Technical Information

Brief overview of the risk:
This security update resolves a vulnerability in Microsoft Windows. The vulnerability could allow information disclosure when Secure Channel (Schannel) allows the use of a weak Diffie-Hellman ephemeral (DFE) 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.

Detailed Information on the risk:

An information disclosure vulnerability exists in Secure Channel (Schannel) when it allows the use of a weak Diffie-Hellman ephemeral (DFE) 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.

Further information on this exploit is available at : MS15-055

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 Service Pack 1 
Windows 7 for x64-based Systems Service Pack 1 
Windows Server 2008 R2 for x64-based Systems Service Pack 1 
Windows Server 2008 R2 for Itanium-based Systems Service Pack 1 
Windows 8 for 32-bit Systems 
Windows 8 for x64-based Systems 
Windows 8.1 for 32-bit Systems 
Windows 8.1 for x64-based Systems 
Windows Server 2012 and Windows Server 2012 R2
Windows Server 2012 R2 
Windows RT and Windows RT 8.1
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 Service Pack 1 (Server Core installation) 
Windows Server 2012 (Server Core installation) 
Windows Server 2012 R2 (Server Core installation)