Security researchers have cracked the code for version 2 of the Waledac worm, used by attackers to build botnets, and found that it carries numerous credentials that enable it to bypass spam filters and security defenses.

"We found that the botmasters have a tremendous amount of stolen credentials," blogged Brett Stone-Gross, a developer and threat analyst at LastLine, a cybercrime intelligence and network monitoring firm.

For starters, the most recent version of Waledac -- the successor to the Storm worm -- contains 123,920 login credentials for FTP servers, through which Waledac's botmasters -- or rather, their automated attack toolkits, issue instructions to worm-infected (aka zombie) PCs.

Oftentimes, these instructions involve rerouting the browsers of infected machines to Web sites that serve malware or advertisements for discount pharmaceuticals. In January alone, LastLine saw the worm redirecting to nearly 10,000 compromised or malicious Web pages across 222 Web sites.

LastLine also discovered that Waledac contains 489,528 credentials for POP3 email accounts. "These credentials are known to be used for 'high-quality' spam campaigns," Stone-Gross said. "The technique abuses legitimate mail servers by authenticating as the victim through the SMTP-AUTH protocol to send spam messages. This method makes IP-based blacklist filtering considerably more difficult."

In December 2010, the previous version of Waledac went dark for about a week. After its hiatus, the updated version appeared, containing not only the built-in credentials, but also enhanced command-and-control capabilities, including improvements in the proprietary Ad Hoc Network Management Protocol (ANMP) the worm uses to communicate with a "bootstrap server" that issues its instructions.

Although Waledac has been relatively quiet recently, the worm still has cybercrime potential to spare. "The Waledac botnet remains just a shadow of its former self for now, but that's likely to change given the number of compromised accounts that the Waledac crew possesses," said Stone-Gross. As of January 2011, LastLine said that the Waledac botnet appeared to comprise about 12,000 infected machines.