SUIT B. Moran
Internet-Draft M. Meriac
Intended status: Informational H. Tschofenig
Expires: September 6, 2018 Arm Limited
March 05, 2018
A Firmware Update Architecture for Internet of Things Devices
draft-moran-suit-architecture-03
Abstract
Vulnerabilities with Internet of Things (IoT) devices have raised the
need for a solid and secure firmware update mechanism that is also
suitable for constrained devices. Incorporating such update
mechanism to fix vulnerabilities, to update configuration settings as
well as adding new functionality is recommended by security experts.
This document lists requirements and describes an architecture for a
firmware update mechanism suitable for IoT devices. The architecture
is agnostic to the transport of the firmware images and associated
meta-data.
This version of the document assumes asymmetric cryptography and a
public key infrastructure. Future versions may also describe a
symmetric key approach for very constrained devices.
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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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 6, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Agnostic to how firmware images are distributed . . . . . 6
3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 6
3.3. Uses state-of-the-art security mechanisms . . . . . . . . 6
3.4. Rollback attacks must be prevented . . . . . . . . . . . 6
3.5. High reliability . . . . . . . . . . . . . . . . . . . . 6
3.6. Operates with a small bootloader . . . . . . . . . . . . 7
3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 7
3.8. Minimal impact on existing firmware formats . . . . . . . 7
3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 7
4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Manifest . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
10. Mailing List Information . . . . . . . . . . . . . . . . . . 15
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11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . 16
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix A. Threat Model, User Stories, Security Requirements,
and Usability Requirements . . . . . . . . . . . . . 17
A.1. Threat Model . . . . . . . . . . . . . . . . . . . . . . 17
A.2. Threat Descriptions . . . . . . . . . . . . . . . . . . . 17
A.2.1. Threat MFT1: Old Firmware . . . . . . . . . . . . . . 17
A.2.2. Threat MFT2: Mismatched Firmware . . . . . . . . . . 18
A.2.3. Threat MFT3: Offline device + Old Firmware . . . . . 18
A.2.4. Threat MFT4: The target device misinterprets the type
of payload . . . . . . . . . . . . . . . . . . . . . 18
A.2.5. Threat MFT5: The target device installs the payload
to the wrong location . . . . . . . . . . . . . . . . 19
A.2.6. Threat MFT6: Redirection . . . . . . . . . . . . . . 19
A.2.7. Threat MFT7: Payload Verification on Boot . . . . . . 19
A.2.8. Threat MFT8: Unauthenticated Updates . . . . . . . . 19
A.2.9. Threat MFT9: Unexpected Precursor images . . . . . . 20
A.2.10. Threat MFT10: Unqualified Firmware . . . . . . . . . 20
A.2.11. Threat MFT11: Reverse Engineering Of Firmware Image
for Vulnerability Analysis . . . . . . . . . . . . . 21
A.3. Security Requirements . . . . . . . . . . . . . . . . . . 21
A.3.1. Security Requirement MFSR1: Monotonic Sequence
Numbers . . . . . . . . . . . . . . . . . . . . . . . 21
A.3.2. Security Requirement MFSR2: Vendor, Device-type
Identifiers . . . . . . . . . . . . . . . . . . . . . 22
A.3.3. Security Requirement MFSR3: Best-Before Timestamps . 22
A.3.4. Security Requirement MFSR4: Signed Payload Descriptor 22
A.3.5. Security Requirement MFSR5: Cryptographic
Authenticity . . . . . . . . . . . . . . . . . . . . 23
A.3.6. Security Requirement MFSR6: Rights Require
Authenticity . . . . . . . . . . . . . . . . . . . . 23
A.3.7. Security Requirement MFSR7: Firmware encryption . . . 23
A.4. User Stories . . . . . . . . . . . . . . . . . . . . . . 23
A.4.1. Use Case MFUC1: Installation Instructions . . . . . . 24
A.4.2. Use Case MFUC2: Reuse Local Infrastructure . . . . . 24
A.4.3. Use Case MFUC3: Modular Update . . . . . . . . . . . 24
A.4.4. Use Case MFUC4: Multiple Authorisations . . . . . . . 25
A.4.5. Use Case MFUC5: Multiple Payload Formats . . . . . . 25
A.4.6. Use Case MFUC6: IP Protection . . . . . . . . . . . . 25
A.5. Usability Requirements . . . . . . . . . . . . . . . . . 25
A.5.1. Usability Requirement MFUR1 . . . . . . . . . . . . . 25
A.5.2. Usability Requirement MFUR2 . . . . . . . . . . . . . 25
A.5.3. Usability Requirement MFUR3 . . . . . . . . . . . . . 26
A.5.4. Usability Requirement MFUR4 . . . . . . . . . . . . . 26
A.5.5. Usability Requirement MFUR5 . . . . . . . . . . . . . 26
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A.6. Manifest Fields . . . . . . . . . . . . . . . . . . . . . 26
A.6.1. Manifest Field: Timestamp . . . . . . . . . . . . . . 27
A.6.2. Manifest Field: Vendor ID Condition . . . . . . . . . 27
A.6.3. Manifest Field: Class ID Condition . . . . . . . . . 27
A.6.4. Manifest Field: Precursor Image Digest Condition . . 27
A.6.5. Manifest Field: Best-Before timestamp condition . . . 27
A.6.6. Manifest Field: Payload Format . . . . . . . . . . . 28
A.6.7. Manifest Field: Storage Location . . . . . . . . . . 28
A.6.8. Manifest Field: URIs . . . . . . . . . . . . . . . . 28
A.6.9. Manifest Field: Digests . . . . . . . . . . . . . . . 28
A.6.10. Manifest Field: Size . . . . . . . . . . . . . . . . 28
A.6.11. Manifest Field: Signature . . . . . . . . . . . . . . 28
A.6.12. Manifest Field: Directives . . . . . . . . . . . . . 29
A.6.13. Manifest Field: Aliases . . . . . . . . . . . . . . . 29
A.6.14. Manifest Field: Dependencies . . . . . . . . . . . . 29
A.6.15. Manifest Field: Content Key Distribution Method . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
When developing IoT devices, one of the most difficult problems to
solve is how to update the firmware on the device. Once the device
is deployed, firmware updates play a critical part in its lifetime,
particularly when devices have a long lifetime, are deployed in
remote or inaccessible areas or where manual intervention is cost
prohibitive or otherwise difficult. The need for a firmware update
may be to fix bugs in software, to add new functionality, or to re-
configure the device.
The firmware update process has to ensure that
- The firmware image is authenticated and attempts to flash a
malicious firmware image are prevented.
- The firmware image can be confidentiality protected so that
attempts by an adversary to recover the plaintext binary can be
prevented. Obtaining the plaintext binary is often one of the
first steps for an attack to mount an attack.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This document uses the following terms:
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- Manifest: The manifest contains meta-data about the firmware
image. The manifest is protected against modification and
provides information about the author.
- Firmware Image: The firmware image is a binary that may contain
the complete software of a device or a subset of it. The firmware
image may consist of multiple images, if the device contains more
than one microcontroller. The image may consist of a differential
update for performance reasons. Firmware is the more universal
term. Both terms are used in this document and are
interchangeable.
The following entities are used:
- Author: The author is the entity that creates the firmware image,
signs and/or encrypts it and attaches a manifest to it. The
author is most likely a developer using a set of tools.
- Device: The device is the recipient of the firmware image and the
manifest. The goal is to update the firmware of the device.
- Untrusted Storage: Firmware images and manifests are stored on
untrusted fileservers or cloud storage infrastructure. Some
deployments may require storage of the firmware images/manifests
to be stored on various entities before they reach the device.
3. Requirements
The firmware update mechanism described in this specification was
designed with the following requirements in mind:
- Agnostic to how firmware images are distributed
- Friendly to broadcast delivery
- Uses state-of-the-art security mechanisms
- Rollback attacks must be prevented.
- High reliability
- Operates with a small bootloader
- Small Parsers
- Minimal impact on existing firmware formats
- Robust permissions
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3.1. Agnostic to how firmware images are distributed
Firmware images can be conveyed to devices in a variety of ways,
including USB, UART, WiFi, BLE, low-power WAN technologies, etc and
use different protocols (e.g., CoAP, HTTP). The specified mechanism
needs to be agnostic to the distribution of the firmware images and
manifests.
3.2. Friendly to broadcast delivery
For an update to be broadcast friendly, it cannot rely on link layer,
network layer, or transport layer security. In addition, the same
message must be deliverable to many devices; both those to which it
applies and those to which it does not without a chance that the
wrong device will accept the update. Considerations that apply to
network broadcasts apply equally to the use of third-party content
distribution networks for payload distribution.
3.3. Uses state-of-the-art security mechanisms
End-to-end security between the author and the device, as shown in
Section 5, is used to ensure that the device can verify firmware
images and manifests produced by authorized authors.
The use of post-quantum secure signature mechanisms, such as hash-
based signatures, should be explored. A mandatory-to-implement set
of algorithms has to be defined offering a key length of 112-bit
symmetric key or security or more, as outlined in Section 20 of RFC
7925. This corresponds to a 233 bit ECC key or a 2048 bit RSA key.
If the firmware image is to be encrypted, it must be done in such a
way that every intended recipient can decrypt it. The information
that is encrypted individually for each device must be an absolute
minimum.
3.4. Rollback attacks must be prevented
A device presented with an old, but valid manifest and firmware must
not be tricked into installing such firmware since a vulnerability in
the old firmware image may allow an attacker gain control of the
device.
3.5. High reliability
A power failure at any time must not cause a failure of the device.
A failure to validate any part of an update must not cause a failure
of the device. One way to achieve this functionality is to provide a
minimum of two storage locations for firmware and one bootable
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location for firmware. An alternative approach is to use a 2nd stage
bootloader with build-in full featured firmware update functionality
such that it is possible to return to the update process after power
down.
Note: This is an implementation requirement rather than a requirement
on the manifest format.
3.6. Operates with a small bootloader
The bootloader must be minimal, containing only flash support,
cryptographic primitives and optionally a recovery mechanism. The
recovery mechanism is used in case the update process failed and may
include support for firmware updates over serial, USB or even a
limited version of wireless connectivity standard like a limited
Bluetooth Smart. Such a recovery mechanism must provide security at
least at the same level as the full featured firmware update
functionalities.
The bootloader needs to verify the received manifest and to install
the bootable firmware image. The bootloader should not require
updating since a failed update poses a risk in reliability. If more
functionality is required in the bootloader, it must use a two-stage
bootloader, with the first stage comprising the functionality defined
above.
All information necessary for a device to make a decision about the
installation of a firmware update must fit into the available RAM of
a constrained IoT device. This prevents flash write exhaustion.
Note: This is an implementation requirement.
3.7. Small Parsers
Since parsers are known sources of bugs they must be minimal.
Additionally, it must be easy to parse only those fields which are
required to validate at least one signature with minimal exposure.
3.8. Minimal impact on existing firmware formats
The design of the firmware update mechanism must not require changes
to existing firmware formats.
3.9. Robust permissions
A device may have many modules that require updating individually.
It may also need to trust several actors in order to authorize an
update. For example, a firmware author may not have the authority to
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install firmware on a device in critical infrastructure without the
authorization of a device operator. In this case, the device should
reject firmware updates unless they are signed both by the firmware
author and by the device operator. To facilitate complex use-cases
such as this, updates require several permissions.
4. Claims
When a simple set of permissions fails to encapsulate the rules
required for a device make decisions about firmware, claims can be
used instead. Claims represent a form of policy. Several claims can
be used together, when multiple actors should have the rights to set
policies.
Some example claims are:
- Trust the actor identified by the referenced public key.
- Three actors are trusted identified by their public keys.
Signatures from at least two of these actors are required to trust
a manifest.
- The actor identified by the referenced public key is authorized to
create secondary policies
The baseline claims for all manifests are described in Appendix A.
In summary, they are:
- Do not install firmware with earlier metadata than the current
metadata.
- Only install firmware with a matching vendor, model, hardware
revision, software version, etc.
- Only install firmware that is before its best-before timestamp.
- Only install firmware with metadata signed by a trusted actor.
- Only allow an actor to exercise rights on the device via a
manifest if that actor has signed the manifest.
- Only allow a firmware installation if all required rights have
been met through signatures (one or more) or manifest dependencies
(one or more).
- Use the instructions provided by the manifest to install the
firmware.
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- Any authorized actor may redirect any URI.
- Install any and all firmware images that are linked together with
manifest dependencies.
- Choose the mechanism to install the firmware, based on the type of
firmware it is.
5. Architecture
We start the architectural description with the security model. It
is based on end-to-end security. Figure 1 illustrates the security
model where a firmware image and the corresponding manifest are
created by an author and verified by the device. The firmware image
is integrity protected and may be encrypted. The manifest is
integrity protected and authenticated. When the author is ready to
distribute the firmware image it is conveyed using some communication
channel to the device, which will typically involve the use of
untrusted storage. Examples of untrusted storage are FTP servers,
Web servers or USB sticks.
+-----------+
+--------+ Firmware Image | | Firmware Image +--------+
| | + Manifest | Untrusted | + Manifest | |
| Device |<-----------------| Storage |<------------------| Author |
| | | | | |
+--------+ +-----------+ +--------+
^ *
* *
************************************************************
End-to-End Security
Figure 1: End-to-End Security.
Whether the firmware image and the manifest is pushed to the device
or fetched by the device is outside the scope of this work and
existing device management protocols can be used for efficiently
distributing this information.
The following assumptions are made to allow the device to verify the
received firmware image and manifest before updating software:
- To accept an update, a device needs to decide whether the author
signing the firmware image and the manifest is authorized to make
the updates. We use public key cryptography to accomplish this.
The device verifies the signature covering the manifest using a
digital signature algorithm. The device is provisioned with a
trust anchor that is used to validate the digital signature
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produced by the author. This trust anchor is potentially
different from the trust anchor used to validate the digital
signature produced for other protocols (such as device management
protocols). This trust anchor may be provisioned to the device
during manufacturing or during commissioning.
- For confidentiality protection of firmware images the author needs
to be in possession of the certificate/public key or a pre-shared
key of a device.
There are different types of delivery modes, which are illustrates
based on examples below.
There is an option for embedding a firmware image into a manifest.
This is a useful approach for deployments where devices are not
connected to the Internet and cannot contact a dedicated server for
download of the firmware. It is also applicable when the firmware
update happens via a USB stick or via Bluetooth Smart. Figure 2
shows this delivery mode graphically.
/------------\ /------------\
/Manifest with \ /Manifest with \
|attached | |attached |
\firmware image/ \firmware image/
\------------/ +-----------+ \------------/
+--------+ | | +--------+
| |<.................| Untrusted |<................| |
| Device | | Storage | | Author |
| | | | | |
+--------+ +-----------+ +--------+
Figure 2: Manifest with attached firmware.
Figure 3 shows an option for remotely updating a device where the
device fetches the firmware image from some file server. The
manifest itself is delivery independently and provides information
about the firmware image(s) to download.
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/------------\
/ \
| Manifest |
\ /
+--------+ \------------/ +--------+
| |<..............................................>| |
| Device | -- | Author |
| |<- --- | |
+--------+ -- --- +--------+
-- ---
--- ---
-- +-----------+ --
-- | | --
/------------\ -- | Untrusted |<- /------------\
/ \ -- | Storage | / \
| Firmware | | | | Firmware |
\ / +-----------+ \ /
\------------/ \------------/
Figure 3: Independent retrieval of the firmware image.
This architecture does not mandate a specific delivery mode but a
solution must support both types.
6. Manifest
In order for a device to apply an update, it has to make several
decisions about the update:
- Does it trust the author of the update?
- Has the firmware been corrupted?
- Does the firmware update apply to this device?
- Is the update older than the active firmware?
- When should the device apply the update?
- How should the device apply the update?
- What kind of firmware binary is it?
- Where should the update be obtained?
- Where should the firmware be stored?
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The manifest encodes the information that devices need in order to
make these decisions. It is a data structure that contains the
following information:
- information about the device(s) the firmware image is intented to
be applied to,
- information about when the firmware update has to be applied,
- information about when the manifest was created,
- dependencies to other manifests,
- pointers to the firmware image and information about the format,
- information about where to store the firmware image,
- cryptographic information, such as digital signatures.
The manifest format is described in a companion document.
7. Example Flow
The following example message flow illustrates the interaction for
distributing a firmware image to a device starting with an author
uploading the new firmware to untrusted storage and creating a
manifest.
+--------+ +-----------------+ +------+
| Author | |Untrusted Storage| |Device|
+--------+ +-----------------+ +------+
| | |
| Create Firmware | |
|--------------- | |
| | | |
|<-------------- | |
| | |
| Upload Firmware | |
|------------------>| |
| | |
| Create Manifest | |
|---------------- | |
| | | |
|<--------------- | |
| | |
| Sign Manifest | |
|-------------- | |
| | | |
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|<------------- | |
| | |
| Upload Manifest | |
|------------------>| |
| | |
| | Query Manifest |
| |<--------------------|
| | |
| | Send Manifest |
| |-------------------->|
| | |
| | | Validate Manifest
| | |------------------
| | | |
| | |<-----------------
| | |
| | Request Firmware |
| |<--------------------|
| | |
| | Send Firmware |
| |-------------------->|
| | |
| | | Verify Firmware
| | |---------------
| | | |
| | |<--------------
| | |
| | | Store Firmware
| | |--------------
| | | |
| | |<-------------
| | |
| | | Reboot
| | |-------
| | | |
| | |<------
| | |
| | | Bootloader validates
| | | Firmware
| | |----------------------
| | | |
| | |<---------------------
| | |
| | | Bootloader activates
| | | Firmware
| | |----------------------
| | | |
| | |<---------------------
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| | |
| | | Bootloader transfers
| | | control to new Firmware
| | |----------------------
| | | |
| | |<---------------------
| | |
Figure 4: Example Flow for a Firmware Upate.
8. IANA Considerations
This document does not require any actions by IANA.
9. Security Considerations
Firmware updates fix security vulnerabilities and are considered to
be an important building block in securing IoT devices. Due to the
importance of firmware updates for IoT devices the Internet
Architecture Board (IAB) organized a 'Workshop on Internet of Things
(IoT) Software Update (IOTSU)', which took place at Trinity College
Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at
the big picture. A report about this workshop can be found at
[RFC8240]. This document (and associated specifications) offer a
standardized firmware manifest format providing end-to-end security
from the author to the device.
There are, however, many other considerations raised during the
workshop. Many of them are outside the scope of standardization
organizations since they fall into the realm of product engineering,
regulatory frameworks, and business models. The following
considerations are outside the scope of this document, namely
- installing firmware updates in a robust fashion so that the update
does not break the device functionality of the environment this
device operates in.
- installing firmware updates in a timely fashion considering the
complexity of the decision making process of updating devices,
potential re-certification requirements, and the need for user's
consent to install updates.
- the distribution of the actual firmware update, potentially in an
efficient manner to a large number of devices without human
involvement.
- energy efficiency and battery lifetime considerations.
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- key management required for verifying the digitial signature
protecting the manifest.
- incentives for manufacturers to offer a firmware update mechanism
as part of their IoT products.
10. Mailing List Information
The discussion list for this document is located at the e-mail
address suit@ietf.org [1]. Information on the group and information
on how to subscribe to the list is at
https://www1.ietf.org/mailman/listinfo/suit
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/suit/current/index.html
11. Acknowledgements
We would like to thank the following persons for their feedback:
- Geraint Luff
- Amyas Phillips
- Dan Ros
- Thomas Eichinger
- Michael Richardson
- Emmanuel Baccelli
- Ned Smith
- David Brown
- Jim Schaad
- Carsten Bormann
- Cullen Jennings
- Olaf Bergmann
- Suhas Nandakumar
- Phillip Hallam-Baker
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- Marti Bolivar
- Andrzej Puzdrowski
- Markus Gueller
We would also like to thank the WG chairs, Russ Housley, David
Waltermire, Dave Thaler and the responsible security area director,
Kathleen Moriarty, for their support and their reviews.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
12.2. Informative References
[RFC8240] Tschofenig, H. and S. Farrell, "Report from the Internet
of Things Software Update (IoTSU) Workshop 2016",
RFC 8240, DOI 10.17487/RFC8240, September 2017,
<https://www.rfc-editor.org/info/rfc8240>.
[STRIDE] Microsoft, "The STRIDE Threat Model", January 2018.
12.3. URIs
[1] mailto:suit@ietf.org
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Appendix A. Threat Model, User Stories, Security Requirements, and
Usability Requirements
A.1. Threat Model
This appendix aims to provide information about the threats that were
considered, the security requirements that are derived from those
threats and the fields that permit implementation of the security
requirements. This model uses the S.T.R.I.D.E. [STRIDE] approach.
Each threat is classified according to:
- Spoofing Identity
- Tampering with data
- Repudiation
- Information disclosure
- Denial of service
- Elevation of privilege
This threat model only covers elements related to the transport of
firmware updates. It explicitly does not cover threats outside of
the transport of firmware updates. For example, threats to an IoT
device due to physical access are out of scope.
A.2. Threat Descriptions
A.2.1. Threat MFT1: Old Firmware
Classification: Escalation of Privilege
An attacker sends an old, but valid manifest with an old, but valid
firmware image to a device. If there is a known vulnerability in the
provided firmware image, this may allow an attacker to exploit the
vulnerability and gain control of the device.
Threat Escalation: If the attacker is able to exploit the known
vulnerability, then this threat can be escalated to ALL TYPES.
Mitigated by: MFSR1
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A.2.2. Threat MFT2: Mismatched Firmware
Classification: Denial of Service
An attacker sends a valid firmware image, for the wrong type of
device, signed by an actor with firmware installation permission on
both types of device. The firmware is verified by the device
positively because it is signed by an actor with the appropriate
permission. This could have wide-ranging consequences. For devices
that are similar, it could cause minor breakage, or expose security
vulnerabilities. For devices that are very different, it is likely
to render devices inoperable.
Mitigated by: MFSR2
A.2.3. Threat MFT3: Offline device + Old Firmware
Classification: Escalation of Privilege
An attacker targets a device that has been offline for a long time
and runs an old firmware version. The attacker sends an old, but
valid manifest to a device with an old, but valid firmware image.
The attacker-provided firmware is newer than the installed one but
older than the most recently available firmware. If there is a known
vulnerability in the provided firmware image then this may allow an
attacker to gain control of a device. Because the device has been
offline for a long time, it is unaware of any new updates. As such
it will treat the old manifest as the most current.
Threat Escalation: If the attacker is able to exploit the known
vulnerability, then this threat can be escalated to ALL TYPES.
Mitigated by: MFSR3
A.2.4. Threat MFT4: The target device misinterprets the type of payload
Classification: Denial of Service
If a device misinterprets the type of the firmware image, it may
cause a device to install a firmware image incorrectly. An
incorrectly installed firmware image would likely cause the device to
stop functioning.
Threat Escalation: An attacker that can cause a device to
misinterpret the received firmware image may gain escalation of
privilege and potentially expand this to all types of threat.
Mitigated by: MFSR4
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A.2.5. Threat MFT5: The target device installs the payload to the wrong
location
Classification: Denial of Service
If a device installs a firmware image to the wrong location on the
device, then it is likely to break. For example, a firmware image
installed as an application could cause a device and/or an
application to stop functioning.
Threat Escalation: An attacker that can cause a device to
misinterpret the received code may gain escalation of privilege and
potentially expand this to all types of threat.
Mitigated by: MFSR4
A.2.6. Threat MFT6: Redirection
Classification: Denial of Service
If a device does not know where to obtain the payload for an update,
it may be redirected to an attacker's server. This would allow an
attacker to provide broken payloads to devices.
Mitigated by: MFSR4
A.2.7. Threat MFT7: Payload Verification on Boot
Classification: All Types
An attacker replaces a newly downloaded firmware after a device
finishes verifying a manifest. This could cause the device to
execute the attacker's code. This attack likely requires physical
access to the device. However, it is possible that this attack is
carried out in combination with another threat that allows remote
execution.
Mitigated by: MFSR4
A.2.8. Threat MFT8: Unauthenticated Updates
Classification: All Types
If an attacker can install their firmware on a device, by
manipulating either payload or metadata, then they have complete
control of the device.
Mitigated by: MFSR5
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A.2.9. Threat MFT9: Unexpected Precursor images
Classification: Denial of Service
An attacker sends a valid, current manifest to a device that has an
unexpected precursor image. If a payload format requires a precursor
image (for example, delta updates) and that precursor image is not
available on the target device, it could cause the update to break.
Threat Escalation: An attacker that can cause a device to install a
payload against the wrong precursor image could gain escalation of
privilege and potentially expand this to all types of threat.
Mitigated by: MFSR4
A.2.10. Threat MFT10: Unqualified Firmware
Classification: Denial of Service, Escalation of Privilege
This threat can appear in several ways, however it is ultimately
about interoperability of devices with other systems. The owner or
operator of a network needs to approve firmware for their network in
order to ensure interoperability with other devices on the network,
or the network itself. If the firmware is not qualified, it may not
work. Therefore, if a device installs firmware without the approval
of the network owner or operator, this is a threat to devices and the
network.
Example 1: We assume that OEMs expect the rights to create firmware,
but that Operators expect the rights to qualify firmware as fit-for-
purpose on their networks.
An attacker obtains a manifest for a device on Network A. They send
that manifest to a device on Network B. Because Network A and
Network B are different, and the firmware has not been qualified for
Network B, the target device is disabled by this unqualified, but
signed firmware.
This is a denial of service because it can render devices inoperable.
This is an escalation of privilege because it allows the attacker to
make installation decisions that should be made by the Operator.
Example 2: Multiple devices that interoperate are used on the same
network. Some devices are manufactured by OEM A and other devices by
OEM B. These devices communicate with each other. A new firmware is
released by OEM A that breaks compatibility with OEM B devices. An
attacker sends the new firmware to the OEM A devices without approval
of the network operator. This breaks the behaviour of the larger
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system causing denial of service and possibly other threats. Where
the network is a distributed SCADA system, this could cause
misbehaviour of the process that is under control.
Threat Escalation: If the firmware expects configuration that is
present in Network A devices, but not Network B devices, then the
device may experience degraded security, leading to threats of All
Types.
Mitigated by: MFSR6
A.2.11. Threat MFT11: Reverse Engineering Of Firmware Image for
Vulnerability Analysis
Classification: All Types
An attacker wants to mount an attack on an IoT device. To prepare
the attack he or she retrieves the provided firmware image and
performs reverse engineering of the firmware image to analyze it for
specific vulnerabilities.
Mitigated by: MFSR7
A.3. Security Requirements
The security requirements here are a set of policies that mitigate
the threats described in the previous section.
A.3.1. Security Requirement MFSR1: Monotonic Sequence Numbers
Only an actor with firmware installation authority is permitted to
decide when device firmware can be installed. To enforce this rule,
Manifests MUST contain monotonically increasting sequence numbers.
Manifests MAY use UTC epoch timestamps to coordinate monotonically
increasting sequence numbers across many actors in many locations.
Devices MUST reject manifests with sequence numbers smaller than any
onboard sequence number.
N.B. This is not a firmware version. It is a manifest sequence
number. A firmware version may be rolled back by creating a new
manifest for the old firmware version with a later sequence number.
Mitigates: Threat MFT1 Implemented by: Manifest Field: Timestamp
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A.3.2. Security Requirement MFSR2: Vendor, Device-type Identifiers
Devices MUST only apply firmware that is intended for them. Devices
MUST know with fine granularity that a given update applies to their
vendor, model, hardware revision, software revision. Human-readable
identifiers are often error-prone in this regard, so unique
identifiers SHOULD be used.
Mitigates: Threat MFT2 Implemented by: Manifest Fields: Vendor ID
Condition, Class ID Condition
A.3.3. Security Requirement MFSR3: Best-Before Timestamps
Firmware MAY expire after a given time. Devices MAY provide a secure
clock (local or remote). If a secure clock is provided and the
Firmware manifest has a best-before timestamp, the device MUST reject
the manifest if current time is larger than the best-before time.
Mitigates: Threat MFT3 Implemented by: Manifest Field: Best-Before
timestamp condition
A.3.4. Security Requirement MFSR4: Signed Payload Descriptor
All descriptive information about the payload MUST be signed. This
MUST include:
- The type of payload (which may be independent of format)
- The location to store the payload
- The payload digest, in each state of installation (encrypted,
plaintext, installed, etc.)
- The payload size
- The payload format
- Where to obtain the payload
- All instructions or parameters for applying the payload
- Any rules that identify whether or not the payload can be used on
this device
Mitigates: Threats MFT4, MFT5, MFT6, MFT7, MFT9 Implemented by:
Manifest Fields: Vendor ID Condition, Class ID Condition, Precursor
Image Digest Condition, Payload Format, Storage Location, URIs,
Digests, Size
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A.3.5. Security Requirement MFSR5: Cryptographic Authenticity
The authenticity of an update must be demonstrable. Typically, this
means that updates must be digitally signed. Because the manifest
contains information about how to install the update, the manifest's
authenticity must also be demonstrable. To reduce the overhead
required for validation, the manifest contains the digest of the
firmware image, rather than a second digitial signature. The
authenticity of the manifest can be verified with a digital
signature, the authenticity of the firmware image is tied to the
manifest by the use of a fingerprint of the firmware image.
Mitigates: Threat MFT8 Implemented by: Signature
A.3.6. Security Requirement MFSR6: Rights Require Authenticity
If a device grants different rights to different actors, exercising
those rights MUST be accompanied by proof of those rights, in the
form of proof of authenticity. Authenticity mechanisms such as those
required in MFSR5 are acceptable but need to follow the end-to-end
security model.
For example, if a device has a policy that requires that firmware
have both an Authorship right and a Qualification right and if that
device grants Authorship and Qualification rights to different
parties, such as an OEM and an Operator, respectively, then the
firmware cannot be installed without proof of rights from both the
OEM and the Operator.
Mitigates: MFT10 Implemented by: Signature
A.3.7. Security Requirement MFSR7: Firmware encryption
Firmware images must be encrypted to prevent third parties, including
attackers, from reading the content of the firmware image and to
reverse engineer the code.
Mitigates: MFT11 Implemented by: Manifest Field: Content Key
Distribution Method
A.4. User Stories
User stories provide expected use cases. These are used to feed into
usability requirements.
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A.4.1. Use Case MFUC1: Installation Instructions
As an OEM for IoT devices, I want to provide my devices with
additional installation instructions so that I can keep process
details out of my payload data.
Some installation instructions might be:
- Specify a package handler
- Use a table of hashes to ensure that each block of the payload is
validate before writing.
- Run post-processing script after the update is installed
- Do not report progress
- Pre-cache the update, but do not install
- Install the pre-cached update matching this manifest
- Install this update immediately, overriding any long-running
tasks.
Satisfied by: MFUR1
A.4.2. Use Case MFUC2: Reuse Local Infrastructure
As an Operator of IoT devices, I would like to tell my devices to
look at my own infrastructure for payloads so that I can manage the
traffic generated by firmware updates on my network and my peers'
networks.
Satisfied by: MFUR2, MFUR3
A.4.3. Use Case MFUC3: Modular Update
As an OEM of IoT devices, I want to divide my firmware into
frequently updated and infrequently updated components, so that I can
reduce the size of updates and make different parties responsible for
different components.
Satisfied by: MFUR3
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A.4.4. Use Case MFUC4: Multiple Authorisations
As an Operator, I want to ensure the quality of a firmware update
before installing it, so that I can ensure a high standard of
reliability on my network. The OEM may restrict my ability to create
firmware, so I cannot be the only authority on the device.
Satisfied by: MFUR4
A.4.5. Use Case MFUC5: Multiple Payload Formats
As a OEM or Operator of devices, I want to be able to send multiple
payload formats to suit the needs of my update, so that I can
optimise the bandwidth used by my devices.
Satisfied by: MFUR5
A.4.6. Use Case MFUC6: IP Protection
As an OEM or developer for IoT devices, I want to protect the IP
contained in the firmware image, such as the utilized algorithms.
The need for protecting IP may have also been imposed on my due to
the use of some third party code libraries.
Satisfied by: MFSR7
A.5. Usability Requirements
The following usability requirements satisfy the user stories listed
above.
A.5.1. Usability Requirement MFUR1
It must be possible to write additional installation instructions
into the manifest.
Satisfies: Use-Case MFUC1 Implemented by: Manifest Field: Directives
A.5.2. Usability Requirement MFUR2
It must be possible to redirect payload fetches. This applies where
two manifests are used in conjunction. For example, an OEM manifest
specifies a payload and signs it, and provides a URI for that
payload. An Operator creates a second manifest, with a dependency on
the first. They use this second manifest to override the URIs
provided by the OEM, directing them into their own infrastructure
instead.
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Satisfies: Use-Case MFUC2 Implemented by: Manifest Field: Aliases
A.5.3. Usability Requirement MFUR3
It MUST be possible to link multiple manifests together so that a
multi-component update can be described. This allows multiple
parties with different permissions to collaborate in creating a
single update for the IoT device, across multiple components.
Satisfies: Use-Case MFUC2, MFUC3 Implemented by: Manifest Field:
Dependencies
A.5.4. Usability Requirement MFUR4
It MUST be possible to sign a manifest multiple times so that
signatures from multiple parties with different permissions can be
required in order to authorise installation of a manifest.
Satisfies: Use-Case MFUC4 Implemented by: COSE Signature (or similar)
A.5.5. Usability Requirement MFUR5
The manifest format MUST accommodate any payload format that an
operator or OEM wishes to use. Some examples of payload format would
be:
- Binary
- Elf
- Differential
- Compressed
- Packed configuration
Satisfies: Use-Case MFUC5 Implemented by: Manifest Field: Payload
Format
A.6. Manifest Fields
Each manifest field is anchored in a security requirement or a
usability requirement. The manifest fields are described below and
justified by their requirements.
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A.6.1. Manifest Field: Timestamp
A monotonically increasing sequence number. For convenience, a
timestamp implements the requirement of a monotonically increasing
sequence number. This allows global synchronisation of sequence
numbers without any additional management.
Implements: Security Requirement MFSR1.
A.6.2. Manifest Field: Vendor ID Condition
Vendor IDs MUST be unique. This is to prevent similarly, or
identically named entities from different geographic regions from
colliding in their customer's infrastructure. Recommended practice
is to use type 5 UUIDs with the vendor's domain name and the UUID DNS
prefix. Other options include type 1 and type 4 UUIDs.
Implements: Security Requirement MFSR2, MFSR4.
A.6.3. Manifest Field: Class ID Condition
Class Identifiers MUST be unique within a Vendor ID. This is to
prevent similarly, or identically named devices colliding in their
customer's infrastructure. Recommended practice is to use type 5
UUIDs with the model, hardware revision, etc. and use the Vendor ID
as the UUID prefix. Other options include type 1 and type 4 UUIDs.
A device "Class" is defined as any device that can run the same
firmware without modification. Classes MAY be implemented in a more
granular way. Classes MUST NOT be implemented in a less granular
way. Class ID can encompass model name, hardware revision, software
revision. Devices MAY have multiple Class IDs.
Implements: Security Requirement MFSR2, MFSR4.
A.6.4. Manifest Field: Precursor Image Digest Condition
When a precursor image is required by the payload format, a precursor
image digest condition MUST be present in the conditions list.
Implements: Security Requirement MFSR4
A.6.5. Manifest Field: Best-Before timestamp condition
This field tells a device the last application time. This is only
usable in conjunction with a secure clock.
Implements: Security Requirement MFSR3
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A.6.6. Manifest Field: Payload Format
The format of the payload must be indicated to devices is in an
unambiguous way. This field provides a mechanism to describe the
payload format, within the signed metadata.
Implements: Security Requirement MFSR4, Usability Requirement MFUR5
A.6.7. Manifest Field: Storage Location
This field tells the device which component is being updated. The
device can use this to establish which permissions are necessary and
the physical location to use.
Implements: Security Requirement MFSR4
A.6.8. Manifest Field: URIs
This field is a list of weighted URIs that the device uses to select
where to obtain a payload.
Implements: Security Requirement MFSR4
A.6.9. Manifest Field: Digests
This field is a map of digests, each for a separate stage of
installation. This allows the target device to ensure authenticity
of the payload at every step of installation.
Implements: Security Requirement MFSR4
A.6.10. Manifest Field: Size
The size of the payload in bytes.
Implements: Security Requirement MFSR4
A.6.11. Manifest Field: Signature
This is not strictly a manifest field. Instead, the manifest is
wrapped by a standardised authentication container, such as a COSE or
CMS signature object. The authentication container MUST support
multiple actors and multiple authentications.
Implements: Security Requirement MFSR5, MFSR6, MFUR4
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A.6.12. Manifest Field: Directives
A list of instructions that the device should execute, in order, when
installing the payload.
Implements: Usability Requirement MFUR1
A.6.13. Manifest Field: Aliases
A list of URI/Digest pairs. A device should build an alias table
while paring a manifest tree and treat any aliases as top-ranked URIs
for the corresponding digest.
Implements: Usability Requirement MFUR2
A.6.14. Manifest Field: Dependencies
A list of URI/Digest pairs that refer to other manifests by digest.
The manifests that are linked in this way must be acquired and
installed simultaneously in order to form a complete update.
Implements: Usability Requirement MFUR3
A.6.15. Manifest Field: Content Key Distribution Method
Encrypting firmware images requires symmetric content encryption
keys. Since there are several methods to protect or distribute the
symmetric content encryption keys, the manifest contains a field for
the Content Key Distribution Method. One examples for such a Content
Key Distribution Method is the usage of Key Tables, pointing to
content encryption keys, which themselves are encrypted using the
public keys of devices.
Implements: Security Requirement MFSR7.
Authors' Addresses
Brendan Moran
Arm Limited
EMail: Brendan.Moran@arm.com
Milosch Meriac
Arm Limited
EMail: Milosch.Meriac@arm.com
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Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@gmx.net
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