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With the deprecation of TLS 1.0 on the horizon, I'm curious how others are handling connectivity and mail delivery issues for dated client environments.

Lets face it, there are more environments live on the internet running deprecated protocols than there are environments running the latest and greatest in updates. How do you handle "important business emails" from these users and still maintain security?

I'd considered setting up a secondary gateway to accept incoming mail under less secure, but more heavily scrutinized conditions, but I've been told this might negate disabling it on the primary environments.

Ultimately my question is whether "you need to upgrade" is a valid response when we start receiving reports that end users can't send us emails.

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    Postel's Law may apply here. One wonders how many MTAs support distinct server and client TLS configurations. – gowenfawr Aug 19 at 21:46
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    Having seen cases where attackers took advantage of situations where security exceptions were made (and hidden) to accommodate a small number of users, I think I vote for "you need to upgrade". It will be good for them in the long run – Conor Mancone Aug 19 at 22:15
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    The consensus in infosec is to do the opposite of Postel's Law. For example: queue.acm.org/detail.cfm?id=1999945 and tools.ietf.org/id/draft-thomson-postel-was-wrong-03.html – Z.T. Aug 19 at 23:02
  • On the case of MTAs you can combine the TLS version seen on the TCP connection with the policies (sieve) that you have in place on the MTA, for example you can use geoip in combination of TLS version, forexample if TLS version < 1.2 and geoip is not europe then mark that network flow as suspicious and in other stage of the SMTP use other police. – camp0 Aug 20 at 7:28
  • "On the horizon"??? For many, it's long past. – Swashbuckler Aug 20 at 18:49
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"You need to upgrade" is absolutely a valid response. TLS 1.0 was defined in 1999 and has been around for 20 years now. TLS 1.2, which is considered secure by today's standards, has been around since 2008, which is plenty of time for even slow companies to migrate to it.

But version numbers alone isn't what gets managers to order an upgrade of their ancient systems. It's a cost/benefit analysis, and in this case it's very simple:

Sticking with TLS 1.0 is a high security risk. Several well-developed attacks against TLS 1.0, such as POODLE or BEAST, exist. If the servers are not upgraded, they risk their e-mails being compromised.


Personally, I would consider supporting provably weak cryptography a security risk. And engaging in such a security risk - especially when there have been so many warnings that you should upgrade - just because some company is unwilling or unable to so, is difficult to justify.

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    POODLE is an attack against SSLv3, and BEAST is a client side attack to which MTAs are not vulnerable. Most TLS attacks, in fact, are not going to be useful against a mail server. The advice is still good, however it should be rationally supported. When the basis for the recommendation is tangential, inapplicable threats, justification becomes significantly more tenuous. – Xander Aug 20 at 14:33
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    @Xander I am not extremely knowledgeable about TLS 1.0 attacks, because it's so rarely encountered in the wild. Please feel free to edit my answer and add better evidence – MechMK1 Aug 20 at 14:36
  • It's rarely encountered in the web space, but is much more common in server based applications, particularly over distributed protocols like email. I'll try to find a more recent study, but as of 2016 only ~50% of mail servers supported either TLS v1.1 or v1.2. Well over 90% supported v1.0 at that point. I'm sure the numbers have shifted substantially since then, but I don't have a clear idea of how much. – Xander Aug 20 at 14:46
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I will describe the techniques that we used on a MTA that I was working on the past and you will get the idea of things that you can make. We got the same problems of old MTAs that didn't support TLS1.2 versions and only support old TLS versions. This MTA got openssl library for TLS and we used to have some security policies that depending on destination IPs, domains, and TLS versions used the admin can accept or reject the connections. This allow admins to deal with old MTAs without too much impact on other systems. So for inbound traffic some customers just allow TLS1.2 on the connections and for outbound they where a bit less restrictive by using the tls versions in combination with the IP/domain.

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