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Date: Wed, 04 Feb 2015 11:50:30 -0600
From: Mark Felder <feld@...d.me>
To: oss-security@...ts.openwall.com
Subject: Re: Apache 2.4 mod_ssl SSLSessionTickets -- others
vulnerable?
On Wed, Feb 4, 2015, at 10:55, Florent Daigniere wrote:
> On Wed, 2015-02-04 at 10:35 -0600, Mark Felder wrote:
> > From the 2.4.12 changelog:
> >
> >
> > *) mod_ssl: New directive SSLSessionTickets (On|Off).
> > The directive controls the use of TLS session tickets (RFC 5077),
> > default value is "On" (unchanged behavior).
> > Session ticket creation uses a random key created during web
> > server startup and recreated during restarts. No other key
> > recreation mechanism is available currently. Therefore using
> > session
> > tickets without restarting the web server with an appropriate
> > frequency
> > (e.g. daily) compromises perfect forward secrecy. [Rainer Jung]
> >
> >
> > So if you use Apache 2.4 and care about PFS protecting your data, you
> > should turn this feature off. This appears to be an implementation issue
> > because there is no other way for Apache to recreate keys. I don't know
> > a lot about the fine details of Session Tickets, but can anyone care to
> > comment if there are other known bad implementations of session tickets
> > out there? Does this affect Apache 2.2? Nginx? Lighttpd?
> >
> >
> > Thanks
> > I find this bizarre that a known security weakness like this is left
> > "on" by default...
>
> You're right, it's "bizarre"
>
> I've tried to make some noise about it two years ago [1] ...
>
> IMHO it's OpenSSL's default that should be changed. The server
> implementation shouldn't give a ticket if it's picked a PFS enabled
> cipher (or a cipher which aims at providing better security than
> AES128-CBC) unless explicitly told to do so (the case where there is
> more than one server).
>
> Apache HTTPd's new setting (SSLSessionTicketKeyFile), allowing you to
> set the ticket key is *DANGEROUS* as documented [1]. It encourages users
> explicitly to store the key on a forensically carvable medium...
> "The ticket key file contains sensitive keying material and should be
> protected with file permissions similar to those used for
> SSLCertificateKeyFile."
> Which is exactly what you shouldn't do!
>
Thanks for the details, Florent. After reviewing this blog post [1] it's
much clearer now, but I'm still a bit fuzzy on if "session caching" and
"session IDs" (RFC 5246, TLS 1.2) -- also as identified by Qualys
SSLLabs line item "Session resumption (caching)" -- are the same; is the
Session Cache caching session IDs? I only ask because I know that
webservers have had SSL Session Cache features for years, but RFC 5246
is TLS 1.2 in its entirety and I believe I've seen this feature predate
TLS 1.2. Was session caching / IDs always part of the SSL/TLS spec, now
superseded by the newer TLS 1.2 RFC?
If I'm understanding that correctly the following would be true: the use
of session caching is not a known vulnerability, but the use of session
tickets is a potential vulnerability. The design of the session tickets
(RFC 5077) appears to solve a specific problem: reducing expensive TLS
renegotiation when you have a cluster of servers and the session is not
guaranteed to stick to a specific server/load balancer. Additionally
OpenSSL lacks key rotation for session tickets, so it seems safe to
assume all software using OpenSSL with session tickets enabled are
likely not working around this problem by enforcing their own key
rotation.
This feels like a feature that should always be turned off unless your
environment absolutely requires it; especially if you have measurable
performance impact / negative client experience without it.
[1]
http://vincent.bernat.im/en/blog/2011-ssl-session-reuse-rfc5077.html
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