Resources

People often ask me "How did you learn how to hack?" The answer: by reading. This page is a collection of the blog posts and other articles that I have accumulated over the years of my journey. Enjoy!

Many laptops come with fingerprint sensors that are used with the Windows Hello platform. The sensors use the Secure Device Connection Protocol (SDCP) for usage. This protocol is used in order to ensure that the fingerprint device and communication between them is valid. The modules are loaded in via secure boot on the Windows side.

To create a secure connection, the host and device perform a key agreement to derive shared session keys. An attestation is sent over, confirming that this is indeed the proper device. Each fingerprint is set up with a unique ID that is associated with the scanner.

To identify users the host generates a nonce and sends it to the sensor. The sensor does the biometric matching at this point. If it's a valid profile, the unique ID of the user is sent over with a MAC using the shared secret.

Eventually, they decided to pick some targets. As most researchers should be, they prioritized things with more support, easier to reverse engineer and bad code quality. Their first target was the Dell Inspiron. To intercept the USB traffic, they used a Linux driver and added some additional functionality.

The Dell Inspiron on Windows did everythign correctly. However, the Linux side did not implement SDCP for whatever reason. So, a user could generate arbitrary unique IDs and ask them to be stored, unlike the regular flow where the host chooses the key. Their plan was to enroll the attackers unique key on Linux to be the same as the Windows box. In practice, they learned that the stores for these are different.

But, how does it know which database to use? By executing a successful MitM and modifying the type, an attacker can get the module to use their fingerprint! Since the IDs were the same, Windows thought it was valid and processed to unlock the computer. Originally, they just tested this in WinDbg. Eventually, they wrote a USB tool in Linux to intercept the traffic.

On the Lenovo Thinkpad, they rolled their own TLS stack instead of using SDCP. Weird! The client certificate and key are encrypted when going across the wire using the device name and serial number (lolz). After this, a TLS session is made. This can be MitM'ed, since we know the private key of the certificate in user. Eventually, they reimplemented enough of the TLS stack to pwn it.

On the Microsoft Surface Pro, it was a joke. They implemented no SDCP, no authentication and completely clear text USB communication. Any USB device can claim to be the proper sensor and it will be accepted. The process of authenticating the senor is a must on this.

What is better: passwords or biometrics? You see, the biometrics opens up an entirely new attack surface that we've never seen before. Although it's convinent, it is also terrifying. Overall, an awesome post on SDCP, hardware hacking, USB tool writing and so much more. One of my favorite articles of the year!