One of the common problems appearing when scanning secure websites is a reported vulnerability in TLSv1 with cipher-block chaining (CBC); see the sample report generated by scanning tools about this problem.
Summary:
SSL/TLS Protocol Initialization Vector Implementation Information Disclosure Vulnerability
Synoposis:
It may be possible to obtain sensitive information from the remote host with SSL/TLS-enabled services.
Impact:
Vulnerability exists in SSL 3.0 and TLS 1.0 that could allow information disclosure if an attacker intercepts encrypted traffic served from an affected system. TLS 1.1, TLS 1.2, and all cipher suites that do not use CBC mode are not affected.
Resolution:
• Configure SSL/TLS servers to only use TLS 1.1 or TLS 1.2 if supported.
• Configure SSL/TLS servers to only support cipher suites that do not use block ciphers.
• Apply patches if available.
Note that additional configuration may be required after the installation of the MS12-006 security update in order to enable the split-record countermeasure
The problem with configuring the server to use TLS 1.1 or TLS 1.2 only is that XP with IE8 only supports TLS 1.0 and SSL 2.0 and 3.0. Whilst Windows 7 with IE8 supports TLS 1.0, 1.1 and 1.2 it is enabled by default. This can affect the users of a website; XP is still used by around 42% of all clients as measured by Net Marketshare.
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Operating System
|
Market Share
|
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Windows 7
|
42.76%
|
|
Windows XP
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42.52%
|
|
Windows Vista
|
6.15%
|
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Mac OS X 10.7
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2.45%
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Mac OS X 10.6
|
2.38%
|
A more user friendly method to get around the vulnerability is not to use CBC ciphers on the server such as those listed
• PSK-AES256-CBC-SHA
• EDH-RSA-DES-CBC3-SHA
• EDH-DSS-DES-CBC3-SHA
• ADH-DES-CBC3-SHA
• DES-CBC3-SHA
• DES-CBC3-MD5
• PSK-3DES-EDE-CBC-SHA
• KRB5-DES-CBC3-SHA
• KRB5-DES-CBC3-MD5
• RC2-CBC-MD5
• PSK-AES128-CBC-SHA
• IDEA-CBC-SHA
• EDH-RSA-DES-CBC-SHA
• EDH-DSS-DES-CBC-SHA
• ADH-DES-CBC-SHA
• DES-CBC-SHA
• DES-CBC-MD5
• KRB5-DES-CBC-SHA
• KRB5-DES-CBC-MD5
• EXP-EDH-RSA-DES-CBC-SHA
• EXP-EDH-DSS-DES-CBC-SHA
• EXP-ADH-DES-CBC-SHA
• EXP-DES-CBC-SHA
• EXP-RC2-CBC-MD5
• EXP-KRB5-RC2-CBC-SHA
• EXP-KRB5-DES-CBC-SHA
• EXP-KRB5-RC2-CBC-MD5
• EXP-KRB5-DES-CBC-MD5
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