Mailserver does not enforce SSL ciphersuite order preference

janipewter

Active Member
Jan 2, 2013
44
3
8
cPanel Access Level
Root Administrator
My SSL cipersuite list is set as follows:

ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256

This is Mozilla's recommended "modern" configuration.

However the server does not enforce the order preference, and there is no option in WHM to make it do so. Obviously I would prefer all clients to use AES256 or CHACHA20 if they are capable.
 

cPanelMichael

Administrator
Staff member
Apr 11, 2011
47,904
2,236
463
However the server does not enforce the order preference, and there is no option in WHM to make it do so. Obviously I would prefer all clients to use AES256 or CHACHA20 if they are capable.
Hello,

Here's a quote from the Exim documentation that may relate to your question:

In a server, the order of items in these lists is unimportant. The server advertises the availability of all the relevant cipher suites. However, in a client, the order in the tls_require_ciphers list specifies a preference order for the cipher algorithms. The first one in the client’s list that is also advertised by the server is tried first. The default order is as listed above.
Can you try moving the ciphers you want prioritized to the beginning of the cipher suite entry to see if that does what you are seeking?

Note that for Apache, we have a feature request you can vote for and monitor at:

SSLHonorCipherOrder on Apache

Thank you.
 

janipewter

Active Member
Jan 2, 2013
44
3
8
cPanel Access Level
Root Administrator
Hello,

Here's a quote from the Exim documentation that may relate to your question:

In a server, the order of items in these lists is unimportant. The server advertises the availability of all the relevant cipher suites. However, in a client, the order in the tls_require_ciphers list specifies a preference order for the cipher algorithms. The first one in the client’s list that is also advertised by the server is tried first. The default order is as listed above.
Can you try moving the ciphers you want prioritized to the beginning of the cipher suite entry to see if that does what you are seeking?
Thank you, that is interesting. The ciphers are in the order I would like them to be selected by the client. Whether or not the clients all recognise this order I'm not sure of it, although it would be logical. What I was looking for was an option analogous to HonorCipherOrder, but for Exim. I only discovered this after running the server test on hardenize.com (from the same developers as the Qualys SSLLabs test but much more in depth, and not just for HTTP). It clearly shows that the server advertises the correct cipher suites, but with no order preference.

Note that for Apache, we have a feature request you can vote for and monitor at:

SSLHonorCipherOrder on Apache

Thank you.
You should definitely implement that. I've had it in the PreMainInclude since day one, but it would be nice if there was an option for it. Also, please see my other thread: More customisation in AutoSSL

Thanks.
 

cPanelMichael

Administrator
Staff member
Apr 11, 2011
47,904
2,236
463
Hello,

The ability to configure the priority of the ciphers that are presented to a user's email client could be limited to the extent of what Exim allows, but I still encourage you to open a feature request using the following URL so that our Developers can review the request and determine if it's something we could implement:

Submit A Feature Request

Thank you.