Level 16 - Anticheat Proxy Bypass

Platform: CUSTOM | Category: Web | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: line_by_line_http, raw_socket_http, tcp_timing_bypass

Summary

Walkthrough of the CUSTOM Web Level 16 - Anticheat Proxy Bypass. Task: Bypass an anticheat proxy that detects automated tools by TCP packet timing. Solution: Send HTTP request line by line with delays using raw sockets to mimic human typing in netcat.

Description

A challenge about bypassing an anticheat proxy that detects automated tools by HTTP request timing. The server distinguishes between curl/wget (which send the entire request in one packet) and a human using netcat (who types line by line with delays).

Analysis

Initial Reconnaissance

When accessing the main URL http://kslweb1.spb.ctf.su/second/level16 we get 403 Forbidden:

• Server: nginx/1.18.0 (Ubuntu)
• X-Powered-By: PHP/5.5.9-1ubuntu4.21
• Accept-CH: Sec-CH-UA, Sec-CH-UA-Mobile, Sec-CH-UA-Full-Version-List, Sec-CH-UA-Platform-Version ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

The response contains a message: "This task is only available through Anticheat-Proxy on different port" and a link to http://kslweb1.spb.ctf.su:58080/second/level16.

Anticheat Proxy

When accessing port 58080 we get a page with an explanation:

• The task requires writing HTTP requests by hand
• The anticheat detects automated tools by checking whether all request lines are sent simultaneously (tool) or one at a time (human)
• It suggests using netcat -C kslweb1.spb.ctf.su 58080

Detection Mechanism

The anticheat proxy analyzes TCP packet timing:

• Automated tools (curl, wget, requests) send the entire HTTP request in a single TCP packet
• A human typing in netcat sends each line separately with noticeable delays between them ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Solution

To bypass the anticheat, we need to send the HTTP request line by line with delays between lines, simulating manual input.

 python read-only • copy allowed Copy
#!/usr/bin/env python3
"""
Bypass anticheat proxy by sending HTTP request line by line with delays.
The proxy detects automated tools by checking if all lines arrive at once.
"""
import socket
import time

def main():
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect(('kslweb1.spb.ctf.su', 58080))
    s.settimeout(10)

    # HTTP request lines - each sent separately with delay
    lines = [
        'GET /second/level16/ HTTP/1.1\r\n',
        'Connection: close\r\n',
        'Host: kslweb1.spb.ctf.su\r\n',
        '\r\n'
    ]
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

    # Send each line with 1 second delay to mimic human typing
    for line in lines:
        s.send(line.encode())
        time.sleep(1)

    # Receive response
    response = b''
    while True:
        try:
            data = s.recv(4096)
            if not data:
                break
            response += data
        except:
            break

    s.close()
    print(response.decode('utf-8', errors='replace'))

if __name__ == '__main__':
    main()

Alternatively, you can use netcat with manual input:

 bash read-only • copy allowed Copy
netcat -C kslweb1.spb.ctf.su 58080
# Then manually type each line:
# GET /second/level16/ HTTP/1.1
# Host: kslweb1.spb.ctf.su
# Connection: close
# (empty line)

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Server response:

HTTP/1.1 200 OK
...
Done! Flag: KSL{REDACTED}

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering line_by_line_http, raw_socket_http, tcp_timing_bypass was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-03-08 • CUSTOM • Level 16 - Anticheat Proxy Bypass • Difficulty: Easy