The number of affected accounts identified by Prevx, a maker of computer security software, rose from 66,000 last Wednesday to 74,000 two days later.
According to Jacques Erasmus, director of research at Prevx, the Trojan is highly infectious.
"We rate this infection as critical," he said in a blog post on Monday. "The infection has a 'China Syndrome' potential. It includes a cyclic infection which leverages infected PCs to programmatically modify hi-volume Web sites to infect additional users who become part of the cycle. More users leads to more discovery of Web site admin credentials which in turn leads to more Web sites being modified to serve the infection which leads to more infected users."
If a vulnerability is found and successfully exploited, malware is installed -- a variant of the Zeus family. It scans compromised machines for FTP credentials and then posts those credentials to a Web server in the Cayman Islands. It also enlists the victim's computer to further spread the infection.
Uriel Maimon, senior researcher at EMC's RSA, characterizes Zeus as particularly nefarious. "The Zeus Trojan has many startling capabilities," he explained in a blog post last year. "In addition to listening in on the submission of forms in the browser, the Trojan also has advanced capabilities, for instance the ability to take screenshots of a victim's machine, or control it remotely, or add additional pages to a Web site and monitor it, or steal passwords that have been stored by popular programs."
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