New Worm Infects Through Explorer Flaw

The worm uses an HTML email that exploits the flaw in browsers to cause its dangerous executable file to silently run without the user clicking on it.

The email messages that carry Bugbear.e are blank, use fake "from" addresses and can have one of many subject lines, including "Click on this!", "25 merchants and rising” and "15 FREE Bonus!" It carries an attachment with a name that's randomly chosen from a file found on the infected computer and has either a .zip or .htm ending. Clicking on the attachment also will cause infection by the virus.

As of Wednesday, there is no available fix or "patch" for Bugbear.e, which first appeared on Monday. But antivirus companies have rolled out software updates that can block Bugbear.e and other variants that also have emerged.

The virus isn't prevalent on the Internet, though its auto-execution feature could help it gain ground, according to Network Associates Inc.'s virus-response center, which also warned of a medium-risk virus known as "Netsky.s," which first emerged Sunday.

Such attacks are somewhat common in Trojan horses but worms like Bugbear.e unnerve security experts because large numbers of computers could be vulnerable to attack and quick defenses would be harder to come by.

Bugbear.e's use of a flaw with no available patch illustrates how the gap between the knowledge of a vulnerability and the release of malicious code that brings us ever closer to a zero-day network worm, an attack using a flaw that security experts don't yet know about.

Microsoft wasn't immediately able to comment on the flaw or when a fix might be available, but its support site, support.Microsoft.com, has additional information available.

The flaw that Bugbear.e exploits was disclosed in February and has since been used by several Trojan horses, which are dropped onto PCs by malicious websites. The virus essentially advances the delivery of a Trojan by using email to push PC users into viewing malicious web content.

Bugbear.e finds sensitive personal information and sends it to the attacker, including cookies, text from open windows and data captured by a program that logs keystrokes to grab passwords and credit card numbers.