Re: [OAUTH-WG] MTLS and in-browser clients using the token endpoint

Neil Madden <> Mon, 17 December 2018 21:52 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id D203B129AB8 for <>; Mon, 17 Dec 2018 13:52:16 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -2.001
X-Spam-Status: No, score=-2.001 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Authentication-Results: (amavisd-new); dkim=pass (1024-bit key)
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id rYdewga-nPq4 for <>; Mon, 17 Dec 2018 13:52:14 -0800 (PST)
Received: from ( [IPv6:2a00:1450:4864:20::433]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by (Postfix) with ESMTPS id B35E812426A for <>; Mon, 17 Dec 2018 13:52:13 -0800 (PST)
Received: by with SMTP id v13so13842019wrw.5 for <>; Mon, 17 Dec 2018 13:52:13 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=google; h=mime-version:subject:from:in-reply-to:date:cc :content-transfer-encoding:message-id:references:to; bh=vLEuXWiInsJLGl2GqYhpMf8XeusWuNKL+JnDQhsyGyU=; b=W1wb5n4KYvpPAaeHCckMYXgULXOcM0+lAAjgKO6g26nOqgEsQ5D5Wt+ngH/XnLJum3 199JmArpWSnBelAtM82ehYxSLSU7fNHCK0/b9ivrC1rKqstgNRjeZCSm42fBG+GUixxZ XDUCW++uFgqBbRh8F8/KbMI1di25dT2uTiqXc=
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20161025; h=x-gm-message-state:mime-version:subject:from:in-reply-to:date:cc :content-transfer-encoding:message-id:references:to; bh=vLEuXWiInsJLGl2GqYhpMf8XeusWuNKL+JnDQhsyGyU=; b=jBv7sFdAVvKrFFHPcoFEjP6P5pw630heXVMjp1R0xxO7m5ROJkTnAIoWdDPcZ3TLsn wye9lXBICMYGL1HgRh9vCbm5J3YOqT9tQWtscid3u3wYoslpTBrFy456b+2CK4Fd6a+O cLuF0Bkt4a4FT2zgG1uOMCUeeyOYy778pP+hBXqgBZlVd398pMQheDT95Q+2iPeHtG0B RsIIBEHNG/5CksdlTiQz4E+yvf3ZpoadP0tagQpoBu5Llf6GbSTelQ/mJwEZiMo5eSeB waxLX/KHmgtMXZO5inD5xwGZhzRk0F9JgAZY1L81Cctnt1N+s24eTS8fz/GFKLcpqBEp jIBA==
X-Gm-Message-State: AA+aEWaYNAzrSzgrgnrVPgwV3v0Ci6nDry9ZUuCtFHXpzgPs2mzGgN7I Oid/Gx243s2qK7mRFO9gasTNCenNykI=
X-Google-Smtp-Source: AFSGD/U/gNH4Sv/GtIWF6ffZfjptQgM6U+/+R5nrpOeLDzwlFdpRq5iVMltGnfSpnWLhpiOib1ODMQ==
X-Received: by 2002:adf:c38e:: with SMTP id p14mr12174540wrf.68.1545083531998; Mon, 17 Dec 2018 13:52:11 -0800 (PST)
Received: from guest2s-mbp.lan ( []) by with ESMTPSA id x20sm708179wme.6.2018. (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 17 Dec 2018 13:52:11 -0800 (PST)
Content-Type: text/plain; charset="utf-8"
Mime-Version: 1.0 (Mac OS X Mail 12.2 \(3445.102.3\))
From: Neil Madden <>
In-Reply-To: <>
Date: Mon, 17 Dec 2018 21:52:08 +0000
Cc: oauth <>
Content-Transfer-Encoding: quoted-printable
Message-Id: <>
References: <>
To: Brian Campbell <>
X-Mailer: Apple Mail (2.3445.102.3)
Archived-At: <>
Subject: Re: [OAUTH-WG] MTLS and in-browser clients using the token endpoint
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: OAUTH WG <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Mon, 17 Dec 2018 21:52:17 -0000

I am currently running a Tomcat instance that I have configured to support, but not demand, client certificates using the certificateVerification=“optionalNoCA” setting. With this config I am able to authenticate a confidential client using mTLS, and yet connecting to the same server over HTTPS in either Safari or Chrome on Mac does not prompt me for any certificate. I don’t have any client certificates configured in my browser, so does this only happen if you do?

Depending on the deployment scenario, it may also be possible to terminate TLS at a proxy and use a separate proxy for (intranet) mTLS clients vs public clients, but that may not suit every deployment.

— Neil

> On 17 Dec 2018, at 20:26, Brian Campbell <> wrote:
> While there's been some disagreement about the specific wording etc., there does seem to be general consensus coming out of this WG to, in one form or another, recommend against the use of the implicit grant in favor of authorization code. In order to follow that recommendation, in-browser JavaScript clients will need to use XHR/fetch (and likely CORS) to make requests directly to the token endpoint.
> Meanwhile there is the MTLS document utilizes TLS client certificates at the token endpoint for client authentication and/or certificate bound access tokens. The security BCP draft even recommends sender/key constrained access tokens and MTLS is close to the only viable way to do that at this time. 
> Unfortunately, however, these two things don't play very nice together. When a browser makes a TLS connection where a client cert is requested by the server in the handshake, even when client certificates are optional and even when it's fetch/XHR, most/many/all browsers will throw up some kind of certificate selection interface to the user.  Which is typically a very very bad user experience. From a practical standpoint, this means that a single deployment cannot really support the MTLS draft and have in-browser JavaScript clients using authorization code at the same time. 
> In order to address the conflict here, I'd propose that the MTLS draft introduce a new optional AS metadata parameter that is an MTLS enabled token endpoint alias. Clients that are doing MTLS client authentication and/or certificate bound access tokens would/should/must use the alternative token endpoint when present in the AS's metadata. While all other clients continue to use the standard token endpoint as they always have. This would allow for an AS to deploy an alternative token endpoint alias on a distinct host or port where it will request client certs in the TLS handshake for OAuth clients that use it while keeping the regular token endpoint as it normally is for other clients, especially in-browser JavaScript clients. 
> Thoughts, objections, agreements, etc., on this proposal?
> PS Bikeshedding on a name for the metadata parameter is also welcome. Some ideas to start:
> token_endpoint_mtls_alias
> token_endpoint_mtls
> mtls_token_endpoint_alias
> mtls_token_endpoint
> alt_token_endpoint_mtls
> mtls_token_endpoint_alt
> a_token_endpoint_that_a_client_wanting_to_do_mtls_stuff_a_la_RFC_[TBD]_should_use
> equally_poor_idea_here
> CONFIDENTIALITY NOTICE: This email may contain confidential and privileged material for the sole use of the intended recipient(s). Any review, use, distribution or disclosure by others is strictly prohibited..  If you have received this communication in error, please notify the sender immediately by e-mail and delete the message and any file attachments from your computer. Thank you._______________________________________________
> OAuth mailing list