Microsoft Corp is racing against time to fix a bug in its Internet Explorer browser. The flaw was revealed by a cybersecurity firm, FireEye Inc., over the weekend.

FireEye disclosed that hackers have already managed to exploit the flaw by launching attacks against some American companies.

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On Saturday, Microsoft announced that it plans to fix the bug that targets the versions 9 to 11 of Internet Explorer.

FireEye said these versions comprise more than 26% of the browser market. The said bug reportedly affects the versions 6 to 11 of the browser. Research company NetMarketShare said these versions make up 55% of the whole PC browser market.

The upcoming bug fix will not cover PCs running on Windows XP. This is because Microsoft has recently stopped supporting this operating system (OS). According to security firms, 15% to 25% of the PCs worldwide still run on the 13-year-old OS.

FireEye said some hackers have launched a campaign dubbed Operation Clandestine Fox. The company said these hackers are very sophisticated, making it hard for the firm to track them.

Vitor De Souza, spokesperson of FireEye, said the campaign seems to be focused on some US-based companies in the defense and financial industries. He also said the motives of the attackers have not yet been determined, but it looks to be gathering intel. Although he did not give more details, he said changing browsers could protect users against the attack.

Microsoft released an advisory about the flaw, saying hackers could use the bug to gain control over an affected system. The company also said the flaw could potentially be exploited by hackers to view, change, or delete data, to spread malicious programs across the system, or to gain full user rights to the system.

Seculert chief technology officer, Aviv Raff, said Microsoft should move quickly to prevent the problem from getting bigger. He also cautioned Windows XP users who will not be able to benefit from the OS update that Microsoft is racing to prepare. Microsoft told Reuters that Windows XP users are advised to upgrade to either Windows 7 or 8.