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Date: Wed, 04 Feb 2015 19:17:18 +0100
From: Florent Daigniere <>
Subject: Re: Apache 2.4 mod_ssl SSLSessionTickets -- others

On Wed, 2015-02-04 at 11:50 -0600, Mark Felder wrote:
> 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?

There's two different resumption mechanisms. Both involve accessing the
key material, what differs is where/how it's retained.

session-ids: The server assigns an id to the key material and sends the
id to the client. This requires the server to keep state but is widely
supported by all clients (SSLv3 does that already)

session-tickets: The server creates a ticket containing the encrypted
key material and asks the client to safekeep it for him. This doesn't
require the server to keep any state and therefore scales better... but
is a TLS extension that not all clients support.

In practice, even if key session-ids are in use, the key material might
leave the server (a very common setup is to have a memcache server store
the key material for all servers in a datacenter). This is configured
using SSLSessionCache.

> 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.

Session Tickets are about "making it scale in very large environment
(several datacenters)" or "when memory is limited (think embedded/IoT)"

>  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.

Yes. The problem doesn't just affect browsers... and it's not "just" PFS

Some people go out of their way to try to have more than 128bits of
security or to use non-AES based ciphers... their efforts are likely to
be vain if session tickets as OpenSSL issues them are issued.

> 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.

Don't get me wrong, I think that session tickets are a good thing and
that all clients should have support for it... but as far as the server
side implementations go, I agree with you; they shouldn't be enabled by

Whether it's Google, Facebook, Twitter, youname it, they all had to
reimplement their own session ticket magic; it hopefully does the right
thing (PFS wise).


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