Updated Thursday, Nov. 4, 2010 at 10:17 a.m. EST 

Though Microsoft has added new protection capabilities to its Malicious Software Removal Tool (MSRT) to help organizations fight Zeus, its tool is ineffective at detecting the newest version of the insidious data-stealing malware, according to researchers at security firm Trusteer.

Last month, coinciding with the release of its monthly bundle of security fixes, Microsoft added Zeus detection and removal capabilities to the freely distributed MSRT.

But after testing the tool against hundreds of Zeus files, Trusteer researchers determined that it is able to detect and remove an old version of Zeus 46 percent of the time but is unable to protect against the latest version, Mickey Boodaei, CEO of Trusteer, told SCMagazineUS.com on Wednesday.

“What Microsoft did is a good first step, but in terms of effectiveness and sustainability against Zeus, we think there is a lot more work to do,” Boodaei said.

The most recent version of Zeus contains sophisticated capabilities to evade detection, Boodaei said.

Each time the data-stealing trojan is installed on a machine, its footprint changes, meaning all the filenames and registry keys associated with the malware are unique, he said. Even older versions of the malware contain less sophisticated capabilities that allow it to evade anti-virus detection most of the time.

“The good news is that MSRT has and will be able to kill approximately half of the Zeus population,” Boodaei wrote in a blog post Wednesday. “This detection rate is very respectable since most anti-virus solutions, if not all, have a much lower detection rate.”

One week after adding the protection capabilities, Microsoft said the tool cleaned Zeus, also known as Zbot, 281,491 times from 274,873 computers, making it the most prevalent family of malware removed from machines.

However, according to Trusteer, the tool is not able to prevent Zeus-related fraud in a majority of cases because it does not operate in real time.

Financial fraud typically occurs within hours or days after a computer is infected with Zeus because sensitive information is immediately transmitted to criminals, Boodaei said. Attackers have a “window of opportunity” between the time a computer is infected with Zeus and the next MSRT scan to steal money from a victim's bank account.

“The fraudsters are very quick to act,” Boodaei said. “You get infected, they analyze your computer, the next time you go onto your bank account the fraud occurs.”

The computer security industry as a whole is lagging behind the Zeus authors, he added.

“We need to find more ways of removing the dependency on signatures and the need to know what Zeus looks like and how it behaves in order to block it,” he said. “That's the only way of successfully fighting Zeus in this dynamic world the fraudsters have created.”

Jerry Bryant, Group Manager, Response Communications, Microsoft told SCMagazineUS.com in an email Thursday that the MSRT should not be considered a replacement for real-time protection.

“Because new variants of many families are identified after MSRT is released in any given month, it is the standard practice for Microsoft to add new variants of threat families previously added on a monthly basis to each new release of the Malicious Software Removal Tool,” Bryant said.

In addition to using real-time protection,  the software giant encourages customers to follow best practices such as enabling a firewall, applying all software updates and installing anti-virus and anti-spyware software.