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AUSCERT External Security Bulletin Redistribution
ESB-2015.0567
MS15-031: Vulnerability in Schannel Could Allow Security Feature Bypass
11 March 2015
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AusCERT Security Bulletin Summary
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Product: Microsoft Windows
Publisher: Microsoft
Operating System: Windows
Impact/Access: Unauthorised Access -- Remote/Unauthenticated
Reduced Security -- Remote/Unauthenticated
Resolution: Patch/Upgrade
CVE Names: CVE-2015-1637
Reference: ESB-2015.0528
Original Bulletin:
https://technet.microsoft.com/en-us/library/security/MS15-031
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Bulletin Number: MS15-031
Bulletin Title: Vulnerability in Schannel Could Allow Security Feature Bypass
Severity: Important
KB Article: 3046049
Version: 1.0
Published Date: March 10, 2015
Description: This security update resolves a vulnerability in Microsoft
Windows that facilitates exploitation of the publicly disclosed FREAK
technique, an industry-wide issue that is not specific to Windows operating
systems. The vulnerability could allow a man-in-the-middle (MiTM) attacker to
force the downgrading of the key length of an RSA key to EXPORT-grade length
in a TLS connection. Any Windows system using Schannel to connect to a remote
TLS server with an exploitable cipher suite is affected.
Executive Summary
This security update resolves a vulnerability in Microsoft Windows that
facilitates exploitation of the publicly disclosed FREAK technique, an
industry-wide issue that is not specific to Windows operating systems. The
vulnerability could allow a man-in-the-middle (MiTM) attacker to force the
downgrading of the key length of an RSA key to EXPORT-grade length in a TLS
connection. Any Windows system using Schannel to connect to a remote TLS
server with an insecure cipher suite is affected.
This security update is rated Important for all supported releases of
Microsoft Windows.
Affected Software
The following software versions or editions are affected.
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
Windows Server 2012 R2
Windows RT[1]
Windows RT 8.1[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)
[1]This update is available via Windows Update only.
Vulnerability Information
Schannel Security Feature Bypass Vulnerability - CVE-2015-1637
A security feature bypass vulnerability exists in Secure Channel (Schannel)
that is caused by an issue in the TLS state machine whereby a client system
accepts an RSA key with a shorter key length than the originally negotiated
key length. The vulnerability facilitates exploitation of the publicly
disclosed FREAK technique, which is an industry-wide issue that is not
specific to Windows operating systems.
In a man-in-the-middle (MiTM) attack, an attacker could downgrade the key
length of an RSA key to EXPORT-grade length in an encrypted TLS session. The
attacker could then intercept and decrypt this traffic. Any Windows system
connecting to a TLS server as a client is affected. An attacker who
successfully exploited this vulnerability could perform MiTM attacks that
could decrypt encrypted traffic.
The security update addresses the vulnerability by correcting the cipher suite
enforcement policies that are used when server keys are exchanged between
servers and client systems.
This vulnerability has been publicly disclosed. It has been assigned Common
Vulnerability and Exposure number CVE-2015-1637. When this bulletin was
originally released, Microsoft had not received any information to indicate
that this issue had been publicly used to attack customers.
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