Seculert CTO Aviv Raff reported finding the evolved attack, codenamed Geodo, in a blog post, warning that the new malware has advanced self-spreading powers.
The attacks work using 50,000 stolen credentials to create and send automated emails with malicious attachments, which when clicked infect the victim machine and connect it to the criminal botnet.
"We were able to determine that the second piece of malware (the worm) is provided with approximately 50,000 stolen Simple Mail Transfer Protocol (SMTP) account credentials including the related SMTP servers to connect to. The bot then uses these credentials to target mostly German accounts by impersonating legitimate email," read the post.
"The command and control (C&C) provides the malware with a batch of 20 targeted email addresses. The malware is also given a from address, subject line and email body text unique to this particular batch of emails. Once the malware has run through the batch, it is provided with a new batch of 20 emails."
Cridex is a data-stealing worm that was first discovered in 2012. Raff said it is unclear where the credentials used in the new campaign came from, though it is likely they were stolen during attacks using the older Cridex version.
"There is no definitive information on where the 50,000 stolen credentials came from, but Cridex is the suspected culprit," he said.
"As a data stealer, Geodo can compromise the intellectual property of a corporation, putting its business and reputation at risk. This new email worm capability displayed by Geodo serves to further emphasise the growing threat of advanced malware to today's enterprises."
Cridex is one of many old attack tools to receive a technical upgrade in recent weeks. Researchers at F-Secure uncovered a fresh BlackEnergy hack campaign believed to be targeting European governments with a wave of spear-phishing emails masquerading as IT alerts in June.