Cyber criminals are using sophisticated malware in compromising thousands of Unix-based servers to spew spam and redirect a half million Web users to malicious content per day, a security firm reported. Dubbed Operation Windigo, the attack has been ongoing for more than two and a half years and has compromised as many as 25,000 servers at one time, anti-virus vendor ESET said Tuesday. Systems infected with the backdoor Trojan are used in stealing credentials, redirecting Web traffic to malicious content and sending as many as 35 million spam messages a day.