Friday, May 21, 2010 Preview Add-on Leaks User Activity; Referer Header Considered Harmful

I've been reading a book called "Inside Intel" by Tim Jackson that reports the history of the chip giant up to 1997. At the end of the book, Intel is dealing with the famous flaw in the Pentium's division circuitry. Jackson observes that Intel's big mistake in dealing with the bug was to deal with it as a minor technical issue rather than as the major marketing issue it really was. If Intel's management had promptly addressed consumer concerns by offering to replace chips for any customer that wanted it rather than dismissing the problem as the inconsequential bug it actually was, it could have avoided 90% of the expense it actually incurred. The public doesn't want to deal with arcane technology bugs; they want to know who to trust.

This week FaceBook and MySpace had to deal with the consequences of obscure bugs that leaked personal subscriber information to advertisers. The Wall Street Journal reported that because Facebook and MySpace put user handles in URLs on their sites, these user handles, which can very often be traced back to a user identity, leaked to advertisers via the referer headers sent by browser software.

Reaction on one technology blog reminded me of Intel's missteps. Marshall Kirkpatrick, on ReadWriteWeb, called the the Journal's article "a jaw dropping move of bizarreness", going on to explain that passing referrer information was "just how the Internet works" and accusing the Journal of "anti-technology fear-mongering".

When a web browser requests a file from a website, it sends a bunch of extra information via http headers. One header gives the address of the file, which might be a web page, an image, or a script file. Other headers give the name of the software being used, the language and character sets supported by the browser. The Referer header (yes, that's how it's spelled, blame the RFC for getting the spelling wrong) reports the address of the page that requested or linked to the file. If the request is made to an advertiser's site, the Referer URL identifies the page that the user is looking at. When that page has an address that include private information, the private stuff can leak.

The controversy spurred me to take a look at some library websites to see what sort of data they might leak using referer headers. I used the very handy Firefox add-on called "Live HTTP Headers". I was astounded to see that a well known book database website seemed to be reporting the books I was browsing to, the URL shortening service! In another header, was also getting an identifying cookie. I went to another website, and found the exact same thing. This set off some alarm bells.

I soon realized that a report of EVERY web page I visit is being sent to The culprit turned out to be's Preview add-on for Firefox. It turns out that for every web page I visit, this line of javascript is executed:
This request for a CSS stylesheet has the side effect of causing Firefox to transmit to the address for each and every web page I visit in a referer header.

It's ironic. My last post described how URL shortening services can be abused for evil, but my point was that these abuses were a burden for the services, not that the services were abusive themselves. In fact, has probably done more than any shortening service to combat abuse and the Preview add-on is part of that anti-abuse effort. With Preview installed, users can safely check what's behind any of the short URLs they encounter by hovering over the link in question.

The privacy leak in Preview is almost certainly an unintentional product of sloppy coding and deficient testing rather than an effort to spy on the 100,000 users who have installed the add-on. Nonetheless, it's a horrific privacy leak. There are other add-ons that intentionally leak private information, but typically they disclose their activity as a natural part of the add-on's functionality. One example would be GetGlue, which I've written about, and even preview cannot help but leak some info when it's doing what it's supposed to do (expand and preview shortened URLs).

I'm sure that will fix this bug quickly; their support was amazingly fast when I reported another issue. But a larger question remains. How do we make sure that the services we use everyday aren't leaking our info all over the place? The most widely deployed services- Google, Amazon, Facebook, etc. all deserve a  higher level of scrutiny because of the quantity of data at their fingertips. All the privacy policies in the world aren't worth a dime if web sites can't be held accountable for the effects of sloppy coding. It's high time for popular sites to submit to strict third-party privacy auditing, and for web users to demand it. It doesn't matter whether any advertisers actually used the personal information that Facebook sent them; what matters is whether users can trust Facebook.

It's also time for the internet technology community to recognize that referer headers are as dangerous to privacy as they are to spelling. They should be abolished. Browser software should stop sending them. The referer header was originally devised to help dispersed server admins fix and control broken links. Today, the referer header is used for "analytics", which is a polite word for "spying". The collection of referer headers helps web sites to "improve their service", but you could say the same of informants and totalitarian governments.

The pipe is rusty- that's why it leaks. We need to fix it.


  1. Danny Sullivan has an interesting comment on the referer issue and "the Death of Web Analytics". He also includes suggestions on what you can do now.

  2. I found your post from the link you put in your FF add-on review, though I'll bet your referrer logs have already told you as much. Thanks for your research, and for spreading the word.

    I am uninstalling the preview add-on as soon as I finish this comment. Hopefully will fix the bug soon.

  3. Thanks for the info, Eric. I just found this posting because I realized that if you install this HTTPS everywhere addon in FireFox on a Windows vista 64-bit pc, the preview breaks. This is an addon created by EFF -, which changes your log in automatically to the https version, if available. Once I began to log in to, the preview quit working.

    My addon works at work, however, on firefox with https-everywhere, but on a windows xp computer. I started looking into the issue and found this posting. I am disabling preview for now until this is addressed. It does not appear to have been fixed so far.

  4. bizzarro...when I try to view the privacy Policy via a link from Mozilla's add-on download page, I am pointed to a page that when you click on int generates a warning page that you are about to go to!

  5. Heather, I saw that back in July. At first I thought the page had been hacked, but after a bit of nosing around, I concluded it was just neglect.

  6. Thanks, and it's also a useless link because you cannot link to the privacy info