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SYNTHESIS OF THE VULNERABILITY

An attacker, located as a Man-in-the-Middle, can force the Windows
Schannel client to accept a weak export algorithm, in order to
more easily capture or alter exchanged data.

Impacted products: IE, Windows 2003, Windows 2008 R0, Windows 2008
R2, Microsoft Windows 2012, Windows 7, Windows 8, Windows RT,
Windows Vista

Severity: 2/4

Creation date: 06/03/2015

DESCRIPTION OF THE VULNERABILITY

The TLS protocol uses a series of messages which have to be
exchanged between the client and the server, before establishing a
secured session.

Several cryptographic algorithms can be negotiated, such as
algorithms allowed for USA export (less than 512 bits).

An attacker, located as a Man-in-the-Middle, can inject during the
session initialization a message choosing an export algorithm.
This message should generate an error, however, the Windows
Schannel client accepts it.

Microsoft indicates that this vulnerability is different from
VIGILANCE-VUL-16301, because export algorithms are disabled, but
still used. Excepted this policy difference, this vulnerability is
identical.

An attacker, located as a Man-in-the-Middle, can therefore force
the Windows Schannel client to accept a weak export algorithm, in
order to more easily capture or alter exchanged data.

ACCESS TO THE COMPLETE VIGIL@NCE BULLETIN

http://vigilance.fr/vulnerability/Windows-Schannel-weakening-TLS-encryption-via-FREAK-16332