Microsoft, the world's largest software maker, is racing to solve a flaw in its internet browser that may allow hackers access to computer systems.
Microsoft has had to fix problems with its software before
The net security watchdog, the Computer Emergency Readiness Team (Cert) has issued a warning, saying there were signs it was being exploited.
Problems arise when a user unknowingly clicks on a bogus web link, triggering a download of software.
Malicious hackers could then have access to data and files on the PC.
The danger lies in the fact that the links can be disguised to look like bona fide URLs and the software is installed without raising any alarms.
The US's Computer Emergency Readiness Team has issued a warning about the problem late on Friday.
"Publicly available exploit code exists for this vulnerability, and US-Cert has monitored incident reports that indicate that this vulnerability is being actively exploited.," it said in a statement.
Microsoft's Windows software runs on about 90% of the world's desktop computers.
The company said it hoped to have the problem sorted before its monthly release of security updates.
Since the beginning of 2004, Microsoft has released 17 security warnings.
This compares to 51 issued last year, of which 20 were classified as the highest category of threat.
While it works on a fix to the problems with Internet Explorer, Microsoft is recommending that web surfers adopt so-called "safe browsing" practices.