OPSAWG Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Updates: 8520 (if approved)                                       W. Pan
Intended status: Best Current Practice               Huawei Technologies
Expires: 30 July 2021                                            E. Lear
                                                           Cisco Systems
                                                         26 January 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

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   This Internet-Draft will expire on 30 July 2021.

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   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 MUD URLs vs Updating MUD files . . . . . . . . . . .   3
     2.1.  Updating the MUD file in place  . . . . . . . . . . . . .   3
       2.1.1.  Adding capabilities . . . . . . . . . . . . . . . . .   3
       2.1.2.  Removing capabilities . . . . . . . . . . . . . . . .   4
       2.1.3.  Significant changes to protocols  . . . . . . . . . .   5
     2.2.  Motivation for updating MUD URLs  . . . . . . . . . . . .   5
   3.  Threat model for MUD URLs . . . . . . . . . . . . . . . . . .   5
     3.1.  Trust on First Use (TOFU): leveraging the manufacturer
           signature . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Concerns about same-signer mechanism  . . . . . . . . . .   6
   4.  Outline of proposed mechanism . . . . . . . . . . . . . . . .   6
   5.  Changes to RFC8520  . . . . . . . . . . . . . . . . . . . . .   6
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     7.1.  Updating files vs Updating MUD URLs . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Appendices . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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 describled in a MUD file hosted
   in its manufacture's server.  By providing a MUD URL, the network
   manager can locate this MUD file.  MUD URLs can come from a number of

   *  IDevID Extensions

   *  DHCP option

   *  LLDP TLV

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

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   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 targetted 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.

   While MUD files may include signatures, it is not mandatory to check
   them, and there is not a clear way to connect the entity which signed
   the MUD file to the device itself.  A malicious device does not need
   to make up a MUD file if there is already an available, and already
   trusted MUD file which it can use to impersonate the device.

   One defense against this is to not trust MUD URLs which are different
   from the one that was placed in an IDevID.  Or if the initial MUD URL
   was not taken from an IDevID, it could be trusted on first use.  But,
   if the MUD controller has locked down the URL, then updates to the
   URL are difficult to do.

2.  Updating MUD URLs vs Updating MUD files

   There are two ways in which a manufacturer can change what the is
   processed by the MUD controller: they can change what is in the MUD
   file (update-in-place), and or they change which file is processed by
   the MUD controller by changing the URL (updated-url).

2.1.  Updating the MUD file in place

   One option is for the manufacturer to never change the MUD URL due to
   firmware updates.  The published description is updated whenever the
   behaviour of the firmware changes.

2.1.1.  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 which have been upgraded to the new firmware
   will make use of the new features.  Devices which 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 for which the device is not prepared to respond to (new incoming

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   It is possible that older versions of the firmware have
   vulnerabilities which were not easily exploitable due to the MUD file
   preventing particular kinds of access.  As an example, an older
   firmware could have a 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.

   Useful and needed upgrades to the firmware could add credentials to
   that service, permitting 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, results in a compromised

   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
   undesireable to perform any upgrade to an airplane outside of its
   service facility.  The owner of a vehicle may desire to only perform
   software upgrades when they are at home, and could make other
   arrangements for transporation, rather than when parked at a remote
   cabin.  The situation for upgrades of medical devices has even more
   considerations involving regulatory compliance.

2.1.2.  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 connection which the
   old firmware still expects to do.  As explained in the previous
   section, upgrades may not always occur immediately upon release of
   the new firmware.

   In this case the old device will be performing unwanted connections,
   and the MUD controller when be alerting the device owner that the
   device is mis-behaving.  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

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2.1.3.  Significant changes to protocols

   [I-D.reddy-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.

2.2.  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 suggests 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.  Threat model for MUD URLs

   Only the DHCP and LLDP MUD URL mechanisms are sufficiently close to
   the firmware version that they can be easily updated when the
   firmware is updated.  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.

   [RFC8520] leaves this for a local policy.  There are any 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.  Trust on First Use (TOFU): leveraging the manufacturer signature

   Many MUD controllers currently use a Trust on First Use (TOFU)
   mechanism.  The first time a signature from a particular device-type
   is verified, the identity of the signing authority is recorded.  It
   is pinned.  Subsequent updates to that MUD file must be signed by the
   same entity in order to be accepted.

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   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.  Possibly, all the keys are collected into a single
   PKI, signed by a common certification authority.  In this case, the
   question as to whether the MUD controller should pin the end-entity
   (EE) certificate, or the CA certificate.  Pinning the 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.  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.  Outline of 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.

5.  Changes to RFC8520

   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 another mechanism.

   This first, initially trusted, MUD file will be called the "root" MUD

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   MUD files contain a self-referential MUD-URL attribute that point to
   a MUD file located on the vendor's web site.  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 URL for this
   reference being the MUD-URL attribute.

   Subsequent MUD files are considered valid if:

   *  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.

   For as 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 their file structure, to
   change web server hostnames (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)

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6.  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.

7.  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-manager 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-manager
       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.

   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 producting having its own

   The process described updates only MUD-managers 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.

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

7.1.  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 propogate through the various caches.  An updated MUD
   URL will however, not experience any cache issues, but can not be
   deployed with a firmware update.

8.  References

8.1.  Normative References

   [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,

8.2.  Informative References

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

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              "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>.

              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-

              Reddy.K, T., Wing, D., and B. Anderson, "MUD (D)TLS
              profiles for IoT devices", Work in Progress, Internet-
              Draft, draft-reddy-opsawg-mud-tls-05, 31 August 2020,

              Richardson, M., Latour, J., and H. Gharakheili, "On
              loading MUD URLs from QR codes", Work in Progress,
              Internet-Draft, draft-richardson-opsawg-securehomegateway-
              mud-05, 8 September 2020, <http://www.ietf.org/internet-

Appendix A.  Appendices

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

   Wei Pan
   Huawei Technologies

   Email: william.panwei@huawei.com

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   Eliot Lear
   Cisco Systems

   Email: lear@cisco.com

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