Linksys EA6100 Firmware Reverse Engineering
Have you ever wondered how your router works? Well, I for one have. ISP and WAN technologies are super interesting, we’ll take a look at Modem images later though :)
We need two things to get started:
- Kali (My personal preference. You can use whatever distro you like, however, it may not have all the required tools)
- Linksys EA6100 Router Firmware (Version 1.1.5)
Once you have that downloaded and your Kali environment setup, prepare a Workspace! It’s going to get messy very quickly.
Decompiling the image
Once you have a workspace your happy with, it’s time to use a tool called
Binwalk is a tool written in Python that searches for the “Magic Byte”, or the first roughly 8 bytes that identify a given file. This can be useful for bin,img, and other files that may contain a collection of files, similar to a zip archieve, but not normally extractable. To extract it, we can use this:
binwalk -e FW_EA6100_1.15.img DECIMAL HEXADECIMAL DESCRIPTION -------------------------------------------------------------------------------- 0 0x0 uImage header, header size: 64 bytes, header CRC: 0xEAD12788, created: 2016-04-29 23:24:21, image size: 1765596 bytes, Data Address: 0x80000000, Entry Point: 0x8000C2F0, data CRC: 0x4DA9E76D, OS: Linux, CPU: MIPS, image type: OS Kernel Image, compression type: lzma, image name: "Linksys EA6100 Router" 64 0x40 LZMA compressed data, properties: 0x5D, dictionary size: 33554432 bytes, uncompressed size: 5435184 bytes 1835008 0x1C0000 JFFS2 filesystem, little endian
Reviewing the results, we get multiple files, the JFFS2 being the most interesting. We may re-visit the uImage header at a later date, but for now, we’re going to focus on the JFFS2 Filesystem. To take a peak inside the file we’re going to need to mount it. However, JFFS2 isn’t a standard filesystem, so we’re going to need to do a couple of things to mount it. This is what I have come up with to be a fairly consise guide step by step guide to mounting the JFFS2 File System. If it is not in Little Endian format (which this image is), you may need to convert it with a tool called jffs2dump. Like I said before, JFFS2 is already in the Little Endian format. So now, to mount the Firmware image!
#Step 1 #If /dev/mtdblock0 exists, remove file/directory and re-create the block device rm -rf /dev/mtdblock0 mknod /dev/mtdblock0 b 31 0 #Step 2 #Create a location for the jffs2 filesysystem to mkdir /mnt/jffs2_file/ #Step 3 #Load required kernel modules modprobe jffs2 modprobe mtdram modprobe mtdblock #Step 4 #Write image to /dev/mtdblock0 dd if=root/workspace/FW/decompiled/1C0000.jffs2 of=/dev/mtdblock0 #Step 5 #Mount file system to folder location mount -t jffs2 /dev/mtdblock0 /mnt/jffs2_file/ #Finally cd /mnt/jffs2_file/
If you made it this far without errors, congratulations! You’re almost done! We can finally take a peak at the Filesystem!
On the surface, it looks like a standard Linux filesystem, that’s because it is! A router honestly just runs Linux on it, you can dump the filesystem, view it, do whatever you like! I plan on trying to work on implementing a backdoor on the device so I can gain persistant access to it. I have some ideas, but am not sure if I can successfully execute it yet. I encourage everyone to try this project with this firmware (or a similar/your own router’s firmware). If you go and try this on your own, good luck!