Re: Blocking POP3 and IMAP

[Shumon Huque <shuque () ISC UPENN EDU>]

On Thu, Oct 11, 2007 at 01:33:27PM -0500, Ken Connelly wrote:
> We allow POP and IMAP, but require that they be the secure
> implementations of each.  Those run on TCP/995 and TCP/993 by default.
> We try very hard to eliminate any plain-text authentication scheme,
> including within our on-campus, switched network.
>
> -ken

It is worth noting that the recommended way to support SSL/TLS for
these applications (and generally any application) is not to use an
alternate port, but to make use of the STARTTLS extension on the
standard ports (143, 110 etc). It is possible to configure servers
to allow plaintext mechanisms only after STARTTLS has been successfully
negotiated.

Here's what RFC 2595 ( "Using TLS with IMAP, POP3 and ACAP"
http://www.ietf.org/rfc/rfc2595.txt ) has to say about this:

--Shumon.

Excerpt:

7. imaps and pop3s ports

   Separate "imaps" and "pop3s" ports were registered for use with SSL.
   Use of these ports is discouraged in favor of the STARTTLS or STLS
   commands.

   A number of problems have been observed with separate ports for
   "secure" variants of protocols.  This is an attempt to enumerate some
   of those problems.

   - Separate ports lead to a separate URL scheme which intrudes into
     the user interface in inappropriate ways.  For example, many web
     pages use language like "click here if your browser supports SSL."
     This is a decision the browser is often more capable of making than
     the user.

   - Separate ports imply a model of either "secure" or "not secure."
     This can be misleading in a number of ways.  First, the "secure"
     port may not in fact be acceptably secure as an export-crippled
     cipher suite might be in use.  This can mislead users into a false
     sense of security.  Second, the normal port might in fact be
     secured by using a SASL mechanism which includes a security layer.
     Thus the separate port distinction makes the complex topic of
     security policy even more confusing.  One common result of this
     confusion is that firewall administrators are often misled into
     permitting the "secure" port and blocking the standard port.  This
     could be a poor choice given the common use of SSL with a 40-bit
     key encryption layer and plain-text password authentication is less
     secure than strong SASL mechanisms such as GSSAPI with Kerberos 5.

   - Use of separate ports for SSL has caused clients to implement only
     two security policies: use SSL or don't use SSL.  The desirable
     security policy "use TLS when available" would be cumbersome with
     the separate port model, but is simple with STARTTLS.

   - Port numbers are a limited resource.  While they are not yet in
     short supply, it is unwise to set a precedent that could double (or
     worse) the speed of their consumption.