How to protect your Debian or Ubuntu Server against the Logjam attack
This tutorial describes the steps that need to be taken to protect your Ubuntu or Debian Linux Server against the recently detected Logjam attack. Logjam is an attack against the Diffie-Hellman key exchange which is used in popular encryption protokols like HTTPS, TLS, SMTPS, SSH and others. A detailed description can be found here https://weakdh.org/.
This tutorial is compatible with ISPConfig 3 setups on Debian and Ubuntu.
The following steps have to be executed as root user on the shell.
Generate a unique DH Group
The first step to secure your server is to generate a unique DH Group with the openssl command. I will create the file in the /etc/ssl/private/ directory. When you dont have this directory on your server, then create it with these commands:
mkdir -p /etc/ssl/private
chmod 710 /etc/ssl/private
Now I'll create the dhparams.pem file and set secure permissions:
cd /etc/ssl/private
openssl dhparam -out dhparams.pem 2048
chmod 600 dhparams.pem
Apache
First I will add a secure cipher suite based on the recommendations from weakdh.org. Open the file /etc/apache2/mods-available/ssl.conf with an editor:
nano /etc/apache2/mods-available/ssl.conf
and change or add these lines:
SSLProtocol all -SSLv2 -SSLv3
SSLCipherSuite ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA SSLHonorCipherOrder on
Please note that the SSLCipherSuide is just one long line, so dont add line breaks!
The second part is to set the DH Group in apache. The SSLOpenSSLConfCmd config option is only available in apache 2.4.8 or newer and it also requires openssl 1.0.2 or newer, so we have to test first if our apache and openssl version supports it:
apache2 -v
The output on my Debian 7 server is:
root@server1:/etc/apache2# apache2 -v
Server version: Apache/2.2.22 (Debian)
Server built: Dec 23 2014 22:48:29
Now I'll test openssl:
openssl version
The output on my system is:
root@server1:/# openssl version
OpenSSL 1.0.1e 11 Feb 2013
So I can't set the DH Group on this server. The first and second parts are independent of each other, the first part already disabled weak ciphers to protect your server and it will work without the DH group. In case you want to install the latest OpenSSL form source, check out this guide. If your apache version is > 2.4.8 and OpenSSL > 1.0.2, then edit the /etc/apache2/mods-available/ssl.conf file again:
nano /etc/apache2/mods-available/ssl.conf
Add the line:
SSLOpenSSLConfCmd DHParameters "/etc/ssl/private/dhparams.pem"
and restart apache:
service apache2 restart
Nginx
Edit the Nginx configuration file /etc/nginx/nginx.conf
nano /etc/nginx/nginx.conf
Add or replace the following settings inside the httpd { .... } section:
ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA';
ssl_prefer_server_ciphers on;
ssl_dhparam /etc/ssl/private/dhparams.pem;
And restart Nginx:
service nginx restart
Postfix
Run these commands to set the secure cipher suite and the DH Group:
postconf -e "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"
postconf -e "smtpd_tls_dh1024_param_file = /etc/ssl/private/dhparams.pem"
And restart postfix:
service postfix restart
Dovecot
Edit the dovecot configuration file /etc/dovecot/dovecot.conf
nano /etc/dovecot/dovecot.conf
and add the line:
ssl_cipher_list=ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA
right after the ssl_protocols line.
For the other parameters, we need to know the dovecot version. Run this command to get the dovecot version information on the shell:
dovecot --version
When the version is 2.2.6 or greater, then add this additional line:
ssl_prefer_server_ciphers = yes
When the version is 2.2.7 or greater, then add this third line:
ssl_dh_parameters_length = 2048
Finally restart dovecot
service dovecot restart
Pure-ftpd
Securing pure-ftpd on Debian and Ubuntu is a bit more complicated as the /usr/sbin/pure-ftpd-wrapper script does not support the -J switch out of the box which is used by pure-ftpd to set the SSL Cipher Suite. The first step is to add support for the -J option in the wrapper script. Open the file:
nano /usr/sbin/pure-ftpd-wrapper
And scroll down to the line:
'TLS' => ['-Y %d', \&parse_number_1],
Now add this new line right afterwards:
'TLSCipherSuite' => ['-J %s', \&parse_string],
Then create (or edit when it exists) the file /etc/pure-ftpd/conf/TLSCipherSuite with nano:
nano /etc/pure-ftpd/conf/TLSCipherSuite
and enter the following cipher list:
ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA
When the file exists already and contains some ciphers, then replace the ciphers with the ones above. Then save the file and restart pure-ftpd:
service pure-ftpd-mysql restart
