Re: [TLS] Client certificate alerts

Hubert Kario <> Mon, 19 September 2016 13:00 UTC

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From: Hubert Kario <>
Date: Mon, 19 Sep 2016 15:00:26 +0200
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Subject: Re: [TLS] Client certificate alerts
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On Saturday, 17 September 2016 20:09:40 CEST David Benjamin wrote:
> Hi folks,
> We've run into some problems with client certificate alerts in Chrome that
> I'd like to fix going forward.
> The first is easy. handshake_failure is an unhelpful alert for server which
> required client certs. This confuses users, so we're planning to
> heuristically map it to a client certificate error if it's received after
> declining to send certs. This seems poor, so I've uploaded this PR which
> adds a certificate_required alert:


> Second, there is the access_denied alert. This alert has an unfortunate
> name. Per spec, it is for:
>    access_denied
>       A valid certificate [TLS1.3: or PSK] was received, but when access
> control was
>       applied, the sender decided not to proceed with negotiation.  This
>       message is always fatal.
> Accordingly, Chrome maps it to a client certificate error. But, by the name
> alone, it sounds much more general. We're seeing evidence of some kind of
> network filtering software mistakenly sending it to block a connection. (I
> can't blame them too much, since we ourselves mistook it for an alert to
> send in a server-side DoS handler! This has since been fixed.)
> From an unscientific survey of recent Chrome user complaints about this
> error, this is twice as prominent as actual client cert errors! So we'll
> want to add another heuristic: if we see access_denied without a
> CertificateRequest, map it to a more generic TLS error. This also seems
> worth fixing going forward.
> Does anyone actually send this alert? OpenSSL does not appear to.

I've done a quick survey of about 60 servers, given the different 
intolerancies and limits they appear to have. Just that 60 servers were 
handled by at least a dozen different implementations, 2 of which were unique 
and unless people already deployed 1.1.0 in droves, the dominant 
implementation didn't look like OpenSSL.

So I don't think we should guide ourselves with what OpenSSL does or doesn't 

> My best
> rename is certificate_denied if we drop 1.3's "or PSK", but perhaps we can
> remove the alert completely? You haven't gotten application data yet, so
> you don't really have anything to apply access control on, particularly for
> PSK. (For certs, there's post-handshake auth, but I would think you'd send
> an application-level error there?)

if it was already misused by multiple people, deprecating it and creating a 
new one probably is a good idea to make it unambiguous.

Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Red Hat Czech s.r.o., Purky┼łova 99/71, 612 45, Brno, Czech Republic