Wednesday, September 23, 2009

XSS Is Alive and Well

Not all that long ago, I had an interview where I was asked what cross site scripting was. Now, the thing is, I know very well what it is, and in fact, while I was working for my former employer, I wrote a white paper on the subject that was very widely used by their support and systems engineering personnel.

But have you ever been in the position of knowing something, yet when someone asks you about it, you just go cold? That's what happened to me in the interview. I stammered out a reply that was utterly wrong, and I knew it was utterly wrong, and I've been kicking myself ever since.

In the meantime, on a technical email list, one of my colleagues suggested that cross site scripting -- or XSS -- is no longer that much of an issue (because everyone knows about it and has taken precautions). That position is naive at best, though it's true that XSS is no longer the big deal it was a couple of years ago. However, it's still very much alive and well as a security vulnerability.

One of the problems in understanding XSS, or cross site scripting, is that the term itself is confusing. Originally it meant that a malicious web site could load another web site into a frame or a window, and then use scripting -- usually JavaScript -- to read and/or write data on that site (which is actually close to what I told my interviewer). However, later on, the term changed to mean "code injection" of scripting into a web page.

There are different kinds of XSS vulnerabilities, but the most typical type is where a user is enticed to click on or otherwise activate a URL that includes scripting language. This isn't that hard to do, because often users, especially those who are less technically sophisticated, don't look at where the URL is actually directing them to when they click on it.

Further, and unbeknownst to many users, it's possible to encode an object on a web page with a malicious URL such that just by viewing the page, the user is activating that URL. There's no way for the user to tell what's happened, and the only way to prevent it is to lock down the user's web browser such that it will not execute any scripts it finds on a web page. The problem with this is that locking down the browser to this degree will cause media-rich web sites to malfunction (from the user's point of view). In other words, as has always been the case, security is often sacrificed for ease of use.

XSS vulnerabilities have been exploited since the advent of the World Wide Web, but XSS became a really hot topic in 2005, which is when my former employer asked me to write the white paper about how one of their products addressed the issue. Vulnerability scanning for XSS was all the rage, and web site developers were scrambling to fix their html and scripting such that code injection could no longer work. Gradually, things calmed down to the point where my colleague could declare on a mailing list full of security geeks that it was no longer an issue. Too bad he was wrong.

The other day, one of my friends commented on a LiveJournal™ post I'd made, to the effect that it appeared my post had been hacked. He directed me to a news article on LJ that I'd missed: . You can read the article for yourself, but basically what happened is that someone had managed to infect a Flash™ file with a malicious URL. Anybody who viewed the file would have their latest (at the time) LJ post altered: the infected file would be inserted, any tags or other extra info would be deleted, and (usually) the post's security level (i.e. public/friends only/private, etc.) would be altered.

Sure enough, my post was "infected". My tags and location were removed, a formerly "friends only" post (the default for my journal) was now public, and the infected media had been inserted. However, the site already knew about the problem, and so it had turned off media embedding so that no further users would be affected, and issued a bulletin explaining what had happened, so all I saw were the "boxes" mentioned in the news article, not the infected media. At this point, I have no idea what the media looked like or who I "caught" the infection from, but as it's been contained and fixed (and my post is now "friends only" again), I'm only mildly curious.

The LiveJournal™ problem was caught and mitigated quickly, and while certainly there was a breach of privacy (secure entries becoming public, email addresses being mined), the effects were relatively minor. But it should be pretty obvious that XSS is far from "no longer an issue", given what happened.

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