247CTF - Custom Protocol Writeup

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We are working on our own custom command and control protocol. Can you identify any hidden features in the service? We also included a packet capture of some old sessions so you can learn how it works.

我们正在开发自己的自定义命令与控制协议。您能发现该服务中有哪些隐藏功能吗？我们还提供了一些旧会话的数据包捕获文件，以便您了解其工作原理。

PCAP Investigation with Scapy

The first step was to examine the provided packet capture (custom_protocol_log.pcap) to understand the communication pattern. I used scapy to extract and decode raw data from the TCP streams.

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#!/usr/bin/env python3
from scapy.all import *
from scapy.layers.inet import TCP

PCAP_FILE = "custom_protocol_log.pcap"

def print_hex(pcap_path):
    packets = rdpcap(pcap_path)
    for pkt in packets:
        if TCP in pkt and pkt.haslayer(Raw):
            raw = pkt[Raw].load
            try:
                # Attempt to decode or print as hex
                print(raw.hex())
            except Exception as e:
                print(f"[-] Error: {e}")

if __name__ == "__main__":
    print_hex(PCAP_FILE)

Protocol Reversal

By analyzing the hex data from the traffic between 172.17.0.1 and 172.17.0.2, a clear pattern emerged:

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b925afc1 00 31 00 30 00 323430323435313235
b925afc1 00 32 00 30 00 323032383630353038
b925afc1 00 33 00 32 00 33383232383038323633

The protocol structure appears to be: [Session ID] [Null] [Counter] [Null] [Command] [Null] [Checksum]

  • Session ID: A 4-byte identifier (e.g., b925afc1).
  • Counter: Increments with each request.
  • Command: Hex-encoded numbers (e.g., 30 for 0, 31 for 1, 32 for 2).
  • Checksum: A CRC32 calculation of the preceding bytes, where the result is converted to a decimal string and then hex-encoded.

Checksum Verification

Using Python to verify the CRC32 logic:

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import zlib

def calc_custom_crc_hex(hex_data: str) -> str:
    data_bytes = bytes.fromhex(hex_data)
    crc_val = zlib.crc32(data_bytes)
    # Convert CRC32 integer to decimal string, then hex-encode that string
    return str(crc_val).encode().hex()

base_hex = "b925afc100310030"
print(f"Payload: {base_hex}")
print(f"Custom CRC Hex: {calc_custom_crc_hex(base_hex)}")
# Output: 323430323435313235 (Matches the PCAP!)

Solution

Exploit Script

I developed a brute-force script using pwntools to iterate through potential commands (0-14) and find the one that triggers the flag response.

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#!/usr/bin/env python3
import zlib
from pwn import *

HOST = "35f9359cc67c7983.247ctf.com"
PORT = 50385
context.log_level = "info"

def calc_custom_crc_hex(hex_data: str) -> str:
    data_bytes = bytes.fromhex(hex_data)
    crc_val = zlib.crc32(data_bytes)
    return str(crc_val).encode().hex()

def solve():
    sep = "00"
    counter = "31"
    # Commands 0-14 in hex format
    commands = [f"3{hex(i)[2:]}" for i in range(15)]

    for cmd in commands:
        try:
            conn = remote(HOST, PORT, timeout=5)
            # Receive Session ID
            session_id = conn.recvline(drop=True).decode(errors="replace")
            log.info(f"Session ID: {session_id} | Testing Command: {cmd}")

            # Construct payload: [SessionID][00][Counter][00][Command][00][Checksum]
            base_hex = f"{session_id}{sep}{counter}{sep}{cmd}"
            crc_hex = calc_custom_crc_hex(base_hex)
            payload = f"{base_hex}{sep}{crc_hex}"

            conn.sendline(payload.encode())

            raw_response = conn.recvall(timeout=3).decode(errors="replace")
            try:
                # Server returns hex-encoded response
                decoded_response = bytes.fromhex(raw_response).decode(errors="replace")
                if "247CTF" in decoded_response:
                    log.success(f"Flag found: {decoded_response}")
                    conn.close()
                    return
            except ValueError:
                log.warning(f"Failed to decode hex response: {raw_response}")
            print("-" * 40)

if __name__ == "__main__":
    solve()

Execution Output

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[*] Starting brute-force with commands: ['30', '31', '32', '33', '34', '35', '36', '37', '38', '39', '3a', '3b', '3c', '3d', '3e']
[+] Opening connection to 35f9359cc67c7983.247ctf.com on port 50385: Done
[*] Session ID: 9b2832a1
[+] Receiving all data: Done (54B)
[*] Closed connection to 35f9359cc67c7983.247ctf.com port 50385
[*] Response: (2\x001\x00notroot
    \x003738085711
----------------------------------------
[*] Connection closed.
[+] Opening connection to 35f9359cc67c7983.247ctf.com on port 50385: Done
[*] Session ID: 3907e672
[+] Receiving all data: Done (152B)
[*] Closed connection to 35f9359cc67c7983.247ctf.com port 50385
[*] Response: 9\x07r\x001\x00uid=1000(notroot) gid=1000(notroot) groups=1000(notroot)
    \x003051696603
----------------------------------------
[*] Connection closed.
[+] Opening connection to 35f9359cc67c7983.247ctf.com on port 50385: Done
[*] Session ID: edd0a465
[+] Receiving all data: Done (96B)
[*] Closed connection to 35f9359cc67c7983.247ctf.com port 50385
[*] Response: Фe\x001\x00Sun Mar  1 06:57:51 UTC 2026
    \x003480231763
----------------------------------------
[*] Connection closed.
[+] Opening connection to 35f9359cc67c7983.247ctf.com on port 50385: Done
[*] Session ID: 705136bc
[+] Receiving all data: Done (446B)
[*] Closed connection to 35f9359cc67c7983.247ctf.com port 50385
[*] Response: pQ6\x001\x00              total        used        free      shared  buff/cache   available
    Mem:        7973384     1587756     3913000        2636     2472628     6135804
    Swap:       2097148           0     2097148
    \x002165499562
----------------------------------------
[*] Connection closed.
[+] Opening connection to 35f9359cc67c7983.247ctf.com on port 50385: Done
[*] Session ID: d0f8fe01
[+] Receiving all data: Done (118B)
[*] Closed connection to 35f9359cc67c7983.247ctf.com port 50385
[*] Response: \x01\x001\x00247CTF{e5df2a6497c8733e8dc4679d856591af}\x001655731160
[+] Flag found: \x01\x001\x00247CTF{e5df2a6497c8733e8dc4679d856591af}\x001655731160
[*] Connection closed.

[Process exited 0]

Flag

247CTF{e5df2a6497c8733e8dc4679d856591af}