Jannik Jochem Jannik Jochem - 2 months ago 20
Apache Configuration Question

How do I enable perfect forward secrecy by default on Apache?

In the wake of recent events, I have been reconsidering my Apache setup. Currently, my apache site config looks something like this:

<IfModule mod_ssl.c>
<VirtualHost *:80>
ServerName mydomain.com
ServerAlias www.mydomain.com
Redirect permanent / https://mydomain.com
</VirtualHost>

<VirtualHost *:443>
ServerAdmin webmaster@localhost
ServerName mydomain.com

DocumentRoot /var/www-wordpress
<Directory />
Options FollowSymLinks
AllowOverride None
</Directory>
<Directory /var/www-wordpress>
Options Indexes FollowSymLinks MultiViews
AllowOverride FileInfo
Order allow,deny
allow from all
</Directory>

ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
<Directory "/usr/lib/cgi-bin">
AllowOverride None
Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
Order allow,deny
Allow from all
</Directory>

ErrorLog ${APACHE_LOG_DIR}/error.log
LogLevel warn

CustomLog ${APACHE_LOG_DIR}/ssl_access.log combined
SSLCertificateFile /etc/ssl/certs/mydomain.com.crt
SSLCertificateKeyFile /etc/ssl/private/mydomain.com.key
SSLCertificateChainFile /etc/ssl/certs/sub.class1.server.ca.pem
<FilesMatch "\.(cgi|shtml|phtml|php)$">
SSLOptions +StdEnvVars
</FilesMatch>
<Directory /usr/lib/cgi-bin>
SSLOptions +StdEnvVars
</Directory>

BrowserMatch "MSIE [2-6]" \
nokeepalive ssl-unclean-shutdown \
downgrade-1.0 force-response-1.0
BrowserMatch "MSIE [17-9]" ssl-unclean-shutdown
</VirtualHost>


What do I have to do to support perfect forward secrecy? How can I enable SSL perfect forward secrecy by default? How could I enforce it?

Answer

How about:

SSLProtocol all -SSLv2 -SSLv3
SSLHonorCipherOrder On
SSLCipherSuite EECDH+AES:EDH+AES:-SHA1:EECDH+RC4:EDH+RC4:RC4-SHA:EECDH+AES256:EDH+AES256:AES256-SHA:!aNULL:!eNULL:!EXP:!LOW:!MD5

Note the addition of the -SSLv3 flag to disable SSLv3. This is added to protect against the POODLE attack.

This will prefer perfect forward secrecy, but not at the expense of being vulnerable to the BEAST attack. Since Apache lacks a way to configure cipher preference based on protocol version, I fake it by referring to ciphers only available in the newer protocols. Specifically, AES was only available with SHA1 hashing until TLSv1.2. Thus the list starts with the TLSv1.2 ephemeral Diffie-Hellman ciphers, then RC4 (first with ephemeral DH, then without), and finally a BEAST-vulnerable AES option. Excluding no auth / weak encryption / weak hashing at the end is just for good hygiene and could be omitted since no such ciphers were introduced. If performance is a concern, use EECDH only and omit EDH.

In combination with Apache 2.2 (thus no EECDH as @Bruno says), per https://www.ssllabs.com/ssltest/analyze.html, this achieves PFS for iOS Safari only. IE and Firefox are TLSv1.0 so they get RC4 to avoid BEAST. (Alas, there is no such thing as EDH RC4, so without EECDH, you give up PFS). This is, I believe, the best one could hope for with those browsers on Apache 2.2. Chrome is the only one poorly served, since it supports TLSv1.1 and could use EDH AES without being vulnerable to BEAST. Instead, it gets RC4-RSA like Firefox and IE. Upgrading Apache to enable EECDH RC4 should get PFS for Firefox, IE, and Chrome.

Update 2013-11-09:

I've found a few alternate recommendations around the web. They put less emphasis on BEAST protection (perhaps wise; BEAST is mostly mitigated client-side now) and more emphasis on perfect forward secrecy. To varying degrees they also have stronger preferences for GCM and greater reluctance to accept RC4.

Of particular note are, I think, the following recommendations:

Personally, I'm going to go with Mozilla OpSec's. Their reasoning is well explained on their page. Of note, they prefer AES128 over AES256. In their words: "[AES128] provides good security, is really fast, and seems to be more resistant to timing attacks."

Noteworthy in Ivan Ristic's and Geoffroy Gramaize's recommendation is that SSLv3 is disabled. I think this mostly just breaks IE6, though some security related differences between SSLv3 and TLS v1.0 are mentioned on Wikipedia.

Also before I didn't talk about CRIME and BREACH. To protect against CRIME, disable SSL compression. This is included in the examples linked. To protected against BREACH, you need to disable compression at the HTTP level. For Apache 2.4, just do this once globally:

<Location />
  SetEnvIfExpr "%{HTTPS} == 'on'" no-gzip
</Location>

For older versions of Apache, place this in each VirtualHost where SSLEngine is on:

<Location />
    SetEnv no-gzip
</Location>

Update 2014-10-14: The Mozilla OpSec guide is now split into recommendations for old/intermediate/modern compatibility. With the settings from intermediate or modern, you end up with SSLv3 disabled. That will protect against the POODLE attack.

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