Over half-a-million Apple Mac users have been infected with a variation of the Flashback Trojan malware, according to a Russian anti-virus website.
The Dr. Web site claims the Trojan has exploited a security hole within Java in order to infiltrate 600,000 computers running Mac OS X around the world.
The Flashback Trojan 'botnet' can steal usernames and passwords by monitoring your web browsing activity.
According to ZDnet, Apple issued a security patch for Java on Wednesday, but Dr. Web says the number of computers infected is continuing to rise.
The site says 12.8 per cent of the infiltrated machines are based in the UK, whereas 56 per cent of affected users are located in the US, including almost 250 machines in Apple's home town of Cupertino.
Apple has posted the Java update on its support website, which nullifies the threat, but if you think your Mac may already have fallen victim to Flashback, then F-Secure has posted removal instructions.