Microsoft rushed to release a patch for Internet Explorer on Tuesday after hackers were reportedly exploiting a flaw in the web browser that could download viruses to a user’s machine simply by visiting the wrong website.
Researchers at the security vendor Qualys claimed the vulnerability "is actively being exploited in the wild", and urged users to patch their machines to defend themselves against infection.
"Now that the vulnerability is disclosed we expect the attack code to spread widely and get integrated into exploit kits and attack frameworks," said Wolfgang Kandek, chief technology officer at Qualys.
The attackers were found using a wide variety of methods to take advantage of the exploit, including placing malicious adverts in legitimate ad networks (a tactic known as "malvertising") and hijacking legitimate blogs.
Other strategies employed included the sending of emails with malicious links and the setting up of websites specifically to attack victims, gaming search engines in the process to increase the chances of infection.
"If the current user is logged on with administrative user rights, an attacker who successfully exploited this vulnerability could take complete control of an affected system," Microsoft said.
"An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights."
It added that systems where Internet Explorer is used regularly, including workstations or terminal servers, were most at risk.