Strategies for protecting IMAP (e.g. MFA)

Lefteris Tsintjelis lefty at
Sun Nov 14 18:03:05 UTC 2021

On 14/11/2021 14:50, Kees van Vloten wrote:
> Apart from a really nice firewall firehol also supplies a good set of 
> ip-blacklists.
> For public exposure of email ports, I am using the combination of 
> firehol-firewall, firehol-blacklist, fail2ban and a whitelist based on 
> geo-ip. The mail-client ports exposed are 993 and 465, because starttls 
> is considered flawed nowadays:
> Full access from any IP (except firehol-blacklist and fail2ban) is 
> possible over VPN (openvpn) with MFA (privacyidea).
> Privacyidea also supplies a mobile-app compatible with a.o. TOTP and 
> HOTP but it provides a more secure way of enrollment (2-step).
> Thanks for pointing at, will see if it can tighten security 
> further in cooperation with the above.
> - Kees

The problem I faced over the years, with so many IPs, was that the black 
listing way would reach its limits at some point. Using the classic 
fail2ban expiration dates and method, over time, never actually manages 
to get rid of them as they keep on trying and trying. I needed to expand 
the blacklist expiration time limits way high but that reached firewall 
limitations so I personally switched to a permanent white list 
firewalling, as I could do that, and it really got rid of a lot of my 
headaches with just about all my public services.

Black listing would work in case of central dedicated anf large 
firewalls but for smaller solutions I think country white listing 
firewall is far better method.

What would also be interesting is something similar to the spamcop 
combined with crowdsec reporting system so that it can be used to 
effectively analyze and reduce all those bots.

The Spamhouse DROP list would also be a good permanent black list 
addition to any border routers or stand alone public services.

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