The change in key length requirements came in the wake of an exploit against Windows Update becoming a key component of . Keys less than 1024 bits are deemed too easy to break with brute force attacks.

The patch has been available since August as an option to give customers time to check it out and install it, but after Tuesday it is no longer optional. "There are still a few days left if you haven't tested it, but don't let this be an 'I told you so' moment," says Paul Henry, security and forensic analyst at Lumension.

Affected platforms are: Windows XP, Windows 2003, Windows Server 2003 R2, Windows Vista, Windows Server 2008, Windows 7 and Windows Server 2008 R2 operating systems.

The upgrade does have some wiggle room. Applications signed before Jan. 1, 2010, are allowed to use keys smaller than 1024 bits.

The deadline coincides with October's Patch Tuesday, which for the second month in a row is pretty light, featuring just .