A new Java zero-day security vulnerability is already being actively exploited to compromise PCs. The best way to defend against the attacks is to disable any Java browser plugins on your systems.

The offending bug is present in fully patched and up-to-date installations of the Java platform, now overseen by database giant Oracle, according to Jaime Blasco, head of labs at security tools firm AlienVault.

"The exploit is the same as the zero-day vulnerabilities we have been seeing in the past year in IE, Java and Flash," Blasco warned.

"The hacker can virtually own your computer if you visit a malicious link thanks to this new vulnerability. At the moment, there is no patch for this vulnerability, so the only way to protect yourself is by disabling Java."

The exploit targets Java 7 update 10 and prior versions. No fix is available and early indications suggest that exploitation is widespread. Brian Krebs reckons the exploit has found its way into crimeware toolkits, such as the Blackhole Exploit Kit, which will use the hole to infect victims with software nasties.

Java vulnerabilities were abused by the infamous Flashback Trojan, creating the first botnet on Mac OS X machines in the process last year. In the years before that attacks on Java and Adobe applications have eclipsed browser bugs as hackers' favourite way into a system.

In all but a limited number of cases Java support in web browsers is not mandatory for home users, unless required by a banking website or similar, so disabling plugins even as a temporary measure is a good idea. Businesses, on the other hand, that rely on Java for particular applications are not so fortunate.

While waiting for a patch from Oracle to plug the gaping hole, instructions on how to turn off Java in browsers can be found in this blog post by Graham Cluley of Sophos. ®