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Another SSL major flow – LOGJAM attack

Updated on May 20, 2015

On 20 May 2015, a group of security researchers has discovered a new flow in TLS protocol that allow an attacker to use man-in-the-middle attack to downgrade encrypted connections between a user and a Web or email server to use weak 512-bit keys which can be easily decrypted.

The new attack called LOGJAM, and it’s a Cousin of FREAK attack.

How the attack works?

The FREAK attack was a bad implementation for TLS protocol, but LOGJAM is a flow in the design of TLS protocol.

Logjam can be exploited on a subset of servers that support the widely used cryptographic algorithm called the "Diffie-Hellman key exchange" which allows protocols like HTTPS, SSH, SMTPS, IPsec to negotiate a secret key and create a secure connection.

The flaw affects any server supporting DHE_EXPORT ciphers and all modern browsers.

Who is affected?

Everyone, both end users, and sysadmin!

For end users, all web browsers (Google Chrome, Mozilla Firefox, Microsoft Internet Explorer, and Apple Safari) are vulnerable to the attack, there is nothing you can do except to wait for the vendors to release a new versions.

All vendors have been informed before announcing the new attack to the public; to allow them to work on the fixes.

Google already announced that they all fix the issue on Chrome 45, by increasing the minimum TLS DH group size to 1024 bits.

Microsoft already released an update for both servers and clients. Details below:

What sysadmins should do?

All sysadmins are advised to disable support for the export-grade (DHE_EXPORT) cipher suites that allow Diffie-Hellman connections to be downgraded, and generate a new and unique 2048-bit Diffie-Hellman group, since the new minimum is 1024-bit.

And deploy Elliptic-Curve Diffie-Hellman (ECDHE). Elliptic-Curve Diffie-Hellman (ECDH) key exchange avoids all known feasible cryptanalytic attacks, and modern web browsers now prefer ECDHE over the original, finite field, Diffie-Hellman.

Above both fixes can be achieved by generating a new key, and disable weak ciphers.

First thing to do is to generate a new key from OpenSSL:

openssl dhparam -out dhparams.pem 2048

Then update the settings on specific server applications.


Disable SSLv2, SSLv3, and all weak ciphers, in httpd.conf

SSLProtocol all -SSLv2 -SSLv3


SSLHonorCipherOrder on

Then, change the DH parameters:

SSLOpenSSLConfCmd DHParameters "{path to dhparams.pem}"

And finally, restart Apache server:

sudo service apache2 reload

sudo service httpd restart


In the website configuration server block in /etc/nginx/sites-enabled/default


ssl_prefer_server_ciphers on;

ssl_dhparam {path to dhparams.pem}

sudo nginx -s reload

Microsoft IIS

From Group Policy Object Editor, expand Computer Configuration, Administrative Templates, Network, and then click SSL Configuration Settings.

Under SSL Configuration Settings, open the SSL Cipher Suite Order setting and update the settings based on Microsoft supported ciphers

Microsoft already released a fix to increase the minimum allowable DHE key length to 1024-bit, MS15-055, kb3061518


In /etc/lighttpd/lighttpd.conf


ssl.dh-file="{path to dhparams.pem}"

sudo service lighttpd restart

Postfix SMTP

In /etc/postfix/

smtpd_tls_mandatory_exclude_ciphers = aNULL, eNULL, EXPORT, DES, RC4, MD5, PSK, aECDH, EDH-DSS-DES-CBC3-SHA, EDH-RSA-DES-CDC3-SHA, KRB5-DE5, CBC3-SHA

smtpd_tls_dh1024_param_file = ${config_directory}/dhparams.pem

sudo postfix reload


In /etc/dovecot.conf


ssl_prefer_server_ciphers = yes (Dovecot 2.2.6 or greater)

ssl_dh_parameters_length = 2048

sudo doveadm reload

© 2015 Mohammad Oweis


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