？")) {
        _alert(s);
    }
}
//-->
</script>
<html>
<body>
<input type="button" onclick="javascript: alert('Hello World!')" value="test" />
</body>
</html>

搞過API Hook的同窗們看到這個代碼必定會心的一笑，先保存原函數實現，而後替換爲咱們本身的函數實現，添加咱們本身的處理邏輯後最終再調用原來的函數實現，這樣這個alert函數就被咱們劫持了。原理很是簡單，下面舉些典型的應用來看看咱們能利用它來作些什麼。

2、應用舉例

1. 實現一個簡易的javascript debugger，這裏說是debugger比較標題黨，其實只是有點相似於debugger的功能，主要利用js函數劫持來實現函數的break point，來看看個簡單的例子：數組

<script type="text/javascript">
<!--
var _eval = eval;
eval = function(s) {
    if (confirm("eval被調用\n\n調用函數\n" + eval.caller + "\n\n調用參數\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>

經過js函數劫持中斷函數執行，並顯示參數和函數調用者代碼，來看一個完整例子的效果：安全

>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.

這裏演示設置斷點，察看斷點和刪除斷點，完整代碼在本文附錄[1]給出。

2. 設置陷阱實時捕捉跨站測試者，搞跨站的人總習慣用alert來確認是否存在跨站，若是你要監控是否有人在測試你的網站xss的話，能夠在你要監控的頁面裏hook alert函數，記錄alert調用狀況：app

<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>

固然，你這個函數要加到頁面的最開始，並且還要比較隱蔽一些，赫赫，你甚至可使alert不彈框或者彈個警告框，讓測試者抓狂一把。

3. 實現DOM XSS自動化掃描，目前通常的XSS自動化掃描的方法是從http返回結果中搜索特徵來肯定是否存在漏洞，可是這種方法不適用於掃描DOM XSS，由於DOM XSS是由客戶端腳本形成的，好比前段時間劍心發現的google的跨站(見附錄[2])原理以下：xss

document.write(document.location.hash);

這樣的跨站沒法反映在http response裏，因此傳統掃描方法無法掃描出來。可是若是你從上個例子裏受到啓發的話，必定會想到設置陷阱的辦法，DOM XSS最終致使alert被執行，因此咱們hook alert函數設置陷阱，若是XSS成功則會去調用alert函數，觸發咱們的陷阱記錄結果，這樣就能夠實現DOM XSS的自動化掃描，陷阱代碼相似於上面。

4. 靈活的使用js劫持輔助你的頁面代碼分析工做，好比分析網頁木馬時，常常會有經過eval或者document.write來進行加密的狀況，因而咱們編寫段hook eval和document.write的小工具，輔助解密：

<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框裏輸入加密的代碼：

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

在output框裏輸出解碼後的代碼：

alert(1);

固然你還能想到更多的靈活應用：）

3、反劫持

談到劫持也就必然要談談反劫持的話題，假設你要寫一個健壯的xss playload，就須要考慮反劫持，有兩個問題要解決：

如何判斷是否被劫持了？
若是發現被劫持了，如何反劫持？

1. 判斷某個函數是否被劫持，這個好辦，寫個小程序對比一下一個函數被hook先後，有什麼差異：

<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>

結果：

function eval() {
    [native code]
}

function(s) {
    alert(s);
    _eval(s);
}

咱們發現那些內置函數是[native code]，而自定義的則是具體的函數定義，用這個特徵就能夠簡單的檢測函數是否被劫持：

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

2. 如何反劫持，第一個想法就是恢復被劫持的函數，若是劫持的人把原函數保存在某個變量裏那還好辦，直接調用原函數就能夠了，可是劫持者本身也沒保存副本怎麼辦，只能本身建立個新的環境，而後用新環境裏的乾淨的函數來恢復咱們這裏被hook了的，怎麼建立新環境？整個新的iframe好了，裏面就是個全新的環境。ok，動手吧：

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();
}

綜合一、2節，整個測試代碼以下：

<!--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. 不是上面兩個問題都解決了麼，爲何要有第3節？由於那不是個最好的解決辦法，既然咱們能夠建立全新的iframe，何不把代碼直接放到全新iframe裏執行呢，這樣作的話綠色環保，既不用考慮當前context裏的hook問題，也不用改動當前context，不會影響自己的程序執行。給出兩個比較通用點的函數：

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封裝進一個函數，而後調用這兩個方法來在iframe裏執行：

function payload() {
    // your code goes here
}

var f = createIframe(top);
injectScriptIntoIframe(f, payload);

4、最後

因爲國內不多有見文章說起這個東西，因此才草成這篇，但願可以拋磚引玉。因爲本人水平有限，不免有錯誤或者疏漏之處請諒解，沒有說清楚的地方，歡迎和我交流。

還有就是一些不得不感謝的人，感謝劍心一直以來毫無保留的交流，感謝黑鍋屢次鼓勵我把本身的心得體會寫成文字，感謝幻影全部的朋友們、B.C.T的朋友們以及羣裏那幫常常一塊兒扯淡的朋友們。

廣告一下，無法幻影blog的朋友，能夠添加hosts來突破：
72.14.219.190 pstgroup.blogspot.com

5、附錄

[1] 簡易的javascript inline debugger代碼

<!--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.
*/

//--------------------------------------------------------------------------//
// 公用的函數
//--------------------------------------------------------------------------//
// 判斷是不是IE
function isIE() {
    return document.all && window.external;
}

// 去除字符串兩邊的空格
String.prototype.trim = function() {
    var re = /(^\s*)|(\s*)$/g;
    return this.replace(re, "");
}

// 刪除數組中某個元素
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;
}

// 判斷一個數組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編碼
function htmlEncode(s) {
    s = s.replace(/&/g, "&amp;");
    s = s.replace(/</g, "&lt;");
    s = s.replace(/>/g, "&gt;");
    s = s.replace(/\"/g, "&quot;");
    s = s.replace(/\'/g, "&#34;");

    return s;
}

// js編碼
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);

    // 標題欄
    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);

    // 標題欄拖動窗口功能
    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;
    }

    // 關閉窗口功能
    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;

    // 窗口關閉
    closeButton.onclick = function(evt) {
        isVisible = false;
        panel.style.display = "none";
    }

    // 標題欄文字
    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);

    // 中間的顯示部分
    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);

    // 命令輸入框
    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;

    // 輸出函數
    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);
        }
    }

    // 調用輸入處理回調函數
    cmdTextbox.onkeydown = function(evt) {
        var event = evt || window.event;

        if (event.keyCode == 0x0d) {
            processInputProc(this.value, dbgPrint);
            this.value = "";
        }
    }
}

// 座標類
function Position(x, y) {
    this.x = x;
    this.y = y;
}

//--------------------------------------------------------------------------//
// 內聯調試器類
//--------------------------------------------------------------------------//
function InlineDebugger() {
    var bpList = new Array();
    var id_eval;

    // 設置斷點
    var bp = function(funcName) {
        // 檢查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";
        }
    }

    // 枚舉全部的斷點
    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;
    }

    // 清除某個斷點
    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";
        }
    }

    // 幫助
    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;
    }

    // 處理命令
    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;
        }
    }
}

//-----------------------------------------------------------------------------//
// 主過程
//-----------------------------------------------------------------------------//
/*try {
    debugger;
} catch (e) {}*/
var id = new InlineDebugger();
var console = new Console(document.body, function(s, printProc){printProc(id.exeCmd(s));});