Disabling SMB1 by default

Rowland Penny rpenny at samba.org
Tue Jun 20 16:37:51 UTC 2017


On Tue, 20 Jun 2017 10:15:59 -0600
David Mulder via samba-technical <samba-technical at lists.samba.org>
wrote:

> 
> > Incorrect, there are two flavors of negotiate, the
> > downward-compatible SMB1-style, and the SMB2-only style. The style
> > is chosen by the client, and typically if SMB1 is not supported
> > there, the SMB2-only style will be sent.
> I meant that with SMB1 enabled in the samba client, it will always
> attempt SMB1 first with a multi-protocol negotiate.
> >
> > The server then selects the highest mutually-supported dialect, and
> > responds. If the selected dialect is SMB1, the SMB1 negotiate
> > response is sent. If SMB2, the SMB2 negotiate response is sent.
> >
> > The risk of supporting SMB1 in the client is that a MTIM can simply
> > downgrade the connection to the vulnerable SMB1 flavors, even when
> > the connection would support a higher version. So, completely
> > disabling the SMB1 support in the client is strongly recommended.
> More details here:
> https://blogs.technet.microsoft.com/staysafe/2017/05/17/disable-smb-v1-in-managed-environments-with-ad-group-policy/
> The risk of a downgrade attack is certainly a good reason to raise the
> default minimum dialect.
> But in order to avoid MITM attacks, we really would need to raise the
> minimum dialect to 3.0 to support secure negotiation (the spec says
> 3.0, but I seem to recall it actually works in 2.10 also), not just
> disable SMB1. Are we really ready to disable dialects below 3.0 by
> default? Even then, secure negotiation hasn't been implemented
> correctly in most clients, so pre-auth integrity would be better, and
> it's only available in 3.1.1.
> >> at the lowest supported and moves up. That's how the protocol is
> >> defined.
> > Again, not exactly. Servers simply select the highest mutually
> > supported dialect. There is no "move up" or "move down",
> > SMB2_NEGOTIATE is a single exchange.
> I specifically meant the multi-protocol negotiate, moving from a SMB1
> negotiate to SMB2, exactly as you've stated. You've just explained it
> much more elegantly :)
> >
> > The protocol specification is here:
> > 	https://technet.microsoft.com/en-us/windowsserver/cc246482.aspx
> >
> > Tom.
> >
> >
> >> So, enabling SMB3 by default will allow clients to negotiate up to
> >> SMB3 if supported, but will also continue to support older
> >> versions.
> >>
> >> On 06/20/2017 06:01 AM, Andreas Hasenack via samba-technical wrote:
> >>> On Mon, Jun 19, 2017 at 8:14 PM, Jeremy Allison <jra at samba.org>
> >>> wrote:
> >>>
> >>>> On Tue, Jun 20, 2017 at 10:20:07AM +1200, Andrew Bartlett via
> >>>> samba-technical wrote:
> >>>>> On Mon, 2017-06-19 at 15:39 +0200, Stefan Metzmacher via samba-
> >>>>> technical wrote:
> >>>>>> Hi Andreas,
> >>>>>>
> >>>>>>> we recently had a bug filed against Ubuntu [1] requesting
> >>>>>>> that we
> >>>> disable
> >>>>>>> the SMB1 protocol by default. That is part of a larger
> >>>>>>> campaign [2]
> >>>> to get
> >>>>>>> rid of SMB1 entirely.
> >>>>>>>
> >>>>>>> Has there been any discussion among Samba developers to
> >>>>>>> change the
> >>>> default
> >>>>>>> client and server min protocol level to SMB2? Would you
> >>>>>>> consider
> >>>> making
> >>>>>>> such a change?
> >>>>>> We're recently discussed changing 'client max protocol = SMB3'
> >>>>>> so that smbclient and other utilities work against servers
> >>>>>> with disabled SMB1 by default.
> >>>>>>
> >>>>>> We hope to get this into 4.7, but there's only about 3 weeks
> >>>>>> left to make this change (until 4.7.0rc1 is branched from
> >>>>>> master), so it's not sure if such a change will make it into
> >>>>>> 4.7.0 (released in September).
> >>>>> I had the dates as giving us 2 weeks.  Yes, there isn't much
> >>>>> time.
> >>>> Yeah, that's too short a time to do anything really. IMHO we
> >>>> just need to help people on the list to turn what they can
> >>>> off themselves for now, and work on how to do the migration
> >>>> properly over the next year or so.
> >>>>
> >>> What is the big issue with allowing the client to try SMB3 first?
> >>> Won't it fallback to SMB2, then NT1, and so on?
> >>>
> >>> Won't many server admins have disabled SMB1 in their windows
> >>> servers after the wannacry attack?
> >>>
> >> --
> >> David Mulder
> >> SUSE Labs Software Engineer - Samba
> >> dmulder at suse.com
> >> SUSE Linux GmbH, GF: Felix Imend├Ârffer, Jane Smithard, Graham
> >> Norton, HRB 21284 (AG N├╝rnberg)
> >>
> 

I think that there may be a problem if SMB1 is disabled, probably every
NT4-style domain will stop working after Samba is upgraded.

Is this the time to admit that we need to stop supporting them ?

Rowland



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