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
According to a spokesman quoted by the Business newspaper, "Microsoft is investigating reports of malicious attack exploiting vulnerabilities".
Problems arise when a user unknowingly clicks on a so-called malicious link, triggering a download of software.
Hackers then have full access to data and files on the computer.
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 already issued a warning about the problem.
Microsoft's Windows software runs on about 90% of the world's computer.
The company said it hoped to have the problem sorted before its monthly release of antivirus updates.