Reference: CERT-EU Security Advisory 2015-325

Title: Logjam Attack

Version history: 04.06.2015 Initial publication.

Dear Colleagues,

Last days was published a new vulnerability related to TSL/SSL protocol
called Logjam attack. This vulnerability allows a man-in-the-middle
attacker to downgrade vulnerable TLS connections to 512-bit export-grade
cryptography (which is an old working mode, still there to support
legacy system enforcing former US cryptography exportation
restrictions). This allows the attacker to read and modify any data
passed over the connection. This attack offers a practical "break" of
the TLS protocol against poorly configured servers and affects any
server that supports DHE_EXPORT ciphers, and all modern web browsers.

This attack takes advantages of the fact that the authentication process
takes place only at the end of the TLS handshake and relies on the
ability of the attacker to recover the secret key, quickly enough to
tamper the server exchange Finished messages of the handshake. The use
of 512-bit export-grade cryptography make it easier for the attacker to
recover the secret key[1,2].

Recommendations:
===============

Web or mail server should disable support for export cipher suites and
generate a unique 2048-bit Diffie-Hellman group. Here there is link a
guide to do that in several OS [3]. In any case consider to review the
documentation of your specific provider about this issue. In the case
the affected system only supports DHE_EXPORTS ciphers a risk assessment
has to be done, in order to take a decision about them. Browsers should
be patched to the most recent version.

CERT-EU is probing public ranges of our constituency in order to
discover and report affected systems.

References:
==========

[1]
http://blog.cryptographyengineering.com/2015/05/attack-of-week-logjam.html

[2] https://weakdh.org/

[3] https://weakdh.org/sysadmin.html

[4] https://weakdh.org/imperfect-forward-secrecy.pdf

Best Regards,

CERT-EU Team (http://cert.europa.eu)
Phone: +32.2.2990005 / e-mail: cert-eu@ec.europa.eu
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