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Security implications of WebRTC (part 3): Tracking users across domains

Identifying users on a website

There has been a lot of noise about WebRTC leaking IP addresses (e.g. for de-anonymizing users behind TOR or VPNs) in the past months. The IP addresses of internal interfaces might also be used to aid in browser fingerprinting.

Today, while working on our new AhoyRTC media engine, something different caught my eye. In the past, Google Chrome used to generate a new self-signed certificate for every WebRTC PeerConnection. But now (using Chrome 46, or maybe earlier as i did not check) it generates a self-signed certificate which is valid for one month and uses it for all PeerConnections of a particular domain.

This turns the fingerprint of the certificate into a unique tracking ID for a single user (at least for up to one month). The certificate fingerprint can be accessed from JavaScript by using the PeerConnection statistics api functions, it is not required to set up a full peer-to-peer connection (e.g. no packets will hit the network). No indication is given to the user.

// create a PeerConnection without using a STUN server
var pc = new webkitRTCPeerConnection(null);
  function createOfferSuccess(description) {
      function setLocalDescriptionSuccess() {
        // it takes a moment until the fingerprint shows up in the statistics
        setTimeout(function() {
          pc.getStats(function(stats) {
            var items = stats.result();
            items.forEach(function(item) {
              if (item.type === "googCertificate") {
                console.log("Your fingerprint is " +;
        }, 500);
      function setLocalDescriptionError(error) {
  function createOfferError(error) {
  { offerToReceiveAudio: true }

To test this go to and to

Each of the two links shows a different fingerprint, because they are on different domains ( and Closing the tab, restarting Chrome or even rebooting the computer does not change the fingerprints. :-)

How to track users across multiple domains?

This is pretty straight forward! Let your tracking code insert an iframe into every website you want to track. The iframe will always be loaded from one domain (in my example “” without “www”), resulting in one stable fingerprint.

The only challenge is to extract the fingerprint from the iframe. The browser’s cross origin policy prevents your website from accessing the iframe and also prevents the iframe from accessing the website it is embedded in.

But the point of the cross origin policy is not to prevent leaking information. When inserting the iframe into the website we can pass a (sort of random) unique identifier (a transaction id) to the iframe (as part of the url). Both the website and the iframe will use the same transaction id to load a tracking pixel from a remote server. This way the remote server can combine the information coming from two separate requests.

To test this go to and to Open the network tab of Chrome’s developer console and compare the urls of the requested “tracking.png”. They should contain the same fingerprint, now! FE:24:D9:41:F6:AF:31:72:58:DA:08:F2:55:FC:45:A5:0B:D4:C4:E1:C7:F5:27:0E:3B:DF:F4:3C:88:F9:0D:88
www-fingerprint FE:24:D9:41:F6:AF:31:72:58:DA:08:F2:55:FC:45:A5:0B:D4:C4:E1:C7:F5:27:0E:3B:DF:F4:3C:88:F9:0D:88


What used to be two different fingerprints are now one! To make sure that I am not cheating please check for cookies…

The good news and the bad news

Actually the good news is pretty bad, too. The incognito mode of Chrome does not persist the certificates. But to make it generate a new one you have to close ALL incognito tabs. Otherwise you can be tracked across multiple domains.

The bad new is that in normal browsing mode, only clearing your browser data will reset the certificates. Doing that when going to a different website every time is a bit tedious…


Because Google Chrome still supports SRTP key exchange via SDES (to be backwards compatible) it is even possible to retrieve the unique fingerprint without needing a remote server. The website and the iframe, it has inserted, can establish a bidirectional communication using the WebRTC data channel. This works although the iframe cannot send its SDP answer back to the website (but the website can derive the SDP answer from its own SDP offer).

To test it go to and to Both pages will show a JS alert with the same fingerprint. If you are still not convinced you can include the following into any other website:

 <script src=""></script>

Security implications of WebRTC (part 2): End to end encryption. Well, no… 3

The problem

Some time ago i was watching an episode of the VoIP Users Conference ( about the Jitsi  WebRTC video bridge. The video bridge is automagically maximizing the video of the currently active speaker without processing the audio of each participant. Instead it is utilizing a RTP header extension (RFC 6464) to find out about the audio level of each participant.

WebRTC media streams are encrypted with SRTP, the SRTP key exchange is performed end-to-end with DTLS-SRTP. This ensures that any kind of man-in-the-middle attack can be detected and the two WebRTC endpoints can be sure that nobody can spy on their conversation.

This is great! Except for the fact that SRTP only encrypts the payload part of RTP packets. The RTP header (and all RTP header extensions, like RFC 6464 audio levels) are NOT encrypted. Which means that anybody who can see your SRTP packets (e.g. your ISP or “some three letter agency recording the whole internet with a datacenter in Utah”) knows when you are speaking, when you are silent or even if you have muted your microphone). Considering what can be done with traditional telephony meta data alone, this is a bit scary.

Chrome is enabling the “ssrc-audio-level” RTP header extension by default (by inserting “a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level” into the SDP offer) for every call although Chrome does not use the data from received SRTP packets (__insert_conspiracy_theory_here__).

 See for yourself

To verify my crazy claims (remember, I am just somebody on the internet!) you just need to use one of the gazillion WebRTC demos (make sure you only share your microphone so it’s easier to find the srtp audio session) and capture the packets with wireshark. Then tell wireshark to decrypt those UDP packets as RTP and have a look:


The first 2 of the 4 marked bytes (the other 2 are just padding) are 0×10 (first 4 bit ID, second 4 bit length of the extension – 1) and 0xff (least significant 7bits are the audio level expressed in -dBov). The audio level in this example is -127 dBov, the audio level of a digitally muted source (I muted my microphone).

How to fix this?

There is a RFC for encrypting RTP header extensions (RFC 6904). Chrome should implement this or at least not enable RFC 6464 by default. I have created an issue for this on the WebRTC google code project (issue 3411).

Given the success I had with reporting my STUN gun (issue 2172 still open for over a year..), I am suggesting that WebRTC devs should fix it today with a bit of SDP mangling in javascript. Whenever you get a SDP description feed it through a function to remove the offending RTP header extension:

function processSdp(sdp_str) {
 var sdp = new Array();
 var lines = sdp_str.split("\r\n");
 for (var i = 0; i < lines.length; i++) {
   if (lines[i].indexOf("urn:ietf:params:rtp-hdrext:ssrc-audio-level") != -1) {
     /* drop it like it's hot */
   } else {
     /* keep the rest */
 return sdp.join("\r\n");