Let's All Love UART

Field	Value
Category	IoT
Difficulty	Easy
Points	500
Author	Eun0us
CTF	Espilon 2026

Description

This challenge emulates a UART interface on a Lain router. Open both connections, interact as if it was real hardware.

TX: Read only
RX: Write only

Let's All Love Lain!

TL;DR

Open the TX and RX ports simultaneously. Send flag on the RX (write) port. The flag is immediately printed on the TX (read) port.

Tools

Tool	Purpose
`nc`	Split UART connection
Python 3	Automated solver script

Solution

Step 1 — Open both channels

# Terminal 1 — read device output
nc <host> 1111

# Terminal 2 — send commands
nc <host> 2222

📸 [screenshot: two terminal windows open, TX showing the device banner]

Step 2 — Request the flag

In Terminal 2 (RX):

flag

Step 3 — Read the flag

In Terminal 1 (TX):

ESPILON{LAIN_TrUsT_U4RT}

📸 [screenshot: TX terminal printing the flag immediately after the flag command is sent]

Automated solver

#!/usr/bin/env python3
import socket
import threading

HOST = "<host>"
TX_PORT = 1111
RX_PORT = 2222

def reader():
    with socket.create_connection((HOST, TX_PORT)) as s:
        while True:
            data = s.recv(4096)
            if not data:
                break
            out = data.decode(errors="replace")
            print(out, end="")
            if "ESPILON{" in out:
                break

tx_thread = threading.Thread(target=reader, daemon=True)
tx_thread.start()

with socket.create_connection((HOST, RX_PORT)) as s:
    s.sendall(b"flag\n")

tx_thread.join()

Flag

ESPILON{LAIN_TrUsT_U4RT}

1.8 KiB Executable File Raw Blame History