At 9:38 AM -0500 5/10/04, Eric A. Hall wrote:
On 5/10/2004 3:02 AM, RL 'Bob' Morgan wrote:
So a "secure ports only" policy has very little to do with security and
very much to do with organizational power relationships, and making
your computing environment dysfunctional.
Somebody check my math on this please, but it seems to me that the whole
STARTTLS approach is succeptible to a specific attack which the secure
socket model is not.
Specifically, a man-in-the-middle can "blank out" the STARTTLS feature
advertisement, and thus make the client believe that TLS is not available.
For example:
server-A MitM client-C
| 250-DSN | 250-DSN
+--> 250-AUTH +-> 250-AUTH
250-STARTTLS 250 ok [...pad...]
250 ok
The client, seeing that TLS is not available, dumbs down to cleartext.
Most clients would probably do that invisibly without even barking at the
user, or not doing so in a way that most of them would appreciate.
Using an encrypted port just means an attack can only produce failure,
rather than inducing fallback.
Unless that's wrong for some reason, I'd say that a "secure ports policy"
actually is more secure.
In many cases, a client for a "secure ports policy" protocol will
fall back to the insecure port instead of telling the user "you can't
communicate". That's not true for HTTPS, but it is true for secure
POP, secure SMTP, and so on.
A man-in-the-middle can more easily block the secure port than he/she
can elide the STARTTLS messing in the client's start-up. STARTTLS is
harder to mount than blocking a "secure port". Thus, the "secure
ports policy" makes the MITM's job easier.
BTW, sometimes the MITM in the "secure ports only" scenario is a
clueless firewall admin at any point in the transit. The STARTTLS
method is not susceptible to this unintentional-but-still-deadly
downgrade attack.
Any client that is willing to back down to non-secure mode is
susceptible to a MITM attack, regardless of the protocol.
--Paul Hoffman, Director
--VPN Consortium
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