Eternal Rogue Code Distribution Possible

Thursday, October 6, 2011 @ 02:10 PM gHale


Websites that accidentally distribute rogue code could find it harder to undo the damage if attackers exploit widespread browser support for HTML5 local storage and an increasing tendency for heavy users of Web apps never to close their browser.

If browsers don’t provide a mechanism for websites to securely recover from certain cross-site scripting attacks, the attacks could become invincible and the site at the origin of the attack remain compromised indefinitely, warned vulnerability researcher and Google security engineer Michal Zalewski.

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There are limitations in the scope of client-side programming languages such as JavaScript within browsers by a critical security concept known as the same-origin policy. This prevents scripts running on certain Web pages from interfering with websites opened in separate tabs or windows.

In the case of cross-site scripting (XSS), attackers manage to insert rogue JavaScript code in targeted pages, where it then executes in the context of their origin, defined by the domain, the protocol and the port number.

JavaScript is very powerful and hackers use it in most types of Web-based attacks. Despite this, browsers don’t currently provide a mechanism that can invalidate such code, something that would provide compromised websites with a way to request a clean slate once they had resolved the problem.

A normal response to XSS attacks is to patch the vulnerability, invalidate session cookies so everyone must re-authenticate, and optionally force a password change. But this is not enough, because once compromised a Web origin can stay tainted indefinitely, Zalewski said.

“At the very minimum, the attacker is in full control for as long as the user keeps the once-affected website open in any browser window; with the advent of portable computers, it is not uncommon for users to keep a single commonly used website open for weeks,” he said. “During that period, there is nothing the legitimate owner of the site can do — and in fact, there is no robust way to gauge if the infection is still going on.”

In essence, there is no way for websites to ensure their users no longer feel the affects of an XSS attack. Still, one would think that such an attack would stop at some point without the website’s intervention, such as when closing the tab or the browser, but as it turns out, that’s not necessarily the case.

There are several methods that attackers can use to extend their hold on a compromised origin pretty much indefinitely, Zalewski said.

One method would involve inserting rogue JavaScript code into a popular webmail service or social networking site. This code could run in the background with the ability to obtain window handles for every new tab opened as a result of clicking on a link from the compromised page. If the newly opened pages had the same origin as the original page, or loaded a piece of code from the same domain, the rogue code could copy itself over to them and the process could begin again.

If Facebook was the target of such an exploit, then given the way users constantly open new pages from the site, or external websites carrying Facebook Like buttons, the compromise could go on for as long as one of those pages remained open.

Shutting down the browser should in theory end any such attack. However, there are now ways of overcoming this too, using technologies such as HTML5 local storage or Web Workers, a special API for running JavaScript code in the background.



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