【漏洞分析】Dubbo Pre-auth RCE（CVE-2021-30179）

Timeline Sec

发布于 2021-08-20 16:03:42

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发布于 2021-08-20 16:03:42

文章被收录于专栏：Timeline SecTimeline Sec

作者：lz2y@Timeline Sec

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声明：仅供学习参考使用，请勿用作违法用途，否则后果自负

0x01 漏洞复现

《CVE-2021-30179：Apache Dubbo RCE复现》

0x02 调试准备

暂无

0x03 漏洞分析

Dubbo使用DecodeHandler#received方法来接收来自socket的连接，当接收到请求时会先调用DecodeHandler#decode方法处理请求，其将调用DecodeableRpcInvocation.java#decode方法处理数据

在138行通过方法名和desc获取服务端中的方法描述，如果为空则判断是否为泛类引用

判断方法名是否为invoke或者invokeAsync，desc是否为Ljava/lang/String;[Ljava/lang/String;[Ljava/lang/Object;，如果不满足则直接抛出异常

decode完成之后将调用HeaderExchangeHandler.java#received方法处理请求，若为泛型引用，则将调用GenericFilter#invoke方法

判断是否满足泛型引用条件，通过getArguments()方法获取参数，并取第一个参数为方法名，第二个参数为方法名的类型，第三个参数为args。通过反射寻找服务端提供的方法，如果没有找到则抛出异常。

接下来将通过获取请求中的generic参数来选择通过raw.return/nativejava/bean反序列化参数成pojo对象，这个CVE漏洞的入口就在这里了

我们依次分析一下触发点

1、设置generic为raw.return或者true，将调用PojoUtils#realize方法

PojoUtils.realize(args, params, method.getGenericParameterTypes());

// PojoUtils#realize
    public static Object[] realize(Object[] objs, Class<?>[] types, Type[] gtypes)    {
        ...
        Object[] dests = new Object[objs.length];
        for (int i = 0; i < objs.length; i++) {
            dests[i] = realize(objs[i], types[i], gtypes[i]);
        }

调用PojoUtils#realize

public static Object realize(Object pojo, Class<?> type, Type genericType) {
        return realize0(pojo, type, genericType, new IdentityHashMap<Object, Object>());

调用PojoUtils#realize0，若pojo为Map实例，则从pojo（也就是一开始的第三个参数）获取key为“class”的值，并通过反射得到class所对应的类type，再判断对象的类型进行下一步处理

如果type不是Map的子类、不为Object.class且不是接口，则进入else，在else中，对type通过反射进行了实例化，得到对象dest

再对pojo进行遍历，以键名为name，值为value，调用getSetterMethod(dest.getClass(), name, value.getClass());获取set方法

最后利用反射执行method.invoke(dest, value);，就可以使用org.apache.xbean.propertyeditor.JndiConverter的setAsText发起JNDI注入了

private static Object realize0(Object pojo, Class<?> type, Type genericType, final Map<Object, Object> history) {
      ...
        if (pojo instanceof Map<?, ?> && type != null) {
            Object className = ((Map<Object, Object>) pojo).get("class");
            if (className instanceof String) {
                try {
                    type = ClassUtils.forName((String) className);
                } catch (ClassNotFoundException e) {
                    // ignore
                }
            }
            ...
            Map<Object, Object> map;
            if (!type.isInterface() && !type.isAssignableFrom(pojo.getClass())) {
                try {
                    ...
                } catch (Exception e) {
                    map = (Map<Object, Object>) pojo;
                }
            }
        if (Map.class.isAssignableFrom(type) || type == Object.class) {...}
        else if (type.isInterface()) {...}
        else {
            Object dest = newInstance(type);
                history.put(pojo, dest);
                for (Map.Entry<Object, Object> entry : map.entrySet()) {
                    Object key = entry.getKey();
                    if (key instanceof String) {
                        String name = (String) key;
                        Object value = entry.getValue();
                        if (value != null) {
                            // 获取对应的set方法
                            Method method = getSetterMethod(dest.getClass(), name, value.getClass());
                            Field field = getField(dest.getClass(), name);
                            if (method != null) {
                                if (!method.isAccessible()) {
                                    method.setAccessible(true);
                                }
                                Type ptype = method.getGenericParameterTypes()[0];
                                value = realize0(value, method.getParameterTypes()[0], ptype, history);
                                try {
                                    // 反射执行方法
                                    method.invoke(dest, value);
                                } catch (Exception e) {...}
                                else if (field != null) {...}
                                
        }

调用栈如下：

setAsText:59, AbstractConverter (org.apache.xbean.propertyeditor)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect)
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:497, Method (java.lang.reflect)
realize0:483, PojoUtils (org.apache.dubbo.common.utils)
realize:211, PojoUtils (org.apache.dubbo.common.utils)
realize:99, PojoUtils (org.apache.dubbo.common.utils)
invoke:91, GenericFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
invoke:38, ClassLoaderFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
invoke:41, EchoFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
reply:145, DubboProtocol$1 (org.apache.dubbo.rpc.protocol.dubbo)
received:152, DubboProtocol$1 (org.apache.dubbo.rpc.protocol.dubbo)
received:177, HeaderExchangeHandler (org.apache.dubbo.remoting.exchange.support.header)
received:51, DecodeHandler (org.apache.dubbo.remoting.transport)
run:57, ChannelEventRunnable (org.apache.dubbo.remoting.transport.dispatcher)
runWorker:1142, ThreadPoolExecutor (java.util.concurrent)
run:617, ThreadPoolExecutor$Worker (java.util.concurrent)
run:745, Thread (java.lang)

2、设置generic为bean，将遍历args，如果args[i]为JavaBeanDescriptor的实例，则调用JavaBeanSerializeUtil#deserialize进行处理

JavaBeanSerializeUtil.deserialize((JavaBeanDescriptor) args[i]);

// JavaBeanSerializeUtil#deserialize
    public static Object deserialize(JavaBeanDescriptor beanDescriptor) {
        return deserialize(
                beanDescriptor,
                Thread.currentThread().getContextClassLoader());
    }

调用

JavaBeanSerializeUtil#instantiateForDeserialize以获得对象，name2Class负责把 Class.forName 的返回值转换为 Class，而instantiate则实例化对象，最终返回一个JavaBeanDescriptor描述的对象

private static Object instantiateForDeserialize(JavaBeanDescriptor beanDescriptor, ClassLoader loader,IdentityHashMap<JavaBeanDescriptor, Object> cache) {
        ...
        Object result;
        if (beanDescriptor.isArrayType()) {...}
        else {
            try {
                Class<?> cl = name2Class(loader, beanDescriptor.getClassName());
                result = instantiate(cl);
                cache.put(beanDescriptor, result);
            } catch (Exception e) {
                throw new RuntimeException(e.getMessage(), e);
            }
        }
        return result;
    }

接着调用JavaBeanSerializeUtil#deserializeInternal进行反序列化，如果beanDescriptor.isBeanType()（只需要实例化JavaBeanDescriptor时指定即可），则将遍历beanDescriptor，获取property及value，调用getSetterMethod获取对应的set方法

最后利用反射执行method.invoke(dest, value);，就可以使用org.apache.xbean.propertyeditor.JndiConverter的setAsText发起JNDI注入了

private static void deserializeInternal(Object result, JavaBeanDescriptor beanDescriptor, ClassLoader loader,IdentityHashMap<JavaBeanDescriptor, Object> cache) {
        if (beanDescriptor.isEnumType() || beanDescriptor.isClassType() || beanDescriptor.isPrimitiveType()) {
            return;
        }
        if(beanDescriptor.isArrayType()){...}
        ...
        else if (beanDescriptor.isBeanType()) {
            for (Map.Entry<Object, Object> entry : beanDescriptor) {
                String property = entry.getKey().toString();
                Object value = entry.getValue();
                if (value == null) {
                    continue;
                }
                if (value instanceof JavaBeanDescriptor) {...}
                // 获取对应的set方法
                Method method = getSetterMethod(result.getClass(), property, value.getClass());
                boolean setByMethod = false;
                try {
                    if (method != null) {
                        // 反射执行方法
                        method.invoke(result, value);
                        setByMethod = true;
                    }
                }catch (Exception e) {
                    ...
                }
        }

调用栈如下：

setAsText:59, AbstractConverter (org.apache.xbean.propertyeditor)
invoke0:-1, NativeMethodAccessorImpl (sun.reflect)
invoke:62, NativeMethodAccessorImpl (sun.reflect)
invoke:43, DelegatingMethodAccessorImpl (sun.reflect)
invoke:497, Method (java.lang.reflect)
deserializeInternal:282, JavaBeanSerializeUtil (org.apache.dubbo.common.beanutil)
deserialize:215, JavaBeanSerializeUtil (org.apache.dubbo.common.beanutil)
deserialize:204, JavaBeanSerializeUtil (org.apache.dubbo.common.beanutil)
invoke:115, GenericFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
invoke:38, ClassLoaderFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
invoke:41, EchoFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
reply:145, DubboProtocol$1 (org.apache.dubbo.rpc.protocol.dubbo)
received:152, DubboProtocol$1 (org.apache.dubbo.rpc.protocol.dubbo)
received:177, HeaderExchangeHandler (org.apache.dubbo.remoting.exchange.support.header)
received:51, DecodeHandler (org.apache.dubbo.remoting.transport)
run:57, ChannelEventRunnable (org.apache.dubbo.remoting.transport.dispatcher)
runWorker:1142, ThreadPoolExecutor (java.util.concurrent)
run:617, ThreadPoolExecutor$Worker (java.util.concurrent)
run:745, Thread (java.lang)

3、设置generic为nativejava，将遍历args，如果args[i]的类型为byte，以args[]为参实例化一个UnsafeByteArrayInputStream，再通过反射获得NativeJavaSerialization，再调用NativeJavaSerialization#readObject方法

if (byte[].class == args[i].getClass()) {
    try (UnsafeByteArrayInputStream is = new UnsafeByteArrayInputStream((byte[]) args[i])) {
        args[i] = ExtensionLoader.getExtensionLoader(Serialization.class).getExtension(GENERIC_SERIALIZATION_NATIVE_JAVA).deserialize(null, is).readObject();
          } catch (Exception e) {
     throw new RpcException("Deserialize argument [" + (i + 1) + "] failed.", e);
    }
}else{...}

NativeJavaSerialization#readObject调用inputStream的readObject()方法，相当于调用了UnsafeByteArrayInputStream的readObject()方法

由于UnsafeByteArrayInputStream没有readObject()故调用ObjectInputStream#readObject()方法进行反序列化，我们可以通过以此为入口触发CC4的gadget

 public Object readObject() throws IOException, ClassNotFoundException {
        return inputStream.readObject();
    }

调用栈如下：

readObject:371, ObjectInputStream (java.io)
readObject:50, NativeJavaObjectInput (org.apache.dubbo.common.serialize.nativejava)
invoke:98, GenericFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
invoke:38, ClassLoaderFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
invoke:41, EchoFilter (org.apache.dubbo.rpc.filter)
invoke:83, ProtocolFilterWrapper$1 (org.apache.dubbo.rpc.protocol)
reply:145, DubboProtocol$1 (org.apache.dubbo.rpc.protocol.dubbo)
received:152, DubboProtocol$1 (org.apache.dubbo.rpc.protocol.dubbo)
received:177, HeaderExchangeHandler (org.apache.dubbo.remoting.exchange.support.header)
received:51, DecodeHandler (org.apache.dubbo.remoting.transport)
run:57, ChannelEventRunnable (org.apache.dubbo.remoting.transport.dispatcher)
runWorker:1142, ThreadPoolExecutor (java.util.concurrent)
run:617, ThreadPoolExecutor$Worker (java.util.concurrent)
run:745, Thread (java.lang)

0x04 个人总结

个人认为CVE-2021-30179的主要思路就是Apache Dubbo在处理泛类引用时，提供了多种通过反序列化方式得到对象再生成pojo对象的选择。

在进行反序列化过程中没有做好防护，轻易相信用户提供的数据，直接将其进行反序列化操作，导致一些恶意对象的实例化以及相对应Gadget的触发，从而造成RCE。

在2.7.10中，使用了黑名单来阻断raw.return和bean这两条链

org\apache\dubbo\common\utils\PojoUtils.java#realize0