SSLCipherSuite – Disable Weak Encryption, CBC Cipher, and MD5 Algorithm

ssltls

A developer recently ran a PCI Scan with TripWire against our LAMP server. They identified several issues and instructed the following to correct the issues:

Problem: SSL Server Supports Weak Encryption for SSLv3, TLSv1,

Solution: Add the following rule to httpd.conf

SSLCipherSuite ALL:!aNULL:!eNULL:!LOW:!EXP:RC4+RSA:+HIGH:+MEDIUM

Problem: SSL Server Supports CBC Ciphers for SSLv3, TLSv1

Solution: Disable any cipher suites using CBC ciphers

Problem: SSL Server Supports Weak MAC Algorithm for SSLv3, TLSv1

Solution: Disable any cipher suites using MD5 based MAC algorithms

I tried searching google for a comprehensive tutorial on how to construct an SSLCipherSuite directive to meet my requirements, but I didn't find anything I could understand. I see examples of SSLCipherSuite directives, but I need an explanation on what each component of the directive does. So even in the directive SSLCipherSuite ALL:!aNULL:!eNULL:!LOW:!EXP:RC4+RSA:+HIGH:+MEDIUM, I dont understand for example what the !LOW means.

Can someone either a) tell me the SSLCipherSuite directive that will meet my needs or b) show me a resource that clearly explains each segment of a SSLCipherSuite is and how to construct one?

Best Answer

If their only complaint is MD5-based MAC, you should be able to simply add the !MD5 element to your existing cipher suite to meet the recommendation.

That said, I see they complain about the use of the CBC mode as well. Unfortunately, there is no CBC cipher group. The recommendation given to you also does not exclude CBC mode cipherspecs, at least on my version of openSSL (1.0.1e). This is a shame. If you need all such ciphers to be excluded, you could exclude all the CBC ones explicitly, though you will have to update that as they are included. Note that even HIGH includes CBC ciphers.

Including both ALL and RC4+RSA is redundant. I would be loathe to trust a security consultant (even a computerized one) that cannot even construct a well-formed cipherspec that meets their own recommendations.

The SSLCipherSuite takes an OpenSSL cipher spec. You can find this in the openssl documentation (link), but I find that this documentation is usually quite out of date. However, you can test one by running openssl ciphers ${cipherspec} on your server; output will be a :-separated list of ciphers that would be allowed by the given spec, or an error indicating none were allowed.

Similarly, if you want to know what LOW contains, do:

falcon@tiernyn ~ $ openssl ciphers 'LOW'
EDH-RSA-DES-CBC-SHA:EDH-DSS-DES-CBC-SHA:ADH-DES-CBC-SHA:DES-CBC-SHA:DES-CBC-MD5

!LOW means to exclude those ones. +HIGH means to prefer the high-security ones in the ordering.

If you want a line-delimited list of all the ciphers that use CBC in your cipherspec, do:

openssl ciphers ${cipherspec} | sed 's/:/\n/g' | grep CBC

Those are the ones you'd have to exclude. You may, however, find it more reasonable to grep -v CBC and include only those (just set them up in a :-delimited list and use that as the cipherspec).

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