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National Cyber Awareness System

US-CERT Alert TA12-251A
Microsoft Update For Minimum Certificate Key Length

Original release date: September 07, 2012
Last revised: --

Systems Affected

     * Windows XP Service Pack 3
     * Windows XP Professional x64 Edition Service Pack 2
     * Windows Server 2003 Service Pack 2
     * Windows Server 2003 x64 Edition Service Pack 2
     * Windows Server 2003 with SP2 for Itanium-based Systems
     * Windows Vista Service Pack 2
     * Windows Vista x64 Edition Service Pack 2
     * Windows Server 2008 for 32-bit Systems Service Pack 2
     * Windows Server 2008 for x64-based Systems Service Pack 2
     * Windows Server 2008 for Itanium-based Systems Service Pack 2
     * Windows 7 for 32-bit Systems and Windows 7 for 32-bit Systems Service Pack 1
     * Windows 7 for x64-based Systems and Windows 7 for x64-based Systems Service Pack 1
     * Windows Server 2008 R2 for x64-based Systems and Windows Server 2008 R2 for x64-based Systems Service Pack 1
     * Windows Server 2008 R2 for Itanium-based Systems
     * Windows Server 2008 R2 for Itanium-based Systems Service Pack 1
     * Windows Server 2008 for 32-bit Systems Service Pack 2 (Server Core installation)
     * Windows Server 2008 for x64-based Systems Service Pack 2 (Server Core installation)
     * Windows Server 2008 R2 for x64-based Systems and Windows Server 2008 R2 for x64-based Systems Service Pack 1 (Server Core installation)


Overview

   Microsoft has announced the availability of an update to Windows
   that restricts the use of certificates with RSA keys that are less
   than 1024 bits in length. Microsoft is planning to release this
   update through Microsoft Update in October 2012. System
   administrators of Microsoft Windows platforms should assess the
   impact of this update on their environment before any wide-scale
   deployment.


Description

   Microsoft's KB2661254 article states in part:

   "The strength of public-key-based cryptographic algorithms is
   determined by the time that it takes to derive the private key by
   using brute-force methods. The algorithm is considered to be strong
   enough when the time that it takes to derive private key is
   prohibitive enough by using the computing power at disposal. The
   threat landscape continues to evolve. Therefore, Microsoft is
   further hardening the criteria for the RSA algorithm with key
   lengths that are less than 1024 bits long.

   After the update is applied, only certificate chains that are built
   by using the CertGetCertificateChain function are affected. The
   CryptoAPI builds a certificate trust chain and validates that chain
   by using time validity, certificate revocation, and certificate
   policies (such as intended purposes). The update implements an
   additional check to make sure that no certificate in the chain has
   an RSA key length of less than 1024 bits."


Impact

   The private keys used in certificates with RSA keys that are less
   than 1024 bits in length can be derived and could allow an attacker
   to duplicate the certificates and use them fraudulently to spoof
   content, perform phishing attacks, or perform man-in-the-middle
   attacks.


Solution

   US-CERT recommends that system administrators of Microsoft Windows
   platforms read Microsoft's KB2661254 article and perform an
   extensive test of the update before doing any wide-scale deployment
   in their environment. The update will be sent to Microsoft Update
   for the October 2012 patch cycle. System administrators can obtain
   the update now from Microsoft's Download Center.


References

 * Microsoft Security Advisory: Update for minimum certificate key
   length
   <http://support.microsoft.com/kb/2661254>

 * Microsoft Security Advisory (2661254) Update For Minimum
   Certificate Key Length
   <http://technet.microsoft.com/en-us/security/advisory/2661254>

 * Windows PKI Blog: RSA keys under 1024 bits are blocked
   <http://blogs.technet.com/b/pki/archive/2012/06/12/rsa-keys-under-1024-bits-are-blocked.aspx>

 * Windows PKI Blog: Blocking RSA Keys less than 1024 bits (part 2)
   <http://blogs.technet.com/b/pki/archive/2012/07/13/blocking-rsa-keys-less-than-1024-bits-part-2.aspx>

 * Microsoft Download Center: Search results for KB2661254
   <http://www.microsoft.com/en-us/download/search.aspx?q=kb2661254>


Revision History

  September 07, 2012: Initial release

 ____________________________________________________________________

   Feedback can be directed to US-CERT Technical Staff. Please send
   email to <cert@cert.org> with "TA12-251A Feedback VU#221532" in
   the subject.
 ____________________________________________________________________

   Produced by US-CERT, a government organization.
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