Re: [Openvpn-devel] Questions related to the SSL renegotiation vulnerability

[Dunc <du...@le...>]

I see,

Thanks very much for clearing that up James.

Cheers,

Dunc



James Yonan wrote:
> Well the problem is that even though OpenVPN doesn't rely on OpenSSL
> renegotiations, it does not explicitly disable them.  So to be safe,
> it's better to upgrade to the fixed version of OpenSSL (0.9.8l).
> 
> Also note that using tls-auth prevents the cited MITM attack
> (CVE-2009-3555) even when using a vulnerable version of OpenSSL, as long
> as the MITM attacker doesn't possess the tls-auth key.
> 
> James
> 
> Dunc wrote:
>> Hi James,
>>
>> Thanks for getting back to me.
>>
>> I was starting to wonder the same myself, but when I found this thread
>>
>> http://article.gmane.org/gmane.network.openvpn.user/28105
>>
>> I thought I must be missing something.
>>
>> So if OpenVPN always uses a new session, what would be the point of
>> adding an option to disable renegotiation at the server side?
>>
>>
>> Cheers,
>>
>> Dunc
>>
>>
>> James Yonan wrote:
>>> OpenVPN uses a fresh SSL/TLS session for each of its mid-session
>>> renegotiations.  This means that when you see:
>>>
>>>   TLS: soft reset sec=0 bytes=314/0 pkts=6/0
>>>
>>> OpenVPN is actually creating a brand new SSL/TLS session.  So the
>>> important point here is that OpenVPN does not rely on the session
>>> renegotiation capability that is built into SSL/TLS, and therefore if
>>> OpenVPN is linked against an OpenSSL library that disables SSL/TLS
>>> renegotiation, there should be no loss of functionality.
>>>
>>> James
>>>
>>> Dunc wrote:
>>>> Hi all,
>>>>
>>>> Apologies in advance if I'm just not understanding something here.
>>>>
>>>> Following on from the recent SSL renegotiation problem, we're assessing
>>>> what we should do with all our SSL services, and as we use OpenVPN in
>>>> several places, this is on the list.
>>>>
>>>> I thought that OpenVPN does renegotiations when re-keying, so at
>>>> first I
>>>> thought I'd try and turn it off at the server end. From reading the
>>>> docs
>>>> and testing I now know that it's not good enough as by default clients
>>>> will want to re-key after 1 hour unless it is turned off in the client
>>>> config too.
>>>>
>>>> It might be hard to ensure that all our customers adjust their config
>>>> properly, so I'd rather deal with this at the server end only, so my
>>>> next thought was to install openssl-0.9.8l which bans renegotiation. I
>>>> figured this would make the VPN drop once an hour, but figured that's
>>>> not so bad in the grand scheme of things, and if it's really a problem
>>>> for anyone we can fix it by having them adjust the client config. This
>>>> was round seems more favourable as I can be sure renegotiations are
>>>> disabled, and work around the fallout.
>>>>
>>>> So, I installed the latest openssl on a test box, and compiled openvpn.
>>>> I set the reneg-sec option to 40s on my client and fired up the VPN,
>>>> fully expecting it to bounce after 40s. Instead, what I see is this
>>>> message in the logs:-
>>>>
>>>> Nov 11 14:13:51 2009 us=763149 TLS: soft reset sec=0 bytes=314/0
>>>> pkts=6/0
>>>>
>>>> and then both ends seem to agree on some new crypto, and everything
>>>> carries on.
>>>>
>>>>
>>>> At first I thought maybe what OpenVPN does isn't the same as SSL
>>>> renegotiation and I had no need to worry anyway, but then I found this
>>>> thread...
>>>>
>>>> http://article.gmane.org/gmane.network.openvpn.user/28105
>>>>
>>>> where there is discussion about adding an option to openvpn to disable
>>>> it, so I now think I should indeed be concerned, but I must be missing
>>>> something obvious, and wondered if anybody here can help me.
>>>>
>>>> I've checked with openssl s_server and s_client that my new openssl
>>>> does
>>>> indeed ban renegotiation, so I wonder exactly what OpenVPN is doing
>>>> during rekeying.
>>>>
>>>> Thanks in advance if anyone can shed light on this for me, and once
>>>> again sorry if I'm just misunderstanding, which is quite possible :-)
>>>>
>>>> Cheers,
>>>>
>>>> Dunc
>>>>
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