We are going to show you how to set up strong SSL security on the apache and nginx webserver. We do this by disabling SSL Compression to mitigate the CRIME attack, disable SSLv3 and SSLv2 and below because of vulnerabilities in the protocol and we will set up a strong ciphersuite that enables Forward Secrecy when possible. We also enable HSTS and HPKP. This way we have a strong and future proof ssl configuration and we get an A on the Qually Labs SSL Test at https://www.ssllabs.com/ssltest/.
The BEAST attack and RC4
In short, by tampering with an encryption algorithm’s CBC – cipher block chaining – mode’s, portions of the encrypted traffic can be secretly decrypted. More info on the above link.
Recent browser versions have enabled client side mitigation for the beast attack. The recommendation was to disable all TLS 1.0 ciphers and only offer RC4. However, [RC4 has a growing list of attacks against it],(http://www.isg.rhul.ac.uk/tls/) many of which have crossed the line from theoretical to practical. Moreover, there is reason to believe that the NSA has broken RC4, their so-called “big breakthrough.”
Disabling RC4 has several ramifications. One, users with shitty browsers such as Internet Explorer on Windows XP will use 3DES in lieu. Triple-DES is more secure than RC4, but it is significantly more expensive. Your server will pay the cost for these users. Two, RC4 mitigates BEAST. Thus, disabling RC4 makes TLS 1.0 users susceptible to that attack, by moving them to AES-CBC (the usual server-side BEAST “fix” is to prioritize RC4 above all else). I am confident that the flaws in RC4 significantly outweigh the risks from BEAST. Indeed, with client-side mitigation (which Chrome and Firefox both provide), BEAST is a nonissue. But the risk from RC4 only grows: More cryptanalysis will surface over time.
SSL Compression (CRIME attack)
The CRIME attack uses SSL Compression to do its magic, so we need to disable that. On Apache 2.2.24+ we can add the following line to the SSL config file we also edited above:
If you are using al earlier version of Apache and your distro has not backported this option then you need to recompile OpenSSL without ZLIB support. This will disable the use of OpenSSL using the DEFLATE compression method. If you do this then you can still use regular HTML DEFLATE compression.
SSLv2 and SSLv3
SSL v2 is insecure, so we need to disable it. We also disable SSLv3, as TLS 1.0 suffers a downgrade attack, allowing an attacker to force a connection to use SSLv3 and therefore disable forward secrecy.
SSLv3 allows exploiting of the POODLE bug. This is one more major reason to disable this!
Again edit the config file:
SSLProtocol All -SSLv2 -SSLv3 (For Apache)
ssl_protocols TLSv1 TLSv1.1 TLSv1.2; (For Nginx)
All is a shortcut for +SSLv2 +SSLv3 +TLSv1 or – when using OpenSSL 1.0.1 and later – +SSLv2 +SSLv3 +TLSv1 +TLSv1.1 +TLSv1.2, respectively. The above line enables everything except SSLv2 and SSLv3. More info on the apache website
Poodle and TLS-FALLBACK-SCSV
SSLv3 allows exploiting of the POODLE bug. This is one more major reason to disable this.
Google have proposed an extension to SSL/TLS named TLSFALLBACKSCSV that seeks to prevent forced SSL downgrades. This is automatically enabled if you upgrade OpenSSL to the following versions:
OpenSSL 1.0.1 has TLSFALLBACKSCSV in 1.0.1j and higher.
OpenSSL 1.0.0 has TLSFALLBACKSCSV in 1.0.0o and higher.
OpenSSL 0.9.8 has TLSFALLBACKSCSV in 0.9.8zc and higher.
The Cipher Suite
(Perfect) Forward Secrecy ensures the integrity of a session key in the event that a long-term key is compromised. PFS accomplishes this by enforcing the derivation of a new key for each and every session.
This means that when the private key gets compromised it cannot be used to decrypt recorded SSL traffic.
The cipher suites that provide Perfect Forward Secrecy are those that use an ephemeral form of the Diffie-Hellman key exchange. Their disadvantage is their overhead, which can be improved by using the elliptic curve variants.
The following two ciphersuites are recommended by me, and the latter by the Mozilla Foundation.
The recommended cipher suite:
If your version of OpenSSL is old, unavailable ciphers will be discarded automatically. Always use the full ciphersuite above and let OpenSSL pick the ones it supports.
The ordering of a ciphersuite is very important because it decides which algorithms are going to be selected in priority. The recommendation above prioritizes algorithms that provide perfect forward secrecy.
Older versions of OpenSSL may not return the full list of algorithms. AES-GCM and some ECDHE are fairly recent, and not present on most versions of OpenSSL shipped with Ubuntu or RHEL.