| Internet-Draft | mud-acceptable-urls | July 2022 |
| Richardson, et al. | Expires 12 January 2023 | [Page] |
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: https://github.com/mcr/iot-mud-acceptable-urls¶
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[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.¶
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.¶
Three scenarios for updating the MUD file and the corresponding potential issues are discussed below.¶
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.¶
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.¶
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].¶
[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.¶
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.¶
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 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.)¶
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.¶
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.¶
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 valid.¶
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.¶
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 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 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 certificate.¶
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 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 the new file.¶
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.¶
A manufacturer has been hosting a MUD file at https://example.com/household/products/mudfiles/toaster.json and wishes to move it to https://example.com/mudfiles/toasters/model1945/mud.json¶
The manufacturer simply changes the MUD-URL contained with the files at the old location to have a value of https://example.com/mudfiles/toasters/model1945/mud.json. 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.¶
A manufacturer has been hosting a MUD file at https://example.com/household/products/mudfiles/toaster.json and wishes to move it to https://mud.example/example.com/toasters/model1945/mud.json¶
The manufacturer simply changes the MUD-URL contained with the files at the old location to have a value of https://example.com/mudfiles/toasters/model1945/mud.json. 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 controllers.¶
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 5.2.2.8 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.¶
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.¶
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:¶
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 directory/pathname.¶
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 compatibility.¶
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 hold.¶
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.¶