Feeds

Major IE8 flaw makes 'safe' sites unsafe

Microsoft's XSS buster busted

Securing Web Applications Made Simple and Scalable

Exclusive The latest version of Microsoft's Internet Explorer browser contains a bug that can enable serious security attacks against websites that are otherwise safe.

The flaw in IE 8 can be exploited to introduce XSS, or cross-site scripting, errors on webpages that are otherwise safe, according to two Register sources, who discussed the bug on the condition they not be identified. Microsoft was notified of the vulnerability a few months ago, they said.

Ironically, the flaw resides in a protection added by Microsoft developers to IE 8 that's designed to prevent XSS attacks against sites. The feature works by rewriting vulnerable pages using a technique known as output encoding so that harmful characters and values are replaced with safer ones. A Google spokesman confirmed there is a "significant flaw" in the IE 8 feature but declined to provide specifics.

It's not clear how the protections can cause XSS vulnerabilities in websites that are otherwise safe. Michael Coates - a senior application security engineer at Aspect Security who has closely studied the feature but was unaware of the vulnerability - speculates it may be possible to cause IE 8 to rewrite pages in such a way that the new values trigger an attack on a clean site.

"If the attacker can figure out a flaw in the way IE 8 is actually doing that output encoding and then create a specific string the attacker will know will be transformed into an actual attack, they could use that to input a value ... that actually results in an attack firing on the page," he said. "This could be a way to introduce an attack into a page that didn't have a vulnerability otherwise."

XSS attacks are a way of manipulating a site's URL to inject malicious code or content into a trusted webpage. Many security watchers have come to view the IE 8 protections as Microsoft's answer to NoScript, a popular extension that helps prevent XSS and other types of attacks against users of the Firefox browser.

Late on Thursday afternoon, Microsoft told The Register: "Microsoft is investigating new public claims of a vulnerability in Internet Explorer. We're currently unaware of any attacks trying to use the claimed vulnerability or of customer impact."

Once its investigation is finished, the company will "take appropriate action," including issuing a patch or guidance on how users can protect themselves against exploits.

When Microsoft introduced the protections, it also created a way for webmasters to override the feature (by adding the response header "X-XSS-Protection: 0"). A review of the top 50 most visited websites shows that only web properties owned by Google have actually opted to do so. The small number of sites blocking the protection calls into question how widespread the vulnerability is.

Asked why Google was forgoing the protection, a company spokesman wrote in an email:

"We're aware of a significant flaw affecting the XSS Filter in IE8, and we've taken steps to help protect our users by disabling the mechanism on our properties until a fix has been released." He didn't elaborate.

In addition to potentially introducing serious vulnerabilities into webpages, the XSS protections can bring other undesirable results. That's because its engine frequently flags perfectly acceptable characters as potentially harmful. An examples of such a false positive is here.

David Ross, a senior software security engineer for Microsoft, has saiddevelopers designing the feature aimed to strike strike a pragmatic balance between protecting users and not breaking the web.

"We needed to find a way to make the filtering automatic and painless and thus provide maximum benefit to users," he wrote. "In summary, the XSS Filter will prove its worth by raising the bar and mitigating the types of XSS most commonly found across the web today, by default;, for users of Internet Explorer 8." ®

Mobile application security vulnerability report

More from The Register

next story
NEW, SINISTER web tracking tech fingerprints your computer by making it draw
Have you been on YouPorn lately, perhaps? White House website?
HIDDEN packet sniffer spy tech in MILLIONS of iPhones, iPads – expert
Don't panic though – Apple's backdoor is not wide open to all, guru tells us
LibreSSL RNG bug fix: What's all the forking fuss about, ask devs
Blow to bit-spitter 'tis but a flesh wound, claim team
Black Hat anti-Tor talk smashed by lawyers' wrecking ball
Unmasking hidden users is too hot for Carnegie-Mellon
Manic malware Mayhem spreads through Linux, FreeBSD web servers
And how Google could cripple infection rate in a second
Own a Cisco modem or wireless gateway? It might be owned by someone else, too
Remote code exec in HTTP server hands kit to bad guys
British data cops: We need greater powers and more money
You want data butt kicking, we need bigger boots - ICO
prev story

Whitepapers

Reducing security risks from open source software
Follow a few strategies and your organization can gain the full benefits of open source and the cloud without compromising the security of your applications.
Consolidation: The Foundation for IT Business Transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.
Application security programs and practises
Follow a few strategies and your organization can gain the full benefits of open source and the cloud without compromising the security of your applications.
Boost IT visibility and business value
How building a great service catalog relieves pressure points and demonstrates the value of IT service management.
Consolidation: the foundation for IT and business transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.