safe renegotiation in client side

Tomas Mraz tmraz at
Mon Mar 15 22:31:30 CET 2010

On Mon, 2010-03-15 at 21:46 +0100, Nikos Mavrogiannopoulos wrote: 
> As you may have noticed there was a big fuss lately about a bug in the
> TLS protocol that could cause a client to connect to the wrong server
> via a renegotiation. There is a fix to the protocol that is
> unfortunately incompatible with previous versions (if security is
> required). Thus a gnutls client implementing the fix cannot connect to
> any non-patched server[0]. To achieve compatibility one has to to
> explicitly allow unsafe renegotiation with a priority string. This is
> not always possible since gnutls might be used unintentionally by a
> program via another library.
> With some trials in my system I noticed that the current behavior causes
> denial of service and a simple user might not even have control over the
> priority string for gnutls.
> Given your experiences (as system packager, user, implementor or so),
> what do you think is the adoption of priority strings in programs? Given
> a program that uses gnutls is it easy to set a string with the
> algorithms etc. needed for the negotiation?

The OpenSSL upstream decided to allow the client to talk to the
unpatched servers by default. Of course it means that if the client
talks to such server it is vulnerable to the attack. They've also added
a function call so an application can query whether the connection is
protected by the safe renegotiation or not.

I, as maintainer of OpenSSL and gnutls packages in Fedora and Red Hat
Enterprise Linux, decided when backporting the safe renegotiation
patches to the old gnutls packages in released distributions, that the
client has to be tolerant to missing safe renegotiation support on
connected servers for now and so I have removed the strict client side
check from the backported patches. If the adoption of the safe
renegotiation extension gets better, we will release updated packages
which will contain the strict client side check.
Tomas Mraz
No matter how far down the wrong road you've gone, turn back.
                                              Turkish proverb

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