[Ach] Suggested Postfix config allows some weak ciphers - please review
cm at coretec.at
Sat May 3 12:53:45 CEST 2014
On Sat, May 03, 2014 at 09:37:10AM +0100, Thomas Preissler wrote:
> I had a go at your recommended Postfix settings. I am on Debian Wheezy,
> Postfix 2.9.6-2.
> When testing these settings with https://starttls.info/, I get the
> following report:
> Key exchange
> Anonymous Diffie-Hellman is accepted. This is suspectible to
> Man-in-the-Middle attacks.
> Weakest accepted cipher: 0.
I think the grading used by starttls.info is sub-optimal.
The issue is that nobody verifies certificates with opportunistic SMTP
encryption, which in turn means there is no security
against MITM anyways.
After all, it is *opportunistic* encryption, and whatever you do to
limit the choices would increase the risk of connections falling back
to good old plain text.
A quick analysis of my log files says:
63% of incoming and 84% of outgoing TLS connections are of the
anonymous DH variant.
Of the outgoing connections, only 10% are ones where a certificate
could be verified.
Disabling RC4 ciphers would lose 3% of the incoming and 0.04% of
outgoing TLS connections.
tl;dr: there's a reason the settings in the paper are what they are.
Christian Mock Wiedner Hauptstr. 15
Senior Security Engineer 1040 Wien
CoreTEC IT Security Solutions GmbH +43-1-5037273
FN 214709 z
CoreTEC: Web Application Audit - Damit so etwas nicht passiert!
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