Changing the keys the Microsoft software uses isn't that tricky, but finding all the customer and third-party software in corporate networks that use smaller keys could require some searching.
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Users should download and test whether it breaks connections with existing before putting it into full production, experts say.
Also, automated tools such as those sold by ExtraHop and Venafi can help track down applications that won't comply with the Microsoft update.
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 .
It apparently addresses a long-standing flaw in code that was repeatedly used in SQL Server from 2000 on. Unpatched, it can allow a remote-code execution on affected platforms.
Since so many consecutive versions of SQL server are being patched by the same update, it indicates the update is addressing the same problem in all the versions, says Alex Horan, a product manager at Core Security. Generally one patch deals with a single flaw. "So from that I conclude this is the same issue (and therefore the same code) across these versions," he writes in an email.
Tim Greene covers Microsoft for Network World and writes the Mostly Microsoft blog. Reach him at tgreene@nww.com and follow him on Twitter @Tim_Greene.
in Network World's Wide Area Network section.