Major Internet Explorer vulnerability could lead to convincing phishing attacks

A security researcher has uncovered what appears to be a serious security flaw in Internet Explorer that could be exploited by malicious hackers to launch convincing phishing attacks and inject malicious code into users’ browsers as they visit websites.

David Leo published details of the flaw, including a link to a proof-of-concept exploit that demonstrates the attack working against the popular Daily Mail website, this weekend on the Full Disclosure mailing list.

The bug, which works on Internet Explorer 11 running Windows 7 or Wondows 8.1, is a universal cross-site scripting (XSS) vulnerability, and bypasses what is known as the Same-Origin Policy.

Same-Origin Policy (SOP) is an important cornerstone of web application security, and is how browsers limit the functionality of scripts running in users’ web browsers. Specifically, is intended to prevent a script running on a webpage one website to read or write from anything other than pages on the same website.

However, Leo appears to have found a way to waltz past the restriction in Internet Explorer.

In Leo’s example exploit page, users running Internet Explorer are invited to click a link opening the Daily Mail website – which opens normally but after seven seconds is replaced with content reading “Hacked by Deusen”.

It’s worth noting that the Daily Mail’s website has not itself been hacked, but the content displayed in the user’s browser has been surreptitiously altered.

What’s so convincing about such an attack, of course, is that the URL displayed inside the browser’s address bar does not change during the attack – meaning it would be possible for an attacker to easily embed code (such as a fake login page) or run malicious code from an external page without the user being aware that anything suspicious has happened.

Joey Fowler, a senior security engineer at Tumblr, confirmed the seriousness of the vulnerability in a later posting on the Full Disclosure mailing list:

As long as the page(s) being framed don’t contain X-Frame-Options headers (with `deny` or `same-origin` values), it executes successfully. Pending the payload being injected, most Content Security Policies are also bypassed (by injecting HTML instead of JavaScript, that is).

It looks like, through this method, all viable XSS tactics are open!

According to The Register, the flaw has been reported to Microsoft, and it is hoped that a security patch will be forthcoming – although no timeline for that has yet been provided.

The public disclosure of such a serious security flaw without giving Microsoft a chance to fix it in advance is sure to once again raise the question of whether researchers are acting responsibly – or whether their actions could be putting the majority of Internet Explorers at risk.