Security researchers have warned that a rash of malware for Mac OS X systems is now being used to run a botnet.

The Trojan malware was first spotted in January, and had been bundled into pirated copies of Mac OS software.

Researchers noted at the time that the payload included tools which could allow an attacker to remotely take control of an infected system. It now appears as if those components are being put to use.

Symantec researchers Mario Barcena and Alfredo Pesoli said that systems infected by the Trojan have been used in at least one denial-of-service attack. Other users are also reporting that their systems are displaying activity caused by the malware.

News of the botnet marks what experts have warned is a small but growing crop of malware targeting OS X systems.

"Quite frankly there is no functionality in this 'bot' that we have not seen before," said Dave Marcus, head of research and communications at McAfee Avert Labs, in a blog post. "The only thing of concern is that it affects the Mac platform, which certainly is fresh territory."

Marcus recommended that Mac users avoid installing pirated software applications and run anti-virus software to guard against infection.