OPSAWG Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Updates: 8520 (if approved)                                       W. Pan
Intended status: Best Current Practice               Huawei Technologies
Expires: December 17, 2020                                       E. Lear
                                                           Cisco Systems
                                                           June 15, 2020

                     Authorized update to MUD URLs


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

Status of This Memo

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   This Internet-Draft will expire on December 17, 2020.

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   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  . . . . . . . . . .   4
     2.2.  Motivation for updating MUD URLs  . . . . . . . . . . . .   5
   3.  Threat model for MUD URLs . . . . . . . . . . . . . . . . . .   5
     3.1.  leveraging the manufacturer signature . . . . . . . . . .   5
     3.2.  Concerns about same-signer mechanism  . . . . . . . . . .   5
   4.  Changes to RFC8520  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Appendices . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC8520] provides a standardized way to describe how a specific
   purpose device makes use of Internet resources.  Access Control Lists
   (ACLs) can be defined in an RFC8520 Manufacturer Usage Description
   (MUD) file that permit a device to access Internet resources by DNS

   MUD URLs can come from a number of sources:

   o  IDevID Extensions

   o  DHCP

   o  LLDP

   o  [I-D.richardson-opsawg-securehomegateway-mud] 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

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   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

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

   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.

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   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
   downtown, 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 medical devices is even more complex.

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 [boycrieswolf] situation,
   leading to real security issues being ignored.  This is a serious
   issue as documented also in [boywolfinfosec], and [falsemalware].

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.  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 is not indicated.

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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 require updates to the MUD URL.  A strategy is do
   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 are also
   easily changed by malware.

   There are mitigating mechanisms which may be enough.  The MUD files
   are signed by the manufacturer.  [RFC8520] has not established a
   trust model for MUD controllers to determine whether a signature from
   a specific entity is legitimate as a signature for a particular
   device.  [RFC8520] leaves this to the industry to work out.

3.1.  leveraging the manufacturer signature

   Many MUD controllers currently use a Trust on First Use mechanism
   where the first time a signature from a device is verified, the
   signatory is recorded.  Subsequent updates to that MUD file MUST be
   signed by the same entity to be accepted.

   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.

4.  Changes to RFC8520

   The first MUD file which is defined for a device can come from an
   IDevID, or via Trust on First Use with DHCP or LLDP or another

   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 transmit an entire MUD file.

   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:

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

   o  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
   that 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.

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

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   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 Internet 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.

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

7.  References

7.1.  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,

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   [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", January 2020,

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

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

              Reddy.K, T., Wing, D., and B. Anderson, "MUD (D)TLS
              profiles for IoT devices", draft-reddy-opsawg-mud-tls-03
              (work in progress), January 2020.

              Richardson, M., Latour, J., and H. Gharakheili, "Loading
              MUD URLs from QR codes", draft-richardson-opsawg-
              securehomegateway-mud-03 (work in progress), March 2020.

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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