Firefox engineers are busy trying to fix a bug that prevents the browser from flagging Web addresses that have been disguised.

Disguising or obfuscating a Web address (URL) is a tactic employed by malware distributors to increase the likelihood of enticing potential victims to malicious Web sites.

Modern browsers commonly attempt to see through obfuscation techniques to identify unsafe Web addresses. But Firefox misses obfuscated URLs inside of iFrames, blocks of HTML code that add content to a Web page from a third party's server.

"[A] bug has been noticed in all versions of Firefox [that] fails to generate an alert when an obfuscated URL is being placed in iFrames," noted security company Armorize in a blog post on Monday. "In certain cases, [the vulnerability] can be used effectively in spreading malware and stealing sensitive information."

Armorize has posted a generic proof-of-concept exploit to demonstrate the flaw.

URL obfuscation and the misuse of iFrames are not new techniques, but they can nevertheless be combined effectively to dupe potential victims, observes Kaspersky Lab's security researcher Dennis Fisher in a blog post.

"Web-based attacks have been employing various forms of URL obfuscation for years now, and iFrames are a favorite of attackers because of their ability to perform malicious actions in the background of a victim's Web session," he wrote. "The new bug in Firefox could allow an attacker to combine these two techniques to prevent the browser from warning the victim that a URL has been modified, removing a key protection mechanism from the equation."

Mozilla's bug tracking system, Bugzilla, has an entry for the vulnerability, 570658, which indicates that a fix is underway.

Johnathan Nightingale, Mozilla's director of Firefox development, downplayed the significance of the issue.

"The concern expressed in the bug is that a page could be constructed with an embedded iFrame that uses a confusing URL," he said in an e-mailed statement. "Most users don't look at the HTML source of the pages they are loading, which is the only way you'd encounter this URL. We do not anticipate this bug would cause user confusion or deception. Firefox ships with built-in phishing and malware protection that warns users if they are attempting to visit a dangerous URL."