OPSAWG Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Updates: 8520 (if approved)                                       W. Pan
Intended status: Best Current Practice               Huawei Technologies
Expires: 23 August 2021                                          E. Lear
                                                           Cisco Systems
                                                        19 February 2021

                     Authorized update to MUD URLs


   This document provides a way for an RFC8520 Manufacturer Usage
   Description (MUD) definitions to declare what are acceptable
   replacement MUD URLs for a device.

   RFCEDITOR-please-remove: this document is being worked on at:

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 23 August 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components

Richardson, et al.       Expires 23 August 2021                 [Page 1]

Internet-Draft             mud-acceptable-urls             February 2021

   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Updating the MUD files in place . . . . . . . . . . . . . . .   3
     2.1.  Adding capabilities . . . . . . . . . . . . . . . . . . .   3
     2.2.  Removing capabilities . . . . . . . . . . . . . . . . . .   4
     2.3.  Significant changes to protocols  . . . . . . . . . . . .   4
     2.4.  Motivation for updating MUD URLs  . . . . . . . . . . . .   5
   3.  Updating the MUD URLs . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Leveraging the manufacturer signature . . . . . . . . . .   6
     3.2.  Concerns about same-signer mechanism  . . . . . . . . . .   6
   4.  Proposed mechanism  . . . . . . . . . . . . . . . . . . . . .   7
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     6.1.  Updating files vs Updating MUD URLs . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Appendices . . . . . . . . . . . . . . . . . . . . .  11
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   [RFC8520] provides a standardized way to describe how a specific
   purpose device makes use of Internet resources and associated
   suggested network behavior, which are described in a MUD file hosted
   in its manufacturer's server.  By providing a MUD URL by the device,
   the network manager can locate this MUD file and determine the
   required network authorization of the device.

   In some cases, e.g., the firmware update, the network behaviors of
   the device may change, and the description in the original MUD file
   will no longer apply.  To solve this problem, there are two common
   ways which the manufacturer can use.

   One is to change what is in the MUD file, i.e., update the MUD file
   in place, whenever the behavior of the firmware changes.
   {updatemudfiles} discusses three scenarios for updating the MUD file
   and the corresponding potential issues.

Richardson, et al.       Expires 23 August 2021                 [Page 2]

Internet-Draft             mud-acceptable-urls             February 2021

   The other is to change which MUD file is processed by changing the
   MUD URL. {updatemudurls} describes the common sources of MUD URLs and
   the problems and threats faced by each type of source when updating
   the MUD URL.  This document proposes an enhanced mechanism of how to
   securely update the MUD URL in {proposedmechanism}.

   There are also some assumptions and prerequisites in this document.

   While MUD files may include signatures, [RFC8520] does not mandate
   checking them, and there is not a clear way to connect the entity
   which signed the MUD file to the device itself.  This document limits
   itself to situations in which the MUD file is signed, and that the
   MUD controller has been configured to always check the signatures,
   rejecting files whose signatures do not match.

   [RFC8520] does not specify how MUD controllers establish their trust
   in the manufacturers' signing key: there are many possible solutions
   from manual configuration of trust anchors, some kind of automatic
   configuration during onboarding, but also including to Trust on First
   Use (TOFU).  How this initial trust is established is not important
   for this document, it is sufficient that some satisfactory initial
   trust is established.

2.  Updating the MUD files in place

   Three scenarios for updating the MUD file and the corresponding
   potential issues are discussed below.

2.1.  Adding capabilities

   For situations where new capabilities are added to the firmware, the
   MUD file will detail the new access that the new firmware requires.
   This may involve new incoming or outgoing connections that should be
   authorized.  Devices that have been upgraded to the new firmware will
   make use of the new features.  Devices that have not been upgraded to
   the new firmware may have new connections that are authorized, but
   which the device does not use (outgoing connections), or which the
   device is not prepared to respond to (new incoming connections).

   It is possible that older versions of the firmware have
   vulnerabilities that were not easily exploitable due to the MUD file
   preventing particular kinds of access.  For example, an older
   firmware could have no credentials required (or default credentials)
   access via telnet on port 23 or HTTP on port 80.  The MUD file
   protected the device such that it could either not be accessed at
   all, or access was restricted to connections from a controller only.

Richardson, et al.       Expires 23 August 2021                 [Page 3]

Internet-Draft             mud-acceptable-urls             February 2021

   Useful and needed upgrades to the firmware could add credentials to
   that service, allowing it to be opened up for more general access.
   The new MUD file would provide for such access, but when combined
   with the weak security of the old firmware, it results in a
   compromised device.

   While there is an argument that old firmware was insecure and should
   be replaced, it is often the case that the upgrade process involves
   downtime, or can introduce risks due to needed evaluations not having
   been completed yet.  As an example: moving vehicles (cars, airplanes,
   etc.) should not perform upgrades while in motion!  It is probably
   undesirable to perform any upgrade to an airplane outside of its
   service facility.  The vehicle owner may desire only to perform
   software upgrades when they are at home, and could make other
   arrangements for transportation, rather than when parked at a remote
   cabin.  The situation for upgrades of medical devices has even more
   considerations involving regulatory compliance.

2.2.  Removing capabilities

   For situations where existing capabilities prove to be a problem and
   are to be turned off or removed in subsequent versions of the
   firmware, the MUD file will be updated to disallow connections that
   previously were allowed.

   In this case, the new MUD file will forbid some connections, which
   the old firmware still expects to do.  As explained in the previous
   section, upgrades may not always occur immediately upon releasing the
   new firmware.

   In this case, the old device will be performing unwanted connections,
   and the MUD controller will be alerting the network owner that the
   device is misbehaving rather than not upgraded.  This causes a false-
   positive situation (see [boycrieswolf]), leading to real security
   issues being ignored.  This is a serious issue as documented also in
   [boywolfinfosec], and [falsemalware].

2.3.  Significant changes to protocols

   [I-D.ietf-opsawg-mud-tls] suggests MUD definitions to allow
   examination of TLS protocol details.  Such a profile may be very
   specific to the TLS library which is shipped in a device.  Changes to
   the library (including bug fixes) may cause significant changes to
   the profile, requiring changes to the profile described in the MUD
   file.  Such changes are likely neither forward nor backward
   compatible with other versions, and in place updates to MUD files are
   therefore not indicated.

Richardson, et al.       Expires 23 August 2021                 [Page 4]

Internet-Draft             mud-acceptable-urls             February 2021

2.4.  Motivation for updating MUD URLs

   While many small tweaks to a MUD file can be done in place, the
   situation described above, particularly when it comes to removing
   capabilities will suggest that changes to the MUD URL.  A strategy
   for doing this securely is needed, and the rest of this document
   provides a mechanism to do this securely.

3.  Updating the MUD URLs

   MUD URLs can come from a number of sources:

   *  IDevID Extensions

   *  DHCP option

   *  LLDP TLV

   *  [I-D.richardson-mud-qrcode] proposes to scan them from QRcodes.

   The IDevID mechanism provides a URL that is asserted
   cryptographically by a manufacturer.  However, it is difficult for
   manufacturers to update the IDevID of a device which is already in a

   The DHCP and LLDP mechanisms are not signed, but are asserted by the
   device.  A firmware update may update what MUD URL is emitted.
   Sufficiently well targeted malware could also change the MUD URL.

   The QRcode mechanism is usually done via paper/stickers, and is
   typically not under the control of the device itself at all.
   However, being applied by a human and not easily changed, a MUD URL
   obtained in this fashion is likely trustworthy.  (It may not, due to
   mixups in labeling represent the correct device, but this is a human
   coordination issue, and is out of scope for this document.)

   The manufacturer can use all the four mechanisms above when
   manufacturing the device.  But when considering updating the
   firmware, it seems like only the DHCP and LLDP mechanisms are
   sufficiently easy to send the new MUD URL.  Because of that
   sensitivity, they may also be easily changed by malware!

   There are mitigating mechanisms which may be enough to deal with this
   problem when MUD files are signed by the manufacturer.

   While [RFC8520] has established a mechanism for signing of MUD files,
   the document does not define a way for a MUD controller to determine
   who should sign the MUD file for a particular device.

Richardson, et al.       Expires 23 August 2021                 [Page 5]

Internet-Draft             mud-acceptable-urls             February 2021

   [RFC8520] leaves this for a local policy.  There are a number of
   processes that could be used, but they require coordination of many
   players.  It is expected that each industrial vertical will work out
   supply chain arrangements or other heuristics.

3.1.  Leveraging the manufacturer signature

   When the first time a signature of the MUD file related to a
   particular device-type is verified by the MUD controller, the
   identity of the signing authority is recorded.  It is pinned.
   Subsequent MUD files must be signed by the same entity in order to be

   The trust and acceptance of the first signer may come from many
   sources, for example, it could be manual configured to trust which
   signer, or using the IDevID mechanism for the first MUD URL and the
   signer of the corresponding MUD file is more trustworthy, or the MUD
   controller can use a Trust on First Use (TOFU) mechanism and trusts
   the first signer by default.

   Based upon this process, an update to the MUD URL would be valid if
   the new MUD file was signed by the same entity that signed the
   previous entry.  This mechanism permits a replacement URL to be any
   URL that the same manufacturer can provide.

3.2.  Concerns about same-signer mechanism

   There is still a potential threat: a manufacturer which has many
   products may have a MUD definition for another product that has the
   privileges that the malware desires.

   The malware could simply change the expressed MUD URL to that of the
   other product, and it will be accepted by the MUD controller as

   This works as long as manufacturers use a single key to sign all
   products.  Some manufacturers could sign each product with a
   different key.  Going logically down this path, if all these product
   keys are collected into a single PKI, signed by a common
   certification authority.

   In this case, the question then becomes whether the MUD controller
   should pin the End-Entity (EE) certificate, or the CA certificate.

   Pinning the End-Entity (EE) certificate defends against malware that
   changes the product type, but keeps the manufacturer from being able
   to cycle the validity of the End-Entity Certificate for cryptographic
   hygiene reasons.

Richardson, et al.       Expires 23 August 2021                 [Page 6]

Internet-Draft             mud-acceptable-urls             February 2021

   Pinning the CA certificate allows the EE certificate to change, but
   may not defend against product type changes.

   It is possible to invent policy mechanisms that would link the EE
   certificate to a value that is in the MUD file.  This could be a
   policy OID, or could involve some content in a subjectAltName.
   Future work could go in this direction.  This document proposes a
   simpler solution.

4.  Proposed mechanism

   The document proposes to limit what MUD URLs are considered valid
   from the device, limiting new MUD URLs to be variations of the
   initial (presumed to be secure) URL.

   The first MUD file which is defined for a device can come from an
   IDevID (which is considered more secure), or via Trust on First Use
   with DHCP or LLDP or other mechanisms.  This first, initially
   trusted, MUD file will be called the "root" MUD file.

   A MUD file contains a self-referential MUD-URL attribute that points
   to the MUD file itself located on the vendor's website.  While the
   IDevID, DHCP and LLDP mechanisms only transmit a URL, there are some
   newer, not yet standardized proposals that would fetch an entire MUD
   file from the device, such as [I-D.jimenez-t2trg-mud-coap].

   The MUD-URL MUST always be an Absolute URI: see [RFC3986] section

   The URL found in the MUD-URL attribute is to be called the canonical
   MUD URL for the device.

   The MUD-SIGNATURE attribute in the MUD file SHOULD be a relative URI
   (see [RFC3986] section 4.2) with the (hierarchical) base URI for this
   reference being the MUD-URL attribute.

   Subsequent MUD files are considered valid if:

   *  they have the same initial Base-URI as the MUD-URL, but may have a
      different final part

   *  they are signed by the same End Entity (same trusted CA and same
      subjectAltName) as the "root" MUD file.

   Section 5.2 of [RFC3986] details many cases for calculating the Base-
   URI.  The test is simplified to: remove everything to the right of
   the last (rightmost) "/" in the URL of "root" MUD file URL, and the
   proposed new URL.  The resulting two strings MUST be identical.

Richardson, et al.       Expires 23 August 2021                 [Page 7]

Internet-Draft             mud-acceptable-urls             February 2021

   For a simple example, if the "root" MUD-URL is
   http://example.com/hello/there/file.json then any URL that starts
   with http://example.com/hello/there/ would be acceptable, such as

   Once the new MUD file is accepted, then it becomes the new "root" MUD
   file, and any subsequent updates MUST be relative to the MUD-URL in
   the new file.

   This process allows a manufacturer to rework its file structure, to
   change web server host names (such as when there is an acquisition or
   merger), etc. so long as they retain the old structure long enough
   for all devices to upgrade at least once.

   (XXX: how should the trust anchor for the signature be updated when
   there is Merger&Acquisition)

5.  Privacy Considerations

   The MUD URL contains sensitive model and even firmware revision
   numbers.  Thus the MUD URL identifies the make, model and revision of
   a device.  [RFC8520] already identifies this privacy concern, and
   suggests use of TLS so that the HTTP requests that retrieve the MUD
   file do not divulge that level of detail.  However, it is possible
   that even observing the traffic to that manufacturer may be
   revealing, and [RFC8520] goes on to suggest use of a proxy as well.

6.  Security Considerations

   Prior to the standardization of the process in this document, if a
   device was infiltrated by malware, and said malware wished to make
   accesses beyond what the current MUD file allowed, the the malware
   would have to:

   1.  arrange for an equivalent MUD file to be visible somewhere on the

   2.  depend upon the MUD controller either not checking signatures, or

   3.  somehow get the manufacturer to sign the alternate MUD

   4.  announce this new URL via DHCP or LLDP, updating the MUD
       controller with the new permissions.

   One way to accomplish (3) is to leverage the existence of MUD files
   created by the manufacturer for different classes of devices.  Such
   files would already be signed by the same manufacturer, eliminating
   the need to spoof a signature.

Richardson, et al.       Expires 23 August 2021                 [Page 8]

Internet-Draft             mud-acceptable-urls             February 2021

   With the standardization of the process in this document, then the
   attacker can no longer point to arbitrary MUD files in step 4, but
   can only make use of MUD files that the manufacturer has already
   provided for this device.

   Manufacturers are advised to maintain an orderly layout of MUD files
   in their web servers, with each unique product having its own

   The process described updates only MUD controllers and the processes
   that manufacturers use to manage the location of their MUD files.

   A manufacturer which has not managed their MUD files in the the way
   described here can deploy new directories of per-product MUD files,
   and then can update the existing MUD files in place to point to the
   new URLs using the MUD-URL attribute.

   There is therefore no significant flag day: MUD controllers may
   implement the new policy without significant concern about backwards

6.1.  Updating files vs Updating MUD URLs

   Device developers need to consider whether to make a change by
   updating a MUD file, or updating the MUD URL.

   MUD URLs can only be updated by shipping a new firmware.  It is
   reasonable to update the MUD URL whenever a new firmware release
   causes new connectivity to be required.  The updated mechanism
   defined in this document makes this a secure operation, and there is
   no practical limitation on the number of files that a web server can

   In place updates to a MUD file should be restricted to cases where it
   turns out that the description was inaccurate: a missing connection,
   an inadvertent one authorized, or just incorrect information.

   Developers should be aware that many enterprise web sites use
   outsourced content distribution networks, and MUD controllers are
   likely to cache files for some time.  Changes to MUD files will take
   some time to propagate through the various caches.  An updated MUD
   URL will however, not experience any cache issues, but can not be
   deployed with a firmware update.

7.  References

7.1.  Normative References

Richardson, et al.       Expires 23 August 2021                 [Page 9]

Internet-Draft             mud-acceptable-urls             February 2021

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,

7.2.  Informative References

              "The Boy Who Cried Wolf", 18 January 2020,

              "Security Alerts - A Case of the Boy Who Cried Wolf?", 18
              January 2020, <https://www.infosecurity-

              "False malware alerts cost organizations $1.27M annually,
              report says", 18 January 2020,
              says/ and http://go.cyphort.com/Ponemon-Report-Page.html>.

              Reddy.K, T., Wing, D., and B. Anderson, "Manufacturer
              Usage Description (MUD) (D)TLS Profiles for IoT Devices",
              Work in Progress, Internet-Draft, draft-ietf-opsawg-mud-
              tls-04, 17 January 2021, <http://www.ietf.org/internet-

              Jimenez, J., "Using MUD on CoAP environments", Work in
              Progress, Internet-Draft, draft-jimenez-t2trg-mud-coap-00,
              9 March 2020, <http://www.ietf.org/internet-drafts/draft-

              Richardson, M., Latour, J., and H. Gharakheili, "On
              loading MUD URLs from QR codes", Work in Progress,
              Internet-Draft, draft-richardson-mud-qrcode-00, 17
              December 2020, <http://www.ietf.org/internet-drafts/draft-

Richardson, et al.       Expires 23 August 2021                [Page 10]

Internet-Draft             mud-acceptable-urls             February 2021

Appendix A.  Appendices


   Jie Yang

   Email: jay.yang@huawei.com

   Tianqing Tang

   Email: tangtianqing@huawei.com

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

   Wei Pan
   Huawei Technologies

   Email: william.panwei@huawei.com

   Eliot Lear
   Cisco Systems

   Email: lear@cisco.com

Richardson, et al.       Expires 23 August 2021                [Page 11]