TL;DR: You can configure Burp to use your PKCS#11 (or Belgian eID) card to set up client-authenticated SSL sessions, which you can then intercept and modify.

This blog post shows how you can easily view and modify your own, local traffic.  In order to complete this tutorial, you still need a valid eID card, and the corresponding PIN code. This article does not mean that there is a vulnerability in PKCS#11 or Belgian eID, it only shows that it is possible for research purposes.

We sometimes come across web applications that are only accessible if you have a valid Belgian eID card, which contains your public/private key. Of course, we still need to intercept the traffic with Burp, so we have to do some setting up. Because I always forget the proper way to do it, I decided to write it down and share it with the internets.

This guide was written for OS X, but I will add information for Windows & Kali too where it differs from OS X. This guide also assumes that you’ve correctly installed your eID card already, which is a whole challenge in itself…

The target

The Belgian Government has created a test page, which can be found here: http://www.test.eid.belgium.be/ . Once you click on the “Start Test”, you will be taken to the HTTPS version, which requires your client certificate. After entering your PIN code, you can view the result of the test:

The first thing to do is set your browser to use Burp as your proxy. This can easily be done by using FoxyProxy (FireFox)/SwitchyOmega (Chrome) or your system-wide proxy (shiver).

After setting up Burp, the site will not be able load your client certificate:

Configuring Burp

Setting up Burp is not too difficult. The main challenge is finding out the location of the PKCS#11 library file. I’ve listed the locations for OS X, Kali and Windows below.

1. Insert your eID card into the reader and restart Burp
2. Go to Project settings -> SSL Tab -> Override user options and click ‘Add’
3. Host: <empty> (or be specific, if you want)
   Certificate type: Hardware token (PKCS#11)
   
4. Supply the path of the correct library. Here are some locations:
   1. OS X
      /usr/local/lib/beid-pkcs11.bundle/Contents/MacOS/libbeidpkcs11.dylib
      (simlinked to /usr/local/lib/libbeidpkcs11.4.3.5.dylib in my case. There might be multiple versions there, if one doesn’t work, try another)
   2. Windows
      C:\Windows\System32\beid_ff_pkcs11.dll or
      C:\Windows\System32\beidpkcs11.dll
      or use the auto search function
   3. Kali
      /usr/local/lib/libbeidpkcs11.so
      (see “building from source” below)
5. If you get “Unable to load library – check file is correct and device is installed”, restart Burp with your eID already plugged in and working in the eID viewer.
6. Enter your PIN, click refresh and select the Authentication certificate:
   
7. Optionally, restart FireFox / Burp (this fixed an issue once)

Et voila, we can now use Burp to intercept the traffic:

Building from source on Kali (Rolling, 2017.3)

Installing the default debian pacakge doesn’t seem to work, so we’ll have to compile from source. You can do this by following the steps listed below.

Note that we’ve had Kali VMs crash on boot after installing the dependencies below and compiling the source. An easy solution is to take a snapshot, compile, and copy over the .so, or use the one we compiled (libbeidpkcs11.so).

> wget https://eid.belgium.be/sites/default/files/software/eid-mw-4.3.3-v4.3.3.tar.gz
> tar -xvf eid-mw-4.3.3-v4.3.3.tar.gz
> cd eid-mw-4.3.3-v4.3.3
> apt-get install pkg-config libpcsclite-dev libgtk-3-dev libxml2-dev libproxy-dev libssl-dev libcurl4-gnutls-dev
> ./configure && make && make install

After these steps, you can find the .so library at /usr/local/lib/libbeidpkcs11.so.

About the author

Jeroen Beckers is a mobile security expert working in the NVISO Cyber Resilience team. He also loves to program, both on high and low level stuff, and deep diving into the Android internals doesn’t scare him. You can find Jeroen on LinkedIn.

“We are happy to share our knowledge with the community and thus hope you enjoyed the content provided on this article! If you’re interested in receiving dedicated support to help you improve your cyber security capabilities, please don’t hesitate to have a look at the services NVISO offers! We can help you define a cyber security strategy / roadmap, but can also offer highly technical services such as security assessments, security monitoring, threat hunting & incident response (https://www.nviso.be). Want to learn similar techniques? Have a look at the SEC599 SANS class, which was co-authored by NVISO’s experts! (https://www.sans.org/course/defeating-advanced-adversaries-kill-chain-defenses)”