The U.S. government issued a serious warning to Windows users that a critical flaw in the operating system could enable a hacker to hijack their computers and install a virus, delete programs, or gain access to private accounts. UPDATE your computer immediately, and spread out the word!

The Department of Homeland Security (DHS) issued the rare advisory after Microsoft announced a dozen patches for vulnerablities in the Windows software as part of the company’s monthly security bulletin. What apparently grabbed the department’s attention is that one of the flaws opens a back door to a computer without any action by the user.

In urging home users and businesses to apply the MS06-040 patch, the DHS said that attempts to exploit the flaw were imminent, and that attacks on Windows could impact government systems, businesses, and critical I.T. infrastructure. In fact, such attacks have already been reported.

According to Microsoft, the flaw allows remote code execution and impacts Windows 2000 Service Pack 4, Windows XP Service Pack 1 and Microsoft Windows XP Service Pack 2, as well as versions of Windows Server 2003.

Users can apply the Microsoft MS06-040 security patch at https://www.microsoft.com/technet/security/bulletin/ms06-040.mspx. Home users may also visit Windows Update at https://update.microsoft.com and select “express” to install critical security updates, including the MS06-040 fix.