Reverse TP-Link Router TL-WR841N

How to start doing reverse on IoT Firmware

Reversing TP-Link Router TL-WR841N

Firmware download URL: https://www.tp-link.com/pt/support/download/tl-wr841n/#Firmware

Extracting the Zip File

unzip unzip TL-WR841N\(EU\)_V14_200903.zip

Inspecting the firmware and arch

Using binwalk tool we can inspect and find the embedded files and executable code inside the firmware binary images.

Tip: use -e option to extract the files into a new folder.

binwalk -e TL-WR841Nv14_EU_0.9.1_4.17_up_boot\[200903-rel58674\].bin

In short, the binwalk structure is composed by three sections:

  • File location in decimal format

  • File location in hexadecimal form

  • Description about what was found and location

As observed above, we got U-Boot at offset 5404B. This is a popular bootload to load the operating system.

Also, an LZMA compressed data was obtained at offset 66560, and finally the squashfs filesystem at 1049088.

Duplicating firmware data with dd

Now, we can copy the filesystem squashfs into a new folder using the dd tool.

  • dd can duplicate data across files, devices, partitions, and volumes.

  • if stands for the input file.

  • of stands for the output file.

  • bs for block size.

Tip: by using -skip you could ignore some data at the beginning of the input stream. So, the -skip command needs to start with the initial offset we want to copy.

dd if=TL-WR841Nv14_EU_0.9.1_4.17_up_boot\[200903-rel58674\].bin skip=1049088 bs=1 of=TP.sfs

We check the new filesystem file using the file command:

file TP.sfs

Unarchive the filesystem with unsquashfs

unsquashfs - tool to uncompress squashfs filesystems

unsquashfs TP.sfs
ls -la unsquashfs-root

Yeah, we got it! 😎

From here, we can start with the analysis of the binaries present and individual files in the filesystem, and so on!

Good luck 🤓

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