怎么滥用IBM WebSphere平台中的Java远程协议漏洞

本篇内容主要讲解"怎么滥用IBM WebSphere平台中的Java远程协议漏洞"，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习"怎么滥用IBM WebSphere平台中的Java远程协议漏洞"吧!

CORBA，即公共对象请求代理体系结构，它是一个由对象管理组织（OMG）定义的标准化规范。它是一个独立与平台的RPC框架，并且早于SOAP和gRPC等标准出现。在分布式环境中，CORBA使用了互联网InterORB协议（IIOP）来实现端点之间的通信。在IBM WebSphere的默认安装配置下，CORBA服务可以运行在TCP端口2809、9100、9402和9403。在调用服务方法之前，Interceptor类将会拦截调用请求，这里我们需要注意的是TxServerInterceptor类。

CVE-2020-4450-snippet-1.java：

public void receive_request(ServerRequestInfo sri) {  // ...snip...    if (TxProperties.SINGLE_PROCESS) {      propagationContext = TxInterceptorHelper.demarshalContext(serviceContext.context_data, (ORB)((LocalObject)sri)._orb()); // <------------      contextType = TxInterceptorHelper.determineContextType(propagationContext);    }    // ...snip...} public static final PropagationContext demarshalContext(byte[] bytes, ORB orb) {  // ...snip...  propContext.implementation_specific_data = inputStream.read_any(); // <-------  // ...snip...} public Any read_any() {  // ...snip...  any.read_value(this.encoderStream, typeCodeImpl); // <------------------------  return any;} private Object simpleReadObjectInternal(Class paramClass, String paramString) throws ClassNotFoundException, IOException {  // ...snip...      return readSerializable(paramClass, objectStreamClass, paramString); // <------  // ...snip...} private Object readSerializable(Class paramClass, ObjectStreamClass paramObjectStreamClass, String paramString) throws IOException, ClassNotFoundException {  // ...snip...  return inputObjectClassDesc(serializable, paramObjectStreamClass); // <-------} private Object inputObjectClassDesc(Object paramObject, ObjectStreamClass paramObjectStreamClass) throws IOException, ClassNotFoundException {  ObjectStreamClass objectStreamClass = processClassHierarchy(paramObject, paramObjectStreamClass);  return inputObjectUsingClassDesc(paramObject, objectStreamClass); // <--------}   Object inputObjectUsingClassDesc(Object paramObject, ObjectStreamClass paramObjectStreamClass) throws IOException, ClassNotFoundException {    // ...snip...      ObjectStreamClass objectStreamClass = this.readObjectOSC;      this.readObjectOSC = paramObjectStreamClass;      paramObjectStreamClass.readObjectMethod.invoke(paramObject, this.readObjectArglist); // <--- calls readObject()      this.readObjectOSC = objectStreamClass;      return true;}

当TxServerInterceptor类成功拦截调用请求时，便会调用receive_request()方法，同时还会试用demarshalContext()方法来从字节流中接收一个ServiceContext对象，而这个对象是攻击者可控的。任何嵌入在这个字节流中的对象都可以通过调用read_any()方法来提取，最终通过调用readObject()方法来获取嵌入的对象类。

远程代码执行漏洞（CVE-2020-4450）

尽管我们可以实现对任何对象进行反序列化操作，但实现远程代码执行并非易事。这是因为IBM Java SDK实现了针对反序列化攻击的安全缓解措施，其中包括：

• 拥有更严格的ClassLoader类，在运行时仅提供必要的类；

• TemplatesImpl类已无法再被序列化；

• IBM SDK不会使用Oracle JDK的Java名命方法以及目录接口（JNDI）。因此，我们无法通过RMI/LDAP来加载远程类并实现漏洞利用；

根据tint0的描述，tint0提供了一个Gadget链来绕过这种缓解方案。这个Gadget使用了WSIFPort_EJB类作为入口点。

CVE-2020-4450-snippet-2.java：

public class WSIFPort_EJB  extends WSIFDefaultPort  implements Serializable {  // ...snip...   private void readObject(ObjectInputStream ois) throws ClassNotFoundException, IOException {    ois.defaultReadObject();    if (this.separatedEJBRefs) {      Object objHome = ois.readObject();      if (objHome != null && objHome instanceof HomeHandle) {        HomeHandle homeHandle = (HomeHandle)objHome;        this.fieldEjbHome = homeHandle.getEJBHome();      }         Object obj = ois.readObject();      if (obj != null && obj instanceof Handle) {        Handle handle = (Handle)obj;        this.fieldEjbObject = handle.getEJBObject();  // <----------------------      }      }  }} public EJBObject getEJBObject() throws RemoteException {  // ...snip...  home = (EJBHome)PortableRemoteObject.narrow(ctx.lookup(this.homeJNDIName), homeClass); // <----  // ...snip...  Method fbpk = findFindByPrimaryKey(homeClass); // <---- returns findFindByPrimaryKey() method  this.object = (EJBObject)fbpk.invoke(home, new Object[] { this.key });   <--  $proxy.findFindByPrimaryKey(Serializable $arg)}

这个类最有趣的地方在于getEJBObject()方法，我们一起来看一看这里面的JNDI查询调用。

CVE-2020-4450-snippet-3.java：

com.sun.jndi.rmi.registry.RegistryContext#lookupcom.sun.jndi.rmi.registry.RegistryContext#decodeObjectjavax.naming.spi.NamingManager#getObjectInstanceorg.apache.aries.jndi.OSGiObjectFactoryBuilder#getObjectInstanceorg.apache.aries.jndi.ObjectFactoryHelper#getObjectInstanceorg.apache.aries.jndi.ObjectFactoryHelper#getObjectInstanceViaContextDotObjectFactories protected Object getObjectInstanceViaContextDotObjectFactories(Object obj, Name name, Context nameCtx, Hashtable environment, Attributes attrs) throws Exception {  Object result = null;  String factories = (String)environment.get("java.naming.factory.object");   if (factories != null && factories.length() > 0) {    String[] candidates = factories.split(":");    ClassLoader cl = (ClassLoader)AccessController.doPrivileged(new PrivilegedAction() {          public ClassLoader run() {            return Thread.currentThread().getContextClassLoader();          }    });     for (String cand : candidates) {      ObjectFactory factory = null;      try {        Class clz = cl.loadClass(cand);        factory = (ObjectFactory)clz.newInstance();      } catch (Exception e) {        if (logger.isLoggable(Level.FINE)) logger.log(Level.FINE, "Exception instantiating factory: " + e);      }       if (logger.isLoggable(Level.FINE)) logger.log(Level.FINE, "cand=" + cand + " factory=" + factory);       if (factory != null) {        if (factory instanceof DirObjectFactory) {          if (logger.isLoggable(Level.FINE)) logger.log(Level.FINE, "its a DirObjectFactory");            DirObjectFactory dirFactory = (DirObjectFactory)factory;          result = dirFactory.getObjectInstance(obj, name, nameCtx, environment, attrs);        }        else {          if (logger.isLoggable(Level.FINE)) logger.log(Level.FINE, "its an ObjectFactory");            result = factory.getObjectInstance(obj, name, nameCtx, environment);        }        }       if (result != null && result != obj)        break;    }  }  if (logger.isLoggable(Level.FINE)) logger.log(Level.FINE, "result = " + result);  return (result == null) ? obj : result;}

我们可以看到，getObjectInstanceViaContextDotObjectFactories()将会调用getObjectInstance的任意ObjectFactory类，其中一个跟我们Gadget链相关的类就是WSIFServiceObjectFactory。

CVE-2020-4450-snippet-4.java：

public Object getObjectInstance(Object obj, Name name, Context context, Hashtable env) throws Exception {  Trc.entry(this, obj, name, context, env);  if (obj instanceof Reference && obj != null) {    Reference ref = (Reference)obj;    if (ref.getClassName().equals(WSIFServiceRef.class.getName())) {      String wsdlLoc = resolveString(ref.get("wsdlLoc"));      String serviceNS = resolveString(ref.get("serviceNS"));      String serviceName = resolveString(ref.get("serviceName"));      String portTypeNS = resolveString(ref.get("portTypeNS"));      String portTypeName = resolveString(ref.get("portTypeName"));      if (wsdlLoc != null) {        WSIFServiceFactory factory = WSIFServiceFactory.newInstance();        WSIFService service = factory.getService(wsdlLoc, serviceNS, serviceName, portTypeNS, portTypeName);        Trc.exit(service);        return service;      }      } else if (ref.getClassName().equals(WSIFServiceStubRef.class.getName())) {      String wsdlLoc = resolveString(ref.get("wsdlLoc"));      String serviceNS = resolveString(ref.get("serviceNS"));      String serviceName = resolveString(ref.get("serviceName"));      String portTypeNS = resolveString(ref.get("portTypeNS"));      String portTypeName = resolveString(ref.get("portTypeName"));      String preferredPort = resolveString(ref.get("preferredPort"));      String className = resolveString(ref.get("className"));      if (wsdlLoc != null) {        WSIFServiceFactory factory = WSIFServiceFactory.newInstance();        WSIFService service = factory.getService(wsdlLoc, serviceNS, serviceName, portTypeNS, portTypeName); // <----        Class iface = Class.forName(className, true, Thread.currentThread().getContextClassLoader());        Object stub = service.getStub(preferredPort, iface);        Trc.exit(stub);        return stub;      }      }    }}

针对getObjectInstance()的调用将会根据一个指向远程XML定义的URL来初始化一个Web服务调用框架（WSIF）服务，而这个XML也是攻击者可控的。在这种场景下，服务的className会被设置为javax.el.ELProcessor，并且会定义一个java:operation元素，然后将findByPrimaryKey()映射为eval()方法。

getObjectInstance()调用将会返回一个WSIFClientProxy Java代理对象，当findByPrimaryKey()方法被调用时，这个代理对象将会调用ELProcessor实例的eval()方法。别忘了我们已经可以通过反序列化来控制this.key参数了，那么这样一来，我们就可以通过表达式语言注入技术来实现远程代码执行了。

信息披露漏洞（CVE-2020-4449）

这个漏洞利用Gadget利用的是一个XXE漏洞，漏洞代码如下所示。

CVE-2020-4450-snippet-5.java：

public static Definition readWSDL(String contextURL, String wsdlLoc) throws WSDLException {  Trc.entry(null, contextURL, wsdlLoc);     initializeProviders();     WSDLFactory factory = WSDLFactory.newInstance("org.apache.wsif.wsdl.WSIFWSDLFactoryImpl");     WSDLReader wsdlReader = factory.newWSDLReader(); // <---------------------  wsdlReader.setFeature("javax.wsdl.verbose", false);  try {    Definition def = wsdlReader.readWSDL(contextURL, wsdlLoc); // <---------    Trc.exitExpandWsdl(def);    return def;  } catch (WSDLException e) {    Trc.exception(e);    MessageLogger.log("WSIF.0002E", wsdlLoc);    throw e;  }  }

这个Gadget还演示了如何在现代JRE中如何绕过类似的缓解方案，尤其是通过FTP来实现越界提取等等。在这种场景下，数据是通过错误信息来提取的，因此当代码在解析XML文档时，如果没有封装在一个try/catch语句中的话，那么该漏洞将产生严重影响。

漏洞修复

为了修复这些漏洞，IBM采取了很多措施来确保TxServerInterceptor类不再会被反序列化为任意对象：

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