If you are an Opera user who hasn't changed the browser homepage or has visited Opera's Portal homepage (portal.opera.com) on Wednesday, you might want to check you computer for malware.

According to Bitdefender, the website in question has been redirecting visitors to a page hosting the infamous Blackhole exploit kit for at least a few hours.

But how did that happen?

The portal would load a malicious obfuscated script - most likely via a third-party ad - and it would insert into the page an iframe that loaded malicious content from an external source, and visitors were automatically redirected to another page.

"This malicious page harbors the BlackHole exploit kit (we got served with the sample via a PDF file rigged with the CVE-2010-0188 exploit) that will infect the unlucky user with a freshly-compiled variant of ZBot, detected by Bitdefender as Trojan.Zbot.HXT," shared the researchers.

"The ZBot malware is on a server in Russia which, most probably, has also fallen victim to a hacking attack, allowing unauthorized access via FTP."

The company has luckily offered a simple way of checking whether you have been saddled with this specific malware - just visit its Quickscan website and speedily test your computer.