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Authorized update to MUD URLs

Document Type Active Internet-Draft (opsawg WG)
Authors Michael Richardson , Wei Pan , Eliot Lear
Last updated 2023-02-24 (Latest revision 2023-01-12)
Replaces draft-richardson-opsawg-mud-acceptable-urls
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Best Current Practice
Additional resources Mailing list discussion
Stream WG state WG Consensus: Waiting for Write-Up
Document shepherd Henk Birkholz
IESG IESG state I-D Exists
Consensus boilerplate Yes
Telechat date (None)
Responsible AD (None)
Send notices to
OPSAWG Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Updates: 8520 (if approved)                                       W. Pan
Intended status: Best Current Practice               Huawei Technologies
Expires: 16 July 2023                                            E. Lear
                                                           Cisco Systems
                                                         12 January 2023

                     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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 16 July 2023.

Copyright Notice

   Copyright (c) 2023 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 (
   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

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised 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  . . . . . . . . . . . .   5
     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
     4.1.  Merger, Acquisitions and Key Changes  . . . . . . . . . .   8
       4.1.1.  Changing file structure . . . . . . . . . . . . . . .   9
       4.1.2.  Changing hosting URLs . . . . . . . . . . . . . . . .   9
       4.1.3.  Changing Signing Authority  . . . . . . . . . . . . .   9
   5.  Polling for changes in MUD files  . . . . . . . . . . . . . .  10
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     7.1.  Updating files vs Updating MUD URLs . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Appendices . . . . . . . . . . . . . . . . . . . . .  13
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   [RFC8520] provides a standardized way to describe how a specific
   purpose device makes use of Internet resources and associated
   suggested network behavior.  The behaviors are described in a MUD
   file hosted in its manufacturer's server.  The device provides a MUD
   URL to the network manager, which 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.

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   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.  Section 2
   discusses three scenarios for updating the MUD file and the
   corresponding potential issues.

   The other is to change which MUD file is processed by changing the
   MUD URL.  Section 3 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 Section 4.

   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)

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   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, 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.  A vehicle owner may desire only to perform
   software upgrades when they are at their residence.  Should there be
   a problem, they could make alternate arrangements for transportation.
   This is constrasted with the situation when the vehicle is parked at,
   for instance, 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].

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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 advised.

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 are in order.
   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 would also be able to change the
   MUD URL that is emitted.

   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.)

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   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.

   [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.  That it, the signing
   authority is pinned.  This policy means that subsequent MUD files
   must be signed by the same entity in order to be accepted.

   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

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   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 prevents 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.  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.

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   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.

   When pinning the signature, the MUD controller SHOULD pin the lowest
   Certification Authority (CA) that was used in the validation of the
   CMS structure, along with the chain of Subject Names leading to the
   signature.  The MUD controller may need additional trust anchors
   (including previously pinned ones) in order to verify that CA

   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 an equivalent End Entity (same trusted CA and
      same Subject Name) 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 a simple example, if the "root" MUD-URL is then any URL that starts
   with 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.

4.1.  Merger, Acquisitions and Key Changes

   The above process allows for a manufacturer to rework its file
   structure.  They can change web server host names, so long as they
   retain the old structure long enough for all devices to upgrade at
   least once.

   The process also allows a manufacturer to change the EE certificate
   and Certification Authority used for signing.

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4.1.1.  Changing file structure

   A manufacturer has been hosting a MUD file at and
   wishes to move it to

   The manufacturer simply changes the MUD-URL contained with the files
   at the old location to have a value of  The
   manufacturer must continue to serve the files from the old location
   for some time, or to return an HTTP 301 (Moved Permanently)
   redirecting to the new location.

4.1.2.  Changing hosting URLs

   A manufacturer has been hosting a MUD file at and
   wishes to move it to

   The manufacturer simply changes the MUD-URL contained with the files
   at the old location to have a value of  The
   manufacturer has to continue to host at the old location until such
   time as it is sure that all MUD controllers have loaded the new data,
   and that all devices in the field have upgraded their URL.  A 301
   Redirect that changed the hostname SHOULD NOT be accepted by MUD

4.1.3.  Changing Signing Authority

   A manufacturer has been signing MUD files using an EE Certificate
   with subjectAltName foo.example, issued by an internal Certification
   Authority BAZ.

   The manufacturer wishes to begin signing with an EE Certificate with
   subjectAltname foo.example, but now signed by a public CA (call it:

   The manufacturer first creates a new MUD file with a new detached
   signature file.  Within this signature file, the manufacturer places
   a certificate chain: Internal-CA BAZ->Fluffy, and then the Fluffy
   Certificate, and then the foo.example certificate issued from Fluffy.

   This supports changing certification authorities, but it does not
   support changing the Subject Name of the signing entity.

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5.  Polling for changes in MUD files

   The MUD file update mechanisms described in Section 2 requires that
   the MUD controller poll for updates.  The MUD controller will receive
   no signal about a change from the device because the URL will not
   have changed.

   The manufacturer SHOULD serve mud files from a source for which ETag
   Section 2.3 of [RFC7232] may be generated.  Static files on disk
   satisfy this requirement.  MUD files generated from a database
   process might not.  The use of ETag allows a MUD controller to more
   efficiently poll for changes in the file.

   A manufacturer should also serve MUD files with an HTTP Max-Age
   header as per Section of [RFC7234].

   The MUD controller should take the Max-Age as an indication of when
   to next poll for updates to the MUD file.  Values of less than 1
   hour, or more than 1 month should be considered out of range, and
   clamped into the range (1 hour, 1 month).

   MUD controllers SHOULD add some random jitter to the timing of their
   requests.  MUD controllers MAY use a single HTTP(S)/1.1 connection to
   retrieve all resources at the same destination.

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.

   The requirement for the MUD controller to poll for changes results in
   multiple interactions between the MUD controller and the
   manufacturer.  Even if HTTPS used, an observer of the traffic to that
   manufacturer will 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:

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   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.

   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

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

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   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.

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,

              "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>.

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              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-11, 12 December 2022,

              Jimenez, J., "Using MUD on CoAP environments", Work in
              Progress, Internet-Draft, draft-jimenez-t2trg-mud-coap-00,
              9 March 2020, <

              Richardson, M., Latour, J., and H. H. Gharakheili,
              "Loading Manufacturer Usage Description (MUD) URLs from QR
              Codes", Work in Progress, Internet-Draft, draft-
              richardson-mud-qrcode-07, 21 March 2022,

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,

Appendix A.  Appendices


   Jie Yang

   Tianqing Tang

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

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   Wei Pan
   Huawei Technologies

   Eliot Lear
   Cisco Systems

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