在今年的四月份，一位名叫tint0的安全研究人员向ZDI提交了两个存在于IBM WebSphere平台中的安全漏洞。其中一个漏洞是一个信息披露漏洞（ZDI-20-690/CVE-2020-4449），另一个则是一个远程代码执行（RCE）漏洞（ZDI-20-689/CVE-2020-4450）。在这篇文章中，我们将对这两个漏洞进行分析。

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#lookup

com.sun.jndi.rmi.registry.RegistryContext#decodeObject

javax.naming.spi.NamingManager#getObjectInstance

org.apache.aries.jndi.OSGiObjectFactoryBuilder#getObjectInstance

org.apache.aries.jndi.ObjectFactoryHelper#getObjectInstance

org.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<ClassLoader>() {

          public ClassLoader run() {

            return Thread.currentThread().getContextClassLoader();

          }

    });

 

    for (String cand : candidates) {

      ObjectFactory factory = null;

      try {

        Class<ObjectFactory> 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：

参考资料

• https://i.blackhat.com/eu-19/Wednesday/eu-19-An-Far-Sides-Of-Java-Remote-Protocols.pdf