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辽宁省赛线下awd,出了那么一道java题:

@PostMapping({"/cmd"})
@ResponseBody
public String cmd(@RequestParam String command) throws Exception {
    Base64.getDecoder();
    StringBuffer sb = new StringBuffer("");
    String[] c = {"/bin/bash", "-c", command};
    Process p = Runtime.getRuntime().exec(c);
    CopyInputStream(p.getInputStream(), sb);
    CopyInputStream(p.getErrorStream(), sb);
    return sb.toString();
}

遗憾的是:只有3个队伍会修。而我们正因为不会修,屈居第四(还好kali上有个jadx-gui,但是为什么jadx-gui只能看不能改啊!!)。

研究了一下,发现了java agent这个东西,可以弄出类似hook的效果(感觉不如。。xposed),但是感觉有很多坑,环境方面,所以记录一下。

首先最不重要的代码部分是从这里参考的,加了点针对函数重载的精确搜索(关于java agent的介绍也可以看这里):https://www.cnblogs.com/rickiyang/p/11368932.html

package agent2;

import java.io.IOException;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;
import javassist.*;

public class PreMainAgent {

    public static void premain(String agentArgs, Instrumentation inst) {
        System.out.println("agentArgs : " + agentArgs);
        inst.addTransformer(new DefineTransformer(), true);
    }

    static class DefineTransformer implements ClassFileTransformer {

        @Override
        public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
            if (!"com/example/ezjava/controller/EvilController".equals(className)) {
                return null;
            }
            System.out.println("premain load Class:" + className);
            try {
                System.out.println("trying!");
                final ClassPool classPool = ClassPool.getDefault();
                final CtClass clazz = classPool.get("com.example.ezjava.controller.EvilController");
                final CtClass str_class = classPool.get("java.lang.String");
                System.out.println(str_class);
                CtMethod cmd = clazz.getDeclaredMethod("cmd", new CtClass[]{str_class});
                //这里对 java.util.Date.convertToAbbr() 方法进行了改写,在 return之前增加了一个 打印操作
                String methodBody = "{return \"stupid!\";}";
                System.out.println("Setting body!");
                cmd.setBody(methodBody);

                // 返回字节码,并且detachCtClass对象
                byte[] byteCode = clazz.toBytecode();
                //detach的意思是将内存中曾经被javassist加载过的Date对象移除,如果下次有需要在内存中找不到会重新走javassist加载
                clazz.detach();
                System.out.println("returning!");
                return byteCode;
            } catch (IOException | CannotCompileException | NotFoundException e) {
                System.out.println("Error!");
                System.out.println(e);
                return null;
            }
        }
    }
}

坑部分:

下载javassist.jar

然后他用的是maven来改manifest,但是我一直没有找到把依赖打进包里的办法,就改用ant了。

IDE我用的netbeans。可以直接在Libraries上右键添加jar。

修改manifest参考这个链接:https://www.javaxt.com/wiki/Tutorials/Netbeans/How_to_Add_Version_Information_to_a_Jar_File_with_Netbeans

但是,其中的这部分不太对

First, you must update your Netbeans “project.properties” file found in the “nbproject” directory. Add the following line to the file:

manifest.file=manifest.mf

应该是把project.properties里的manifest.file=manifest.mf改成manifest.file=MANIFEST.MF

manifest里除了Premain-ClassAgent-Class以外还要再加两行(记得把他写的乱七八糟的属性都删掉):

<attribute name="Can-Redefine-Classes" value="true"/>
<attribute name="Can-Retransform-Classes" value="true"/>

虽然ant也没把依赖打进包里,但他创建了个lib目录并在manifest里写入了classpath的属性,所以可以跑起来。

然后启动命令是java -javaagent:'/home/zfn/NetBeansProjects/agent2/dist/agent2.jar' -jar awd.jar

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