Microsoft Security Bulletin MS15-055 - Important

Published: May 12, 2015

Version: 1.0

This security update resolves a vulnerability in Microsoft Windows that facilitates exploitation of the publicly disclosed Logjam technique, an industry-wide issue that is not specific to Windows operating systems. The vulnerability could allow information disclosure when Secure Channel (Schannel) allows the use of a weak Diffie-Hellman ephemeral (DHE) key length of 512 bits in an encrypted TLS session. Allowing 512-bit DHE keys makes DHE key exchanges weak and vulnerable to various attacks. A server needs to support 512-bit DHE key lengths for an attack to be successful; the minimum allowable DHE key length in default configurations of Windows servers is 1024 bits.

This security update is rated Important for all supported releases of Microsoft Windows. For more information, see the Affected Software section.

The security update addresses the vulnerability by increasing the minimum allowable DHE key length to 1024 bits. For more information about the vulnerability, see the Vulnerability Information section.

For more information about this update, see Microsoft Knowledge Base Article 3061518.

The following software versions or editions are affected. Versions or editions that are not listed are either past their support life cycle or are not affected. To determine the support life cycle for your software version or edition, see Microsoft Support Lifecycle.

^[1]Note that update 3050514 in MS15-052 is releasing concurrently with 3061518 in MS15-055. Customers who intend to install both updates manually on Windows 8 or Windows Server 2012 should install 3050514 in MS15-052 prior to installing 3061518 in MS15-055 (this is taken care of automatically for customers with automatic updating enabled). For more information, see the Known Issues section of Microsoft Knowledge Base Article 3061518.

^[2]This update is available via Windows Update only.

Does this update contain any other security-related changes to functionality?
Yes. This update standardizes TLS False Start ciphers across Windows 8 and Windows 8.1 by removing False Start optimization during cipher negotiation for the following two ciphers on Windows 8 systems:

• TLS_RSA_WITH_RC4_128_SHA
• TLS_RSA_WITH_RC4_128_MD5

It also implements a provision for disallowing False Start during RC4 cipher suite negotiation.

The following severity ratings assume the potential maximum impact of the vulnerability. For information regarding the likelihood, within 30 days of this security bulletin's release, of the exploitability of the vulnerability in relation to its severity rating and security impact, please see the Exploitability Index in the May bulletin summary.

An information disclosure vulnerability exists in Secure Channel (Schannel) when it allows the use of a weak Diffie-Hellman ephemeral (DHE) key length of 512 bits in an encrypted TLS session. Allowing 512-bit DHE keys makes DHE key exchanges weak and vulnerable to various attacks.

The security update addresses the vulnerability by increasing the minimum allowable DHE key length to 1024 bits.

Microsoft received information about this vulnerability through coordinated vulnerability disclosure. When this security bulletin was issued, Microsoft had not received any information to indicate that this vulnerability had been publicly used to attack customers.

The following mitigating factors may be helpful in your situation:

• A server needs to support 512-bit DHE key lengths for an attack to be successful; the minimum allowable DHE key length in default configurations of Windows servers is 1024 bits.

The following workaround may be helpful in your situation:

• Disable DHE cipher suites
  Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

  1. Open Registry Editor.

  2. Access key exchange algorithm settings by navigating to the following registry location:

     HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\KeyExchangeAlgorithms

  3. Select the Diffie-Hellman sub key (if it does not exist, then create it).

  4. Set the Enabled DWORD registry value to 0 (if it does not exist, then create it).

  5. Exit Registry Editor.  

  How to undo the workaround.

  1. Open Registry Editor.

  2. Access key exchange algorithm settings by navigating to the following registry location:  

     HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\KeyExchangeAlgorithms

  3. Select the Diffie-Hellman sub key.

  4. Set the Enabled DWORD registry value to 1.

  5. Exit Registry Editor.

   

  Impact of the workaround: Encrypted TLS sessions that rely on DHE keys will no longer function unless alternative failover options have been implemented.

For Security Update Deployment information, see the Microsoft Knowledge Base article referenced in the Executive Summary.

Microsoft recognizes the efforts of those in the security community who help us protect customers through coordinated vulnerability disclosure. See Acknowledgments for more information.

The information provided in the Microsoft Knowledge Base is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.

• V1.0 (May 12, 2015): Bulletin published.

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