☕ Rooting, Firmware Analysis, and Persistent Credentials of the TP-Link TL-841N

☆｡ﾟjuni's caramel tech café･ﾟ☆ /posts/42/rooting-the-tplink-tl841n-pt1/ (˃ 𖥦 ˂) wowie!

(˶˃ ᵕ ˂˶) .ᐟ.ᐟ Welcome back to the café! Today we are diving deep into the rooting, firmware extraction, and the discovery of hardcoded, reset-persistent credentials on the TP-Link TL-841N.

Date: 2 Aug 2026

[!IMPORTANT] Disclaimer: Please bear with me and this little puppo 🐶 throughout the post; you'll be seeing them quite a bit!

🛠️ The Mission

As part of my ongoing, slightly clumsy exploration of hardware hacking, I scored a budget-friendly $10 TP-Link TL-841N(EU) router from a stranger on Facebook Marketplace. The transaction was a high-stakes encounter conducted in the middle of a local shopping mall.

My objectives for this project were:

Perform a full physical teardown of an IoT device.
Access internal debug logs.
Obtain a root shell.
Practice firmware extraction (via UART and on-chip flash).
Explore the filesystem for security vulnerabilities.

🔍 Phase 1: Preliminary Reconnaissance

Before diving in, I wanted to see what I was dealing with. Spoiler alert: Stay tuned until the end to see the plaintext, hardcoded credentials from the previous owner—some of which survived a factory reset!

Cracking the Case: Finding UART

I started by searching the model number on the FCCID database to locate the datasheet. This provided critical intel on:

The PCB layout.
Operating voltage levels.
The specific chips utilized on the board.

Upon opening the chassis, I found the UART ports for the serial/debug interface were conveniently labeled. One interesting detail: the flash chip, a GD25Q64CSIG, was located on the underside of the board.

Router Shell

Identifying Pins (The Manual Way)

If your device isn't labeled, you can use a multimeter to probe pins during the boot sequence.

TX Pin: You will see a fluctuating voltage $\Delta V$ because the device is transmitting boot logs (binary $1$ s and $0$ s).
GND Pin: This should remain at a steady $0\text{V}$ .

~~I just guessed at first~~ $\rightarrow$ Actually, I probed until I found the fluctuating voltage, confirming the TX port.

🔌 Phase 2: Establishing the Connection

I used a budget-friendly USB-to-UART adapter from AliExpress. I verified it supported both $3.3\text{V}$ and $5\text{V}$ to ensure I didn't accidentally fry the board.

Wiring Configuration

Wire Color	Function	Connection Point
RED	$5\text{V}$	Not connected (to avoid damage)
BLACK	$\text{GND}$	Router $\text{GND}$ (or board metal)
GREEN	$\text{TX}$	USB $\text{RX}$
WHITE	$\text{RX}$	Router $\text{RX}$ (found via trial/error)

To ensure a stable, "plug-and-play" experience 😜, I decided to solder the GND, RX, and TX leads directly to the board.

Soldering

🐚 Phase 3: Gaining Root Access

Once the hardware was ready, I identified the device path (e.g., /dev/tty.usbserial-1210) and connected using picocom at the standard baud rate of 115200.

The Command:

picocom -b 115200 --logfile bootlog-tplink.txt /dev/tty.usbserial-1210

Result: I was dropped straight into a root shell!

I also tested the connectivity by joining the TP-Link Wi-Fi network with my phone, which populated the logs with additional activity. By connecting my laptop to a LAN port, I was assigned an IP and could access the admin console as usual.

💾 Phase 4: Firmware Extraction (Method #1: UART TFTP)

With root access, the next goal was to dump the firmware for local analysis.

1. Analyzing Partitions

I started by checking the flash partitions located at /proc/mtd to understand the memory layout.

2. Setting up the TFTP Server

On my Apple Silicon Mac, I used brew to install and run tftp-now:

tftp-now serve

[!WARNING] Critical Path Note: You must create a local folder in your home directory that matches the router's source path (e.g., ~/var). If you don't, the TFTP agent will return a No such file exists error because it attempts to mirror the remote path /var/ locally.

TFTP Setup

3. The Extraction Process

I connected my analysis machine to the router's LAN port (receiving an IP like 192.168.0.200). Since the router's internal storage was nearly full, I had to use the RAM-backed /var directory (approx. $6.4\text{ MiB}$ ) as a temporary staging area.

To avoid overloading the RAM, I used an incremental dump loop:

Dump the partition to RAM: cat /dev/mtd0 /var/boot.bin
Transfer the file to the laptop: tftp -p -l /var/boot.bin 192.168.1.100
Delete the temporary file: rm /var/boot.bin
Repeat for all remaining sectors.

Summary of the loop:

# Repeat for each mtd partition
cat /dev/mtd[X] /var/dump.bin
tftp -p -l /var/dump.bin [ANALYSIS_IP]
rm /var/dump.bin