Re: [dmarc-ietf] Nitpicky questions about DMARC record syntax

ned+dmarc@mrochek.com Thu, 17 January 2019 13:07 UTC

Return-Path: <ned+dmarc@mrochek.com>
X-Original-To: dmarc@ietfa.amsl.com
Delivered-To: dmarc@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id B231612426E for <dmarc@ietfa.amsl.com>; Thu, 17 Jan 2019 05:07:12 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -2.001
X-Spam-Level:
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, SPF_PASS=-0.001] autolearn=ham autolearn_force=no
Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (1024-bit key) header.d=mrochek.com
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id M5vRi9mL0Y1i for <dmarc@ietfa.amsl.com>; Thu, 17 Jan 2019 05:07:11 -0800 (PST)
Received: from mauve.mrochek.com (mauve.mrochek.com [66.218.59.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 52000124BF6 for <dmarc@ietf.org>; Thu, 17 Jan 2019 05:07:11 -0800 (PST)
Received: from dkim-sign.mauve.mrochek.com by mauve.mrochek.com (PMDF V6.1-1 #35243) id <01R23UK29VR400GOXL@mauve.mrochek.com> for dmarc@ietf.org; Thu, 17 Jan 2019 05:02:08 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=mrochek.com; s=201712; t=1547730128; bh=VlM7UvfSy25nI6xQ3fNc2A+WmqnCom4EOxqbyY9p38w=; h=From:Cc:Date:Subject:In-reply-to:References:To:From; b=dw+rmy4gIb95/XzLr/nIvnNNfY1cUWmq1eVYduB2er9OyFK7PfTU3pFuern1nFiWT BeoCCbjGoycIt/mI2xPjxX556fn/hffJPUongZ/fONBpMkRSfcOFskGmeZly0QdQ7T M1JVzExIz7XggQ+Kx6A3+kKqbz8ATAtywFus0R3k=
MIME-version: 1.0
Content-transfer-encoding: 7BIT
Content-type: TEXT/PLAIN; CHARSET=US-ASCII
Received: from mauve.mrochek.com by mauve.mrochek.com (PMDF V6.1-1 #35243) id <01R1N39ADWKW00004L@mauve.mrochek.com> (original mail from NED@mauve.mrochek.com) for dmarc@ietf.org; Thu, 17 Jan 2019 05:02:05 -0800 (PST)
From: ned+dmarc@mrochek.com
Cc: Grant Taylor <gtaylor=40tnetconsulting.net@dmarc.ietf.org>, dmarc@ietf.org
Message-id: <01R23UK0KSK800004L@mauve.mrochek.com>
Date: Thu, 17 Jan 2019 05:00:01 -0800 (PST)
In-reply-to: "Your message dated Thu, 17 Jan 2019 00:50:07 +0300" <132ef6dc-86f0-3283-71fb-ea80d8800428@corp.mail.ru>
References: <20190116005804.A0A80200CACDA9@ary.qy> <b6d9024b-8a88-66fb-cfe7-800ee463c01c@gmail.com> <alpine.OSX.2.21.1901161029520.36401@ary.qy> <babe5ec6-9ceb-c7e1-1758-8dc20d116b55@gmail.com> <alpine.OSX.2.21.1901161050550.36401@ary.qy> <CABuGu1oqy8NxfpCZOu0v-z2D2MmZUfD43B3diGZ0xQtNwPD8EQ@mail.gmail.com> <alpine.OSX.2.21.1901161222030.38502@ary.qy> <11a5d635-a16b-17b9-0ba6-7713b8f169e2@spamtrap.tnetconsulting.net> <132ef6dc-86f0-3283-71fb-ea80d8800428@corp.mail.ru>
To: Vladimir Dubrovin <dubrovin=40corp.mail.ru@dmarc.ietf.org>
Archived-At: <https://mailarchive.ietf.org/arch/msg/dmarc/rrYTpUJbPzk-nFcg8CO7jkf2k6w>
Subject: Re: [dmarc-ietf] Nitpicky questions about DMARC record syntax
X-BeenThere: dmarc@ietf.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: "Domain-based Message Authentication, Reporting, and Compliance \(DMARC\)" <dmarc.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/dmarc>, <mailto:dmarc-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/dmarc/>
List-Post: <mailto:dmarc@ietf.org>
List-Help: <mailto:dmarc-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/dmarc>, <mailto:dmarc-request@ietf.org?subject=subscribe>
X-List-Received-Date: Thu, 17 Jan 2019 13:07:13 -0000

> I believe in the situation where standard is absolutely clear like this,
> any implementer must strictly follow the standard. Otherwise, it can
> lead to unpredictable behavior and security issues.

Sorry, I meant to mention this in my previous response. It's one thing
when there's wiggle-room or lack of clarity in the standard. It's quite
another when things are clear, as is the case here.

> Example: there are absolutely legal situations where non-trusted or less
> privileged side can partially control the name and/or content of the DNS
> record. I can provide users or customers with possibility to register a
> hostname and TXT record in my zone but I want to prevent them from
> corrupting or changing SPF/DMARC/etc policy. I can rely on filtering TXT
> record content. Non-standard behavior of DMARC implementation can allow
> to bypass this filtering.

> While this example doesn't seem realistic, I can demonstrate quite
> realistic ones. For example, a minor relaxation for ARC version check
> can lead to DKIM signature spoofing, compromising DMARC as a result. For
> DMARC DNS record itself, realistic scenario may appear in the future.

> We already have a lot of problems in-the-wild because of standards
> relaxations, e.g. it's usually possible to bypass DMARC which relies on
> RFC5322 via malformed From: due to fact invalid From: header is
> generally accepted and parsing From: header for DMARC validation and for
> visual representation is usually made by different pieces of code with
> different behavior.

Exactly. This is a security protocol and there are real security implications
of doing stuff like this. Another reason to Just Say No.

				Ned