[tor-talk] New SSLv3 attack: Turn off SSLv3 in your TorBrowser

Nick Mathewson nickm at torproject.org
Wed Oct 15 02:15:26 UTC 2014

Hi!  It's a new month, so that means there's a new attack on TLS.

This time, the attack is that many clients, when they find a server
that doesn't support TLS, will downgrade to the ancient SSLv3.  And
SSLv3 is subject to a new padding oracle attack.

There is a readable summary of the issue at
https://www.imperialviolet.org/2014/10/14/poodle.html .

Tor itself is not affected: all released versions for a long time have
shipped with TLSv1 enabled, and we have never had a fallback mechanism
to SSLv3. Furthermore, Tor does not send the same secret encrypted in
the same way in multiple connection attempts, so even if you could
make Tor fall back to SSLv3, a padding oracle attack probably wouldn't
help very much.

TorBrowser, on the other hand, does have the same default fallback
mechanisms as Firefox.  I expect and hope the TorBrowser team will be
releasing a new version soon with SSLv3 enabled.  But in the meantime,
I think you can disable SSLv3 yourself by changing the value of the
"security.tls.version.min" preference to 1.

To do that:

1.  enter "about:config" in the URL bar.

2. Then you click "I'll be careful, I promise".

3. Then enter "security.tls.version.min" in the preference "search"
field underneath the URL bar.  (Not the search box next to the URL

4. You should see an entry that says "security.tls.version.min" under
"Preference Name".  Double-click on it, then enter the value "1" and
click okay.

You should now see that the value of "security.tls.version.min" is set to one.

(Note that I am not a Firefox developer or a TorBrowser developer: if
you're cautious, you might want to wait until one of them says
something here before you try this workaround.)

Obviously, this isn't a convenient way to do this; if you are
uncertain of your ability to do so, waiting for an upgrade might be a
good move.  In the meantime, if you have serious security requirements
and you cannot disable SSLv3, it might be a good idea to avoid using
the Internet for a week or two while this all shakes out.

best wishes to other residents of interesting times,

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