Daily Archives: 2008-05-18

Twisty little passages, all alike

Last month, on the 4th April, I published a document describing how the Phorm system worked and blogged about what I thought of the scheme. The document had been run past Phorm’s technical people to ensure it was correct, but — it turns out — there were still a handful of errors in it. A number of helpful people pointed out that I’d misdescribed third-party cookies (which didn’t matter much because Phorm specifically uses first-party cookies), and I’d managed to reference RFC2695 rather than RFC2965 !

In my original document, I’d waved my hands a little bit about how the system worked if people had blocked cookies for specific domains, and so I swapped some more email with Phorm to better understand, and then published a revised version on 23rd April — so that the correct information would be available to accompany FIPR’s press release and paper on the various laws that the Phorm system breaks. However, there was one final thing that wasn’t dealt with by press time, and that’s now been explained to me….

The Phorm system does some of its tracking magic by redirecting browser requests using HTTP 307 responses. When this was first explained to me at the meeting with Phorm there were two redirections (a scan of my notes is here), but having thought about this for a while, I asked for it to be explained to me again later on, and it turned out that I had previously been misled, and that there were in fact three redirections (here’s my notes of this part of the meeting).

It now turns out, following my further emails with Phorm, that there are in fact FOUR redirections occurring! This is not because my notes are rubbish — but because Phorm have managed to recall more of the detail of their own system!

For full details of how I understand the system works (at least until some more detail comes to light), see the latest version of my explanatory document, but to give you a flavour of it, consider an example visit to www.cnn.com:

  • The user wants to visit www.cnn.com, but their request does not contain a cookie (for www.cnn.com) with a Phorm unique identifier within it. They are redirected (ONE) by the Phorm system to www.webwise.net.
  • The user visits webwise.net by following the redirection. If they do not have a Phorm identifier cookie, then they will be issued with a new identifier and redirected (TWO) elsewhere on webwise.net.
  • The user visits webwise.net for the second time. If they still don’t have a Phorm identifier cookie then their IP address is marked as wishing to opt-out and they will be redirected to www.cnn.com and they won’t be redirected again for at least 30 minutes. If they do have a cookie (or if they had one at the previous stage) they are redirected (THREE) to a special URL within www.cnn.com.
  • The user visits the special URL, which the Phorm system redirects to a fake version of www.cnn.com that sets a www.cnn.com cookie with their Phorm identifier in it, and redirects (FOUR) them to the URL they wanted to visit all along.

For the moment, this appears to be the grand total; there can be up to four redirections, and it is deducible from this description what happens if you refuse (or delete) cookies in the webwise.net and www.cnn.com domains. It is also apparent that if you resolve webwise.net to that you’ll never get past the first redirection; and you will need to rely on the Phorm system spotting these repeated failures and turning off redirection for your IP address.

direct adjective: Straightforward in manner or conduct; upright, honest.

indirect adjective: Mechanism by which Phorm fools your system into accepting tracking cookies from third-party websites, even when those websites promise never to track you!