Re: [secdir] Security review of draft-ietf-netmod-yang-json-08

Ladislav Lhotka <lhotka@nic.cz> Tue, 08 March 2016 15:31 UTC

Return-Path: <lhotka@nic.cz>
X-Original-To: secdir@ietfa.amsl.com
Delivered-To: secdir@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 6EF3912D793; Tue, 8 Mar 2016 07:31:18 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.9
X-Spam-Level:
X-Spam-Status: No, score=-1.9 tagged_above=-999 required=5 tests=[BAYES_00=-1.9] autolearn=ham autolearn_force=no
Received: from mail.ietf.org ([127.0.0.1]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id TioA4bekokGh; Tue, 8 Mar 2016 07:31:14 -0800 (PST)
Received: from trail.lhotka.name (trail.lhotka.name [77.48.224.143]) by ietfa.amsl.com (Postfix) with ESMTP id AB96D12D791; Tue, 8 Mar 2016 07:31:13 -0800 (PST)
Received: from localhost (unknown [195.113.220.110]) by trail.lhotka.name (Postfix) with ESMTPSA id E73A41CC035F; Tue, 8 Mar 2016 16:31:11 +0100 (CET)
From: Ladislav Lhotka <lhotka@nic.cz>
To: Hilarie Orman <hilarie@purplestreak.com>, iesg@ietf.org, secdir@ietf.org
In-Reply-To: <201603072216.u27MG09g002861@rumpleteazer.rhmr.com>
References: <201603072216.u27MG09g002861@rumpleteazer.rhmr.com>
User-Agent: Notmuch/0.21 (http://notmuchmail.org) Emacs/24.4.51.2 (x86_64-apple-darwin14.0.0)
Date: Tue, 08 Mar 2016 16:31:09 +0100
Message-ID: <m2r3flouki.fsf@birdie.labs.nic.cz>
MIME-Version: 1.0
Content-Type: text/plain
Archived-At: <http://mailarchive.ietf.org/arch/msg/secdir/flu0zRMahnQWBvcssygHgAKxnMw>
X-Mailman-Approved-At: Tue, 08 Mar 2016 07:35:15 -0800
Cc: draft-ietf-netmod-yang-json.all@tools.ietf.org
Subject: Re: [secdir] Security review of draft-ietf-netmod-yang-json-08
X-BeenThere: secdir@ietf.org
X-Mailman-Version: 2.1.17
Precedence: list
List-Id: Security Area Directorate <secdir.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/secdir>, <mailto:secdir-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/secdir/>
List-Post: <mailto:secdir@ietf.org>
List-Help: <mailto:secdir-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/secdir>, <mailto:secdir-request@ietf.org?subject=subscribe>
X-List-Received-Date: Tue, 08 Mar 2016 15:31:19 -0000

Hi Hilarie,

thank you for the review, please se my responses inline.

Hilarie Orman <hilarie@purplestreak.com> writes:

>                      Security review of
> JSON Encoding of Data Modeled with YANG draft-ietf-netmod-yang-json-08
>
> Do not be alarmed.  I have reviewed this document as part of the
> security directorate's ongoing effort to review all IETF documents
> being processed by the IESG.  These comments were written primarily
> for the benefit of the security area directors.  Document editors and
> WG chairs should treat these comments just like any other last call
> comments.
>
> "This document defines encoding rules for representing configuration,
> state data, parameters of RPC operations or actions, and notifications
> defined using YANG as JavaScript Object Notation (JSON) text.  YANG is
> a data modeling language originally designed to model configuration
> and state data manipulated by the Network Configuration Protocol
> (NETCONF), NETCONF remote procedure calls, and NETCONF notifications."
>
> I have no specific recommendation for an action on this, just
> some observations.
>
> I'm not an expert on YANG, JSON, or XML, but I was taken aback to read
> in section 5.5:
>
>      Anydata data node serves as a container for an arbitrary set of nodes
>      that otherwise appear as normal YANG-modeled data.  A data model for
>      anydata content may or may not be known at run time.  In the latter
>      case, converting JSON-encoded instances to the XML encoding defined
>      in [I-D.ietf-netmod-rfc6020bis] may be impossible.

An "anydata" node represents data for which no schema is specified in
the data model. The schema for such data may be known at run time, but
the consensus in the NETMOD WG was to keep the possibility that this is
not the case. And if the schema is not available, translation between
XML and JSON encodings cannot be done because, for example, we use the
data model info for translating namespace identifiers.

As long as we want to keep the option of specifying schema-less data in
the data model, we have to live with this. I don't think it is really
ominous, also because normal configuration and state data should almost
never be modelled with "anydata".

>
> That seems ominous, and there are other warnings about the force
> fitting of JSON and XML:
>
>    "JSON processing is rather different from XML, and JSON parsers may
>    thus suffer from other types of vulnerabilities than their XML
>    counterparts.  To minimize these new security risks, software on the
>    receiving side SHOULD reject all messages that do not comply to the
>    rules of this document and reply with an appropriate error message to
>    the sender."

Implementors might be tempted to apply the Postel Principle and be
liberal on the receiving side. This paragraph warns against doing so,
unless, of course, the implementor knows what he or she is doing.

>
> The security section refers back to the security considerations in
> https://tools.ietf.org/html/draft-ietf-netmod-rfc6020bis-11
> section 16 (should be 17) where we read:
>
>    "Data modeled in YANG might contain sensitive information.  RPCs or
>    notifications defined in YANG might transfer sensitive information."
>
>    "YANG parsers need to be robust with respect to malformed documents.
>    Reading malformed documents from unknown or untrusted sources could
>    result in an attacker gaining privileges of the user running the YANG
>    parser.  In an extreme situation, the entire machine could be
>    compromised."
>
> There being no succinct description of correctness of YANG, JSON, or
> XML for NETCONF data, how would one determine that any of it,
> including mappings from one to another, was "robust"?  If that simply

In fact, it is one of the most important virtues of data modelling that
the correctness of configuration or state data can be reliably
verified. In the context of draft-ietf-netmod-yang-json, correctness has
two aspects:

1. All data is required to be I-JSON compliant [RFC 7493] which helps
   avoid common JSON-specific interoperability and security problems.

2. The data model precisely specifies the schema, data types of all
   values, and also semantic constraints.

If an implementor pays attention to data model definitions (including
textual descriptions), then I believe the risk of any data-induced
issues is effectively minimised.

> means "doesn't cause a buffer overflow or crash", I suppose it's
> achievable (and should be explicit).  But how could anyone be sure
> that sensitive data was not leaked without a full analysis of the
> specifications of all the component parts?  Perhaps this is an
> unaddressable question, but one does hope that the extreme situation
> does not occur.
>

This question isn't specific to this document - sensitive data may be
present in any encoding. It is also one of the duties of a good data
model to identify such data, and sec. 6 in RFC 6087 puts forward specific
requirements in this direction.

Thanks, Lada

-- 
Ladislav Lhotka, CZ.NIC Labs
PGP Key ID: E74E8C0C