Up to now, Web-borne worms couldn't be easily spread from host to host, and they have usually been easy to detect. But two researchers have written a proof-of-concept of a more deadly and persistent Web worm that can run in both the client's browser and on a Web server -- and evades signature-based scans.

The so-called "hybrid Web worm" has more staying power than previous Web worms -- such as the infamous Samy worm that infected MySpace -- that were restricted to specific hosts and domains, and typically only exploited a single vulnerability.

"As [researcher] HD Moore put it, [these older Web worms] were like a smallpox epidemic on a small island," says Billy Hoffman, lead researcher for SPI Dynamics' Labs and co-author of the hybrid worm proof-of-concept. "Samy couldn't leave MySpace."

At the Black Hat conference next month, Hoffman and fellow researcher John Terrill will demonstrate their wily, next-generation Web worm in a session entitled "The Little Hybrid Web Worm that Could." Their new worm mutates to evade signature detection -- it can even use vulnerability information from sites like Secunia to infect other servers and browsers.

"We're going to demonstrate how it can pull new vulnerability information in the wild and start using it" to spread, Hoffman says.

Hoffman and Terrill, who is executive vice president of Enterprise Management Technology, have only built out the client side of the attack, and they won't release the POC, because they don't want the potentially lethal code to fall into the wrong hands.

"We didn't want to build something truly evil," Hoffman says. "This is a proof-of-concept on the client, but it would work on the server, too."

Although current Web worm attacks are easily detectable and restricted to a single host, they are still more sophisticated than predecessor email-based attacks, because they can use JavaScript and Flash to run across disparate operating systems.

But the hybrid Web worm is even more intelligent. When it's injected into a Web server, it can write a JavaScript version of itself into Web pages, so that when a user visits those sites, it infects his or her browser. "We came up with a way for it to update its vulnerabilities while in the wild, " Hoffman says.

It grabs, and then exploits, new vulnerabilities reported on sites such as Secunia and then continues propagating. "Secunia's [vulnerability data] is machine-consumable, so it could pick up new vulnerabilities while it's in the wild" and spread via those bugs as well, Hoffman says.

This polymorphic feature of the next-generation worm is a natural evolution for the malware -- and it will be tough for enterprises to defend against, Hoffman says. "Trying to use signatures on these worms isn't going to work."

One hope for detection is to study the malware's behavior for worm-like characteristics, Hoffman says. Even though the hybrid worm is mutating, it still behaves like any other worm: "We are trying to think about what a worm does, versus its source code."

He says he and Terrill also will discuss at Black Hat ways to attempt to "signature" a mutating Web worm, as well as detecting infected machines based on the traffic they generate when they look for and infect other machines.

— Kelly Jackson Higgins, Senior Editor, Dark Reading