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Microsoft: 'Update your security certs this month – or else'

October update to block sites using weak crypto

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The good news is that Microsoft's next Patch Tuesday, due on September 11, should be a breeze, bringing just two security updates. The bad news is that October's Patch Tuesday will be a game changer, and Microsoft has cautioned Windows admins to take advantage of the lull to make sure their security houses are in order.

Beginning in October, the minimum RSA key length for certificates used in Public Key Infrastructure (PKI) will increase to 1024 bits for all supported versions of Windows, going back to XP Service Pack 3. That means admins may need to update the certificates on their secure servers to avoid widespread problems.

For starters, once the patch is applied Internet Explorer will block access to SSL websites that use certificates with keys less than 1024 bits long. Similarly, Outlook 2010 will not be able to connect to an Exchange Server that uses a key that's too short, and it will no longer be able to encrypt or digitally sign mail using such keys. Applications and ActiveX controls that were signed with less than 1024 bit signatures may not install correctly, either, among other potential problems.

The change has been a long time coming. Microsoft first pledged to up its security requirements in 2011, and it issued a security advisory of the new policy this August. A patch has been available through the Download Center since August, too, for customers who want to get a head start.

But beginning with the October 9 Patch Tuesday, Redmond says it will push the patch out to all customers through Windows Update – ready or not – so admins better be sure their certificates aren't signed with short keys. As Angela Gunn of Microsoft's Trusted Computing division writes:

Though many have already moved away from such certificates, customers will want to take advantage of September's quiet bulletin cycle to review their asset inventories – in particular, examining those systems and applications that have been tucked away to collect dust and cobwebs because they "still work" and have not had any cause for review for some time.

The fix for any related problems that arise will be to reissue new certificates to any servers that are currently running with certificates signed using 1024-bit or shorter keys. As Gunn points out, 1024-bit keys should be considered the new baseline standard, and most experts recommend keys of 2048 bits or longer.

The trick, of course, will be finding those old certificates before they start causing problems. According to Microsoft, one way to spot them is to use CAPI2 logging. But even if you have to check the length of each certificate's key manually, the best plan is to find the bad ones before users start getting locked out of systems.

You have one month. Happy hunting! ®

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