Decreasing Security for Perceived Security -- all in the name of compliance

Today I ran into a little setback for an issue I did not foresee. For the past several months, I've been on a PCI remediation project, of which one of my tasks was to implement a web application firewall to address PCI requirement 6.6. Now, for everyone out there who has been following our posts, knows us dealing with WAFs is worse than pulling teeth.

So, in creating a secure Apache standard build, I seemed to have over-engineered and made our build too secure. How? Well, one of the lines in our httpd.conf included the following SSLCipherSuite directive:

SSLCipherSuite HIGH:MEDIUM:+TLSv1:!LOW:!EXP:!ADH:!aNULL:!eNULL:!NULL:!SSLv2

With support for the following ciphers:

DHE-RSA-AES256-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(256)  Mac=SHA1
DHE-DSS-AES256-SHA      SSLv3 Kx=DH       Au=DSS  Enc=AES(256)  Mac=SHA1
AES256-SHA              SSLv3 Kx=RSA      Au=RSA  Enc=AES(256)  Mac=SHA1
DHE-RSA-AES128-SHA      SSLv3 Kx=DH       Au=RSA  Enc=AES(128)  Mac=SHA1
DHE-DSS-AES128-SHA      SSLv3 Kx=DH       Au=DSS  Enc=AES(128)  Mac=SHA1
AES128-SHA              SSLv3 Kx=RSA      Au=RSA  Enc=AES(128)  Mac=SHA1
EDH-RSA-DES-CBC3-SHA    SSLv3 Kx=DH       Au=RSA  Enc=3DES(168) Mac=SHA1
EDH-DSS-DES-CBC3-SHA    SSLv3 Kx=DH       Au=DSS  Enc=3DES(168) Mac=SHA1
DES-CBC3-SHA            SSLv3 Kx=RSA      Au=RSA  Enc=3DES(168) Mac=SHA1
RC4-SHA                 SSLv3 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=SHA1
RC4-MD5                 SSLv3 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=MD5

Looks good right?

This directive, included the Diffie-Hellman ciphers, which I was reminded by blatant error messages of unsupported ciphers in the management console, that it was not susceptible to MITM attacks due to the nature of the key-exchange. Doh! What's this mean? Our WAF, was rendered useless for monitoring and protecting our application because our SSL encryption strengths being negotiated were the most secure, and thus, the WAF had no visibility into our application. The most common SSL cipher being negotiated was TLS_DHE_RSA_WITH_AES_256_CBC_SHA -- the most secure of all.

So to comply with PCI Requirement 6.6, I had to step down our list of available SSL ciphers with the following SSLCipherSuite directive:

HIGH:MEDIUM:+TLSv1:!DH:!LOW:!EXP:!ADH:!aNULL:!eNULL:!NULL:!SSLv2

which at least provided support for the following ciphers:

AES256-SHA              SSLv3 Kx=RSA      Au=RSA  Enc=AES(256)  Mac=SHA1
AES128-SHA              SSLv3 Kx=RSA      Au=RSA  Enc=AES(128)  Mac=SHA1
DES-CBC3-SHA            SSLv3 Kx=RSA      Au=RSA  Enc=3DES(168) Mac=SHA1
RC4-SHA                 SSLv3 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=SHA1
RC4-MD5                 SSLv3 Kx=RSA      Au=RSA  Enc=RC4(128)  Mac=MD5

So much for PCI Requirement 4.1, which requires the use of strong cryptography and security protocols such as SSL/TLS to safeguard sensitive cardholder data. This post is just a little FYI for those in the same boat. Turns out we can be "too secure," :P

As my manager likes to sometimes put it, "How do you like 'dem apples?"

Posted by Marcin on Thursday, November 20, 2008 in Security and Work.