An IT watchdog has warned against using Microsoft's Internet Explorer web browser because of a possible security flaw.
The German government agency overseeing IT safety said people should use other browsers until the problem is fixed.
The Federal Office for Information Security said the browser's "weak point is already being used for targeted attacks."
It warns the code to attack computers running on Windows XP or Windows 7 operating systems through the browser is freely available online and might therefore spread rapidly.
The agency says users of internet Explorer versions 7, 8 or 9 should switch to alternative browsers until Microsoft updates its browser's security features.
A spokesman for Microsoft said the company is aware of the issue and is working on a software update.
"This is not a massive problem. There have been only a small number of targeted attacks," said the spokesman.
The browser is used by hundreds of millions of consumers and workers around the world.
Attackers lure users to an infected website, for example through an emailed link. Visiting the website then allows hackers to introduce codes to take control of the user's computer, the German agency said.
Microsoft said people should always be vigilant when clicking on links from unknown sources. It added that many antivirus programs might already be updated to protect their users against attacks through the browser loophole.