Re: Channel binding being attempted even when SCRAM PLUS not advertized

[Simon Josefsson]

Manvendra Bhangui <mbhangui@gmail.com> writes:

> On Mon, 15 Aug 2022 at 14:32, Manvendra Bhangui <mbhangui@gmail.com> wrote:
>>
>> "Clients that do not support mechanism negotiation never use a "y"
>> gs2-cbind-flag, they use either "p" or "n" according to whether they
>> require and support the use of channel binding or whether they do not,
>> respectively."
>
> RFC 5802 isn't explicit on what to do when the client supports channel
> binding but the server does not.

For SMTP, the client should select a non-PLUS SCRAM mechanisms and not
supply libgsasl with a channel binding.  The server signals to the
client that it supports channel bindings by announcing PLUS mechanisms.
If the server supports negotiation, it will announce both.  It was the
downgrade-protection with 'y' that kicked in here.

/Simon

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