Alex CTF USB probing Forensics 3 – 150 writeup

usb_challenge_qn

Challenge file: Download.

In fact, this is my first attempt to recover USB traffic from a PCAP file.

The initial 4 packets had the information of the devices involved in the traffic. Using the Product ID and Vendor ID I did some research here to get the device details. It is a flash drive.

usb_1

In the following paragraphs I will try to explain my approach to solve this problem but if you just want to see the solution please check the last 2 paragraphs.

Wireshark doesn’t have an easy option to view the transferred files using USB protocol, on the contrary it’s easy to extract or view transferred files in TCP (using TCP stream).

I made a simple test to understand how a simple file is transferred via USB protocol. I plugged in a USB device and transferred a text file ( with contents “findme”*1000). Of course, wireshark was listening to the usb interface in the background. To capture the USB traffic you must load the USB kernel module (check here).

$ sudo modprobe usbmon

Most of the packet’s sizes were less than 100 bytes and the transferred text file was found in a packet having a length greater than 1000 bytes, check the URB_BULK out.

usb_findme

So as a conclusion check for the packets having size greater than 1000 bytes with flags URB_BULK out/in. Also I found the file names that were present inside the flash drive.

Let’s repeat the same steps to find what was transferred. Load up the challenge file and try to find the packets having length greater than 1000 bytes. Go down a bit and bingo, you can find the PNG image’s header! 😉

usb_flag

Select the stream and press Ctrl + h or you can use File->Export Packet Bytes. Open the saved file in a image viewer and you see the flag!!

contest3

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s