[CentOS] IMAP security

Ned Slider nedslider at f2s.com
Fri Mar 28 11:06:06 UTC 2008



Anne Wilson wrote:
> I have port 143 open so that I can get my mail when away from home.  
> Occasionally, though, my router reports things like
> 
> Thu, 2008-03-27 02:00:11 - TCP Packet - Source:200.122.134.9,3821 
> Destination:88.97.17.41,143 - [IMAP rule match]
> Thu, 2008-03-27 05:39:49 - TCP Packet - Source:140.127.181.141,3461 
> Destination:88.97.17.41,143 - [IMAP rule match]
> Thu, 2008-03-27 16:10:03 - TCP Packet - Source:80.88.161.125,2352 
> Destination:88.97.17.41,143 - [IMAP rule match]
> 

If you open ports, you will see folks scanning them - it's inevitable. A 
public mail server will attract interest from those wishing to exploit it.

> Looking at those addresses in whois, I don't see any good reason for these, 
> and I'm concerned in case they are relays.  Advice?
> 

Those looking for relays would be more interested in the smtp port 25. 
The IMAP port is the port you connect to to receive your mail. As long 
as your imap server (dovecot, courier-imap) is fully patched and 
presumably secure then you should be OK.

Advice - one potential weakness is that by default your username and 
password is likely being sent in plain text (not a good idea!). Someone 
could potentially intercept your username and password and access/use 
your email account. If that username/password is also your system 
account then potentially that could be compromised too.

There are a number of things you can do to harden your security. You 
could set up an additional user account with nologin for email so if the 
username/password does get compromised it's limited to purely email. You 
could run imap services on a non-standard port (security through 
obscurity), or firewall the connection to only allow trusted IP 
addresses (works if you always conect from known trusted IP addresses). 
None of these solutions are perfect, so probably the best method is to 
encrypt the connection using SSl. See howto here (for postfix/dovecot):

http://wiki.centos.org/HowTos/postfix_sasl

Hope that helps,

Ned




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