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.

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Table of Contents

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 name.

MUD URLs can come from a number of sources:

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 box.

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 connections).

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 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 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.

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 mechanism.

This first, initially trusted, MUD file will be called the “root” MUD file.

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 4.3.

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:

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 http://example.com/hello/there/revision2.json.

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 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 directory/pathname.

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 compatibility.

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

7.2. Informative References

[boycrieswolf] "The Boy Who Cried Wolf", January 2020.
[boywolfinfosec] "Security Alerts - A Case of the Boy Who Cried Wolf?", January 2020.
[falsemalware] "False malware alerts cost organizations $1.27M annually, report says", January 2020.
[I-D.reddy-opsawg-mud-tls] Reddy.K, T., Wing, D. and B. Anderson, "MUD (D)TLS profiles for IoT devices", Internet-Draft draft-reddy-opsawg-mud-tls-03, January 2020.
[I-D.richardson-opsawg-securehomegateway-mud] Richardson, M., Latour, J. and H. Gharakheili, "Loading MUD URLs from QR codes", Internet-Draft draft-richardson-opsawg-securehomegateway-mud-03, 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
Eliot Lear Cisco Systems EMail: lear@cisco.com