Network Working Group                                  J. Preuß Mattsson
Internet-Draft                                               J. Fornehed
Intended status: Informational                               G. Selander
Expires: 10 November 2022                                   F. Palombini
                                                                Ericsson
                                                               C. Amsüss
                                             Energy Harvesting Solutions
                                                              9 May 2022


         Attacks on the Constrained Application Protocol (CoAP)
                   draft-ietf-core-attacks-on-coap-00

Abstract

   Being able to securely read information from sensors, to securely
   control actuators, and to not enable distributed denial-of-service
   attacks are essential in a world of connected and networking things
   interacting with the physical world.  This document summarizes a
   number of known attacks on CoAP and show that just using CoAP with a
   security protocol like DTLS, TLS, or OSCORE is not enough for secure
   operation.  Several of the discussed attacks can be mitigated with
   the solutions in draft-ietf-core-echo-request-tag.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-core-attacks-on-coap/.

   Discussion of this document takes place on the Constrained RESTful
   Environments (CoRE) Working Group mailing list
   (mailto:core@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/EricssonResearch/coap-actuators.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.



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   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
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   This Internet-Draft will expire on 10 November 2022.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Attacks on CoAP . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The Block Attack  . . . . . . . . . . . . . . . . . . . .   5
     2.2.  The Request Delay Attack  . . . . . . . . . . . . . . . .   6
     2.3.  The Response Delay and Mismatch Attack  . . . . . . . . .   9
     2.4.  The Request Fragment Rearrangement Attack . . . . . . . .  12
       2.4.1.  Completing an Operation with an Earlier Final
               Block . . . . . . . . . . . . . . . . . . . . . . . .  13
       2.4.2.  Injecting a Withheld First Block  . . . . . . . . . .  14
       2.4.3.  Attack difficulty . . . . . . . . . . . . . . . . . .  15
     2.5.  The Relay Attack  . . . . . . . . . . . . . . . . . . . .  16
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   5.  Informative References  . . . . . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   Being able to securely read information from sensors and to securely
   control actuators are essential in a world of connected and
   networking things interacting with the physical world.  One protocol
   used to interact with sensors and actuators is the Constrained
   Application Protocol (CoAP) [RFC7252].  Any Internet-of-Things (IoT)
   deployment valuing security and privacy would use a security protocol



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   such as DTLS [RFC9147], TLS [RFC8446], or OSCORE [RFC8613] to protect
   CoAP, where the choice of security protocol depends on the transport
   protocol and the presence of intermediaries.  The use of CoAP over
   UDP and DTLS is specified in [RFC7252] and the use of CoAP over TCP
   and TLS is specified in [RFC8323].  OSCORE protects CoAP end-to-end
   with the use of COSE [RFC8152] and the CoAP Object-Security option
   [RFC8613], and can therefore be used over any transport.

   The Constrained Application Protocol (CoAP) [RFC7252] was designed
   with the assumption that security could be provided on a separate
   layer, in particular by using DTLS [RFC6347].  The four properties
   traditionally provided by security protocols are:

   *  Data confidentiality

   *  Data origin authentication

   *  Data integrity checking

   *  Replay protection

   In this document we show that protecting CoAP with a security
   protocol on another layer is not nearly enough to securely control
   actuators (and in many cases sensors) and that secure operation often
   demands far more than the four properties traditionally provided by
   security protocols.  We describe several serious attacks any on-path
   attacker (i.e., not only "trusted intermediaries") can do and
   discusses tougher requirements and mechanisms to mitigate the
   attacks.  In general, secure operation of actuators also requires the
   three properties:

   *  Data-to-data binding

   *  Data-to-space binding

   *  Data-to-time binding

   "Data-to-data binding" is e.g., binding of responses to a request or
   binding of data fragments to each other.  "Data-to-space binding" is
   the binding of data to an absolute or relative point in space (i.e.,
   a location) and may in the relative case be referred to as proximity.
   "Data-to-time binding" is the binding of data to an absolute or
   relative point in time and may in the relative case be referred to as
   freshness.  The two last properties may be bundled together as "Data-
   to-spacetime binding".






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   Freshness is a measure of when a message was sent on a timescale of
   the recipient.  A client or server that receives a message can either
   verify that the message is fresh or determine that it cannot be
   verified that the message is fresh.  What is considered fresh is
   application dependent.  Freshness is completely different from replay
   protection, but most replay protection mechanism use a sequence
   number.  Assuming the client is well-behaving, such a sequence number
   that can be used by the server as a relative measure of when a
   message was sent on a timescale of the sender.  Replay protection is
   mandatory in TLS and OSCORE and optional in DTLS.  DTLS and TLS use
   sequence numbers for both requests and responses.  In TLS the
   sequence numbers are implicit and not sent in the record.  OSCORE use
   sequence numbers for requests and some responses.  Most OSCORE
   responses are bound to the request and therefore, enable the client
   to determine if the response is fresh or not.

   The request delay attack (valid for DTLS, TLS, and OSCORE and
   described in Section 2.2) lets an attacker control an actuator at a
   much later time than the client anticipated.  The response delay and
   mismatch attack (valid for DTLS and TLS and described in Section 2.3)
   lets an attacker respond to a client with a response meant for an
   older request.  The request fragment rearrangement attack (valid for
   DTLS, TLS, and OSCORE and described in Section 2.4) lets an attacker
   cause unauthorized operations to be performed on the server, and
   responses to unauthorized operations to be mistaken for responses to
   authorized operations.

   The goal with this document is motivating generic and protocol-
   specific recommendations on the usage of CoAP.  Mechanisms mitigating
   some of the attacks discussed in this document can be found in
   [RFC9175].  This document is a companion document to [RFC9175] giving
   more information on the attacks motivating the mechanisms.

2.  Attacks on CoAP

   Internet-of-Things (IoT) deployments valuing security and privacy,
   need to use a security protocol such as DTLS, TLS, or OSCORE to
   protect CoAP.  This is especially true for deployments of actuators
   where attacks often (but not always) have serious consequences.  The
   attacks described in this section are made under the assumption that
   CoAP is already protected with a security protocol such as DTLS, TLS,
   or OSCORE, as an attacker otherwise can easily forge false requests
   and responses.








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2.1.  The Block Attack

   An on-path attacker can block the delivery of any number of requests
   or responses.  The attack can also be performed by an attacker
   jamming the lower layer radio protocol.  This is true even if a
   security protocol like DTLS, TLS, or OSCORE is used.  Encryption
   makes selective blocking of messages harder, but not impossible or
   even infeasible.  With DTLS and TLS, proxies can read the complete
   CoAP message, and with OSCORE, the CoAP header and several CoAP
   options are not encrypted.  In all three security protocols, the IP-
   addresses, ports, and CoAP message lengths are available to all on-
   path attackers, which may be enough to determine the server,
   resource, and command.  The block attack is illustrated in Figures 1
   and 2.

                 Client   Foe   Server
                    |      |      |
                    +----->X      |      Code: 0.03 (PUT)
                    | PUT  |      |     Token: 0x47
                    |      |      |  Uri-Path: lock
                    |      |      |   Payload: 1 (Lock)
                    |      |      |

                        Figure 1: Blocking a request

   Where 'X' means the attacker is blocking delivery of the message.

               Client   Foe   Server
                  |      |      |
                  +------------>|      Code: 0.03 (PUT)
                  |      | PUT  |     Token: 0x47
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 1 (Lock)
                  |      |      |
                  |      X<-----+      Code: 2.04 (Changed)
                  |      | 2.04 |     Token: 0x47
                  |      |      |

                       Figure 2: Blocking a response












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   While blocking requests to, or responses from, a sensor is just a
   denial-of-service attack, blocking a request to, or a response from,
   an actuator results in the client losing information about the
   server's status.  If the actuator e.g., is a lock (door, car, etc.),
   the attack results in the client not knowing (except by using out-of-
   band information) whether the lock is unlocked or locked, just like
   the observer in the famous Schrödinger’s cat thought experiment.  Due
   to the nature of the attack, the client cannot distinguish the attack
   from connectivity problems, offline servers, or unexpected behavior
   from middle boxes such as NATs and firewalls.

   Remedy: Any IoT deployment of actuators where synchronized state is
   important need to use confirmable messages and the client need to
   take appropriate actions when a response is not received and it
   therefore loses information about the server's status.

2.2.  The Request Delay Attack

   An on-path attacker may not only block packets, but can also delay
   the delivery of any packet (request or response) by a chosen amount
   of time.  If CoAP is used over a reliable and ordered transport such
   as TCP with TLS or OSCORE (with TLS-like sequence number handling),
   no messages can be delivered before the delayed message.  If CoAP is
   used over an unreliable and unordered transport such as UDP with DTLS
   or OSCORE, other messages can be delivered before the delayed message
   as long as the delayed packet is delivered inside the replay window.
   When CoAP is used over UDP, both DTLS and OSCORE allow out-of-order
   delivery and uses sequence numbers together with a replay window to
   protect against replay attacks against requests.  The replay window
   has a default length of 64 in DTLS and 32 in OSCORE.  The attacker
   can influence the replay window state by blocking and delaying
   packets.  By first delaying a request, and then later, after
   delivery, blocking the response to the request, the client is not
   made aware of the delayed delivery except by the missing response.
   In general, the server has no way of knowing that the request was
   delayed and will therefore happily process the request.  Note that
   delays can also happen for other reasons than a malicious attacker.

   If some wireless low-level protocol is used, the attack can also be
   performed by the attacker simultaneously recording what the client
   transmits while at the same time jamming the server.  The request
   delay attack is illustrated in Figure 3.









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               Client   Foe   Server
                  |      |      |
                  +----->@      |      Code: 0.03 (PUT)
                  | PUT  |      |     Token: 0x9c
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 0 (Unlock)
                  |      |      |
                    ....   ....
                  |      |      |
                  |      @----->|      Code: 0.03 (PUT)
                  |      | PUT  |     Token: 0x9c
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 0 (Unlock)
                  |      |      |
                  |      X<-----+      Code: 2.04 (Changed)
                  |      | 2.04 |     Token: 0x9c
                  |      |      |

                        Figure 3: Delaying a request

   Where '@' means the attacker is storing and later forwarding the
   message (@ may alternatively be seen as a wormhole connecting two
   points in time).

   While an attacker delaying a request to a sensor is often not a
   security problem, an attacker delaying a request to an actuator
   performing an action is often a serious problem.  A request to an
   actuator (for example a request to unlock a lock) is often only meant
   to be valid for a short time frame, and if the request does not reach
   the actuator during this short timeframe, the request should not be
   fulfilled.  In the unlock example, if the client does not get any
   response and does not physically see the lock opening, the user is
   likely to walk away, calling the locksmith (or the IT-support).

   If a non-zero replay window is used (the default when CoAP is used
   over UDP), the attacker can let the client interact with the actuator
   before delivering the delayed request to the server (illustrated in
   Figure 4).  In the lock example, the attacker may store the first
   "unlock" request for later use.  The client will likely resend the
   request with the same token.  If DTLS is used, the resent packet will
   have a different sequence number and the attacker can forward it.  If
   OSCORE is used, resent packets will have the same sequence number and
   the attacker must block them all until the client sends a new message
   with a new sequence number (not shown in Figure 4).  After a while
   when the client has locked the door again, the attacker can deliver
   the delayed "unlock" message to the door, a very serious attack.





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               Client   Foe   Server
                  |      |      |
                  +----->@      |      Code: 0.03 (PUT)
                  | PUT  |      |     Token: 0x9c
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 0 (Unlock)
                  |      |      |
                  +------------>|      Code: 0.03 (PUT)
                  | PUT  |      |     Token: 0x9c
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 0 (Unlock)
                  |      |      |
                  <-------------+      Code: 2.04 (Changed)
                  |      | 2.04 |     Token: 0x9c
                  |      |      |
                    ....   ....
                  |      |      |
                  +------------>|      Code: 0.03 (PUT)
                  | PUT  |      |     Token: 0x7a
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 1 (Lock)
                  |      |      |
                  <-------------+      Code: 2.04 (Changed)
                  |      | 2.04 |     Token: 0x7a
                  |      |      |
                  |      @----->|      Code: 0.03 (PUT)
                  |      | PUT  |     Token: 0x9c
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 0 (Unlock)
                  |      |      |
                  |      X<-----+      Code: 2.04 (Changed)
                  |      | 2.04 |     Token: 0x9c
                  |      |      |

                 Figure 4: Delaying request with reordering

   While the second attack (Figure 4) can be mitigated by using a replay
   window of length zero, the first attack (Figure 3) cannot.  A
   solution must enable the server to verify that the request was
   received within a certain time frame after it was sent or enable the
   server to securely determine an absolute point in time when the
   request is to be executed.  This can be accomplished with either a
   challenge-response pattern, by exchanging timestamps between client
   and server, or by only allowing requests a short period after client
   authentication.






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   Requiring a fresh client authentication (such as a new TLS/DTLS
   handshake or an EDHOC key exchange [I-D.ietf-lake-edhoc]) mitigates
   the problem, but requires larger messages and more processing than a
   dedicated solution.  Security solutions based on exchanging
   timestamps require exactly synchronized time between client and
   server, and this may be hard to control with complications such as
   time zones and daylight saving.  Wall clock time is not monotonic,
   may reveal that the endpoints will accept expired certificates, or
   reveal the endpoint's location.  Use of non-monotonic clocks is
   problematic as the server will accept requests if the clock is moved
   backward and reject requests if the clock is moved forward.  Even if
   the clocks are synchronized at one point in time, they may easily get
   out-of-sync and an attacker may even be able to affect the client or
   the server time in various ways such as setting up a fake NTP server,
   broadcasting false time signals to radio-controlled clocks, or
   exposing one of them to a strong gravity field.  As soon as client
   falsely believes it is time synchronized with the server, delay
   attacks are possible.  A challenge response mechanism where the
   server does not need to synchronize its time with the client is
   easier to analyze but require more roundtrips.  The challenges,
   responses, and timestamps may be sent in a CoAP option or in the CoAP
   payload.

   Remedy: Any IoT deployment of actuators where freshness is important
   should use the mechanisms specified in [RFC9175] unless another
   application specific challenge-response or timestamp mechanism is
   used.

2.3.  The Response Delay and Mismatch Attack

   The following attack can be performed if CoAP is protected by a
   security protocol where the response is not bound to the request in
   any way except by the CoAP token.  This would include most general
   security protocols, such as DTLS, TLS, and IPsec, but not OSCORE.
   CoAP [RFC7252] uses a client generated token that the server echoes
   to match responses to request, but does not give any guidelines for
   the use of token with DTLS and TLS, except that the tokens currently
   "in use" SHOULD (not SHALL) be unique.  In HTTPS, this type of
   binding is always assured by the ordered and reliable delivery, as
   well as mandating that the server sends responses in the same order
   that the requests were received.

   The attacker performs the attack by delaying delivery of a response
   until the client sends a request with the same token, the response
   will be accepted by the client as a valid response to the later
   request.  If CoAP is used over a reliable and ordered transport such
   as TCP with TLS, no messages can be delivered before the delayed
   message.  If CoAP is used over an unreliable and unordered transport



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   such as UDP with DTLS, other messages can be delivered before the
   delayed message as long as the delayed packet is delivered inside the
   replay window.  Note that mismatches can also happen for other
   reasons than a malicious attacker, e.g., delayed delivery or a server
   sending notifications to an uninterested client.

   The attack can be performed by an attacker on the wire, or an
   attacker simultaneously recording what the server transmits while at
   the same time jamming the client.  As (D)TLS encrypts the Token, the
   attacker needs to predict when the Token is resused.  How hard that
   is depends on the CoAP library, but some implementations are known to
   omit the Token as much as possible and others lets the application
   chose the Token.  If the response is a "piggybacked response", the
   client may additionally check the Message ID and drop it on mismatch.
   That doesn't make the attack impossible, but lowers the probability.

   The response delay and mismatch attack is illustrated in Figure 5.

               Client   Foe   Server
                  |      |      |
                  +------------>|      Code: 0.03 (PUT)
                  | PUT  |      |     Token: 0x77
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 0 (Unlock)
                  |      |      |
                  |      @<-----+      Code: 2.04 (Changed)
                  |      | 2.04 |     Token: 0x77
                  |      |      |
                    ....   ....
                  |      |      |
                  +----->X      |      Code: 0.03 (PUT)
                  | PUT  |      |     Token: 0x77
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 0 (Lock)
                  |      |      |
                  <------@      |      Code: 2.04 (Changed)
                  | 2.04 |      |     Token: 0x77
                  |      |      |

             Figure 5: Delaying and mismatching response to PUT

   If we once again take a lock as an example, the security consequences
   may be severe as the client receives a response message likely to be
   interpreted as confirmation of a locked door, while the received
   response message is in fact confirming an earlier unlock of the door.
   As the client is likely to leave the (believed to be locked) door
   unattended, the attacker may enter the home, enterprise, or car
   protected by the lock.



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   The same attack may be performed on sensors.  As illustrated in
   Figure 6, an attacker may convince the client that the lock is
   locked, when it in fact is not.  The "Unlock" request may be also be
   sent by another client authorized to control the lock.

               Client   Foe   Server
                  |      |      |
                  +------------>|      Code: 0.01 (GET)
                  | GET  |      |     Token: 0x77
                  |      |      |  Uri-Path: lock
                  |      |      |
                  |      @<-----+      Code: 2.05 (Content)
                  |      | 2.05 |     Token: 0x77
                  |      |      |   Payload: 1 (Locked)
                  |      |      |
                  +------------>|      Code: 0.03 (PUT)
                  | PUT  |      |     Token: 0x34
                  |      |      |  Uri-Path: lock
                  |      |      |   Payload: 1 (Unlock)
                  |      |      |
                  |      X<-----+      Code: 2.04 (Changed)
                  |      | 2.04 |     Token: 0x34
                  |      |      |
                  +----->X      |      Code: 0.01 (GET)
                  | GET  |      |     Token: 0x77
                  |      |      |  Uri-Path: lock
                  |      |      |
                  <------@      |      Code: 2.05 (Content)
                  | 2.05 |      |     Token: 0x77
                  |      |      |   Payload: 1 (Locked)
                  |      |      |

             Figure 6: Delaying and mismatching response to GET

   As illustrated in Figure 7, an attacker may even mix responses from
   different resources as long as the two resources share the same
   (D)TLS connection on some part of the path towards the client.  This
   can happen if the resources are located behind a common gateway, or
   are served by the same CoAP proxy.  An on-path attacker (not
   necessarily a (D)TLS endpoint such as a proxy) may e.g., deceive a
   client that the living room is on fire by responding with an earlier
   delayed response from the oven (temperatures in degree Celsius).









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           Client   Foe   Server
              |      |      |
              +------------>|      Code: 0.01 (GET)
              | GET  |      |     Token: 0x77
              |      |      |  Uri-Path: oven/temperature
              |      |      |
              |      @<-----+      Code: 2.05 (Content)
              |      | 2.05 |     Token: 0x77
              |      |      |   Payload: 225
              |      |      |
                ....   ....
              |      |      |
              +----->X      |      Code: 0.01 (GET)
              | GET  |      |     Token: 0x77
              |      |      |  Uri-Path: livingroom/temperature
              |      |      |
              <------@      |      Code: 2.05 (Content)
              | 2.05 |      |     Token: 0x77
              |      |      |   Payload: 225
              |      |      |

      Figure 7: Delaying and mismatching response from other resource

   Remedy: Section 4.2 of [RFC9175] formally updates the client token
   processing for CoAP [RFC7252].  Following this updated processing
   mitigates the attack.

2.4.  The Request Fragment Rearrangement Attack

   These attack scenarios show that the Request Delay and Block Attacks
   can be used against block-wise transfers to cause unauthorized
   operations to be performed on the server, and responses to
   unauthorized operations to be mistaken for responses to authorized
   operations.  The combination of these attacks is described as a
   separate attack because it makes the Request Delay Attack relevant to
   systems that are otherwise not time-dependent, which means that they
   could disregard the Request Delay Attack.

   This attack works even if the individual request/response pairs are
   encrypted, authenticated and protected against the Response Delay and
   Mismatch Attack, provided the attacker is on the network path and can
   correctly guess which operations the respective packages belong to.









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   The attacks can be performed on any security protocol where the
   attacker can delay the delivery of a message unnoticed.  This
   incluses DTLS, IPsec, and most OSCORE configurations.  The attacks
   does not work on TCP with TLS or OSCORE (with TLS-like sequence
   number handling) as in these cases no messages can be delivered
   before the delayed message.

2.4.1.  Completing an Operation with an Earlier Final Block

   In this scenario (illustrated in Figure 8), blocks from two
   operations on a POST-accepting resource are combined to make the
   server execute an action that was not intended by the authorized
   client.  This works only if the client attempts a second operation
   after the first operation failed (due to what the attacker made
   appear like a network outage) within the replay window.  The client
   does not receive a confirmation on the second operation either, but,
   by the time the client acts on it, the server has already executed
   the unauthorized action.

   Client   Foe   Server
      |      |      |
      +------------->    POST "incarcerate" (Block1: 0, more to come)
      |      |      |
      <-------------+    2.31 Continue (Block1: 0 received, send more)
      |      |      |
      +----->@      |    POST "valjean" (Block1: 1, last block)
      |      |      |
      +----->X      |    All retransmissions dropped
      |      |      |

   (Client: Odd, but let's go on and promote Javert)

      |      |      |
      +------------->    POST "promote" (Block1: 0, more to come)
      |      |      |
      |      X<-----+    2.31 Continue (Block1: 0 received, send more)
      |      |      |
      |      @------>    POST "valjean" (Block1: 1, last block)
      |      |      |
      |      X<-----+    2.04 Valjean Promoted
      |      |      |

       Figure 8: Completing an operation with an earlier final block

   Remedy: If a client starts new block-wise operations on a security
   context that has lost packets, it needs to label the fragments in
   such a way that the server will not mix them up.




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   A mechanism to that effect is described as Request-Tag [RFC9175].
   Had it been in place in the example and used for body integrity
   protection, the client would have set the Request-Tag option in the
   "promote" request.  Depending on the server's capabilities and setup,
   either of four outcomes could have occurred:

   1.  The server could have processed the reinjected POST "valjean" as
       belonging to the original "incarcerate" block; that's the
       expected case when the server can handle simultaneous block
       transfers.

   2.  The server could respond 5.03 Service Unavailable, including a
       Max-Age option indicating how long it prefers not to take any
       requests that force it to overwrite the state kept for the
       "incarcerate" request.

   3.  The server could decide to drop the state kept for the
       "incarcerate" request's state, and process the "promote" request.
       The reinjected POST "valjean" will then fail with 4.08 Request
       Entity incomplete, indicating that the server does not have the
       start of the operation any more.

2.4.2.  Injecting a Withheld First Block

   If the first block of a request is withheld by the attacker for later
   use, it can be used to have the server process a different request
   body than intended by the client.  Unlike in the previous scenario,
   it will return a response based on that body to the client.

   Again, a first operation (that would go like "Girl stole apple.  What
   shall we do with her?" - "Set her free.") is aborted by the proxy,
   and a part of that operation is later used in a different operation
   to prime the server for responding leniently to another operation
   that would originally have been "Evil Queen poisoned apple.  What
   shall we do with her?" - "Lock her up.".  The attack is illustrated
   in Figure 9.















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   Client   Foe   Server
      |      |      |
      +----->@      |    POST "Girl stole apple. Wh"
      |      |      |        (Block1: 0, more to come)

   (Client: We'll try that one later again; for now, we have something
   more urgent:)

      |      |      |
      +------------->    POST "Evil Queen poisened apple. Wh"
      |      |      |        (Block1: 0, more to come)
      |      |      |
      |      @<-----+    2.31 Continue (Block1: 0 received, send more)
      |      |      |
      |      @------>    POST "Girl stole apple. Wh"
      |      |      |        (Block1: 0, more to come)
      |      |      |
      |      X<-----+    2.31 Continue (Block1: 0 received, send more)
      |      |      |
      <------@      |    2.31 Continue (Block1: 0 received, send more)
      |      |      |
      +------------->    POST "at shall we do with her?"
      |      |      |        (Block1: 1, last block)
      |      |      |
      <-------------+    2.05 "Set her free."
      |      |      |        (Block1: 1 received and this is the result)

                 Figure 9: Injecting a withheld first block

   The remedy described in Section 2.4.1 works also for this case.  Note
   that merely requiring that blocks of an operation should have
   incrementing sequence numbers would be insufficient to remedy this
   attack.

2.4.3.  Attack difficulty

   The success of any fragment rearrangement attack has multiple
   prerequisites:

   *  A client sends different block-wise requests that are only
      distinguished by their content.

      This is generally rare in typical CoRE applications, but can
      happen when the bodies of FETCH requests exceed the fragmentation
      threshold, or when SOAP patterns are emulated.

   *  A client starts later block-wise operations after an earlier one
      has failed.



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      This happens regularly as a consequence of operating in a low-
      power and lossy network: Losses can cause failed operation
      (especially when the network is unavailable for time exceeding the
      "few expected round-trips" they may be limited to per [RFC7959]),
      and the cost of reestablishing a security context.

   *  The attacker needs to be able to determine which packets contain
      which fragments.

      This can be achieved by an on-path attacker by observing request
      timing, or simply by observing request sizes in the case when a
      body is split into precisely two blocks.

   It is _not_ a prerequisite that the resulting misassembled request
   body is syntactically correct: As the server erroneously expects the
   body to be integrity protected from an authorized source, it might be
   using a parser not suitable for untrusted input.  Such a parser might
   crash the server in extreme cases, but might also produce a valid but
   incorrect response to the request the client associates the response
   with.  Note that many constrained applications aim to minimize
   traffic and thus employ compact data formats; that compactness leaves
   little room for syntactically invalid messages.

   The attack is easier if the attacker has control over the request
   bodies (which would be the case when a trusted proxy validates the
   attacker's authorization to perform two given requests, and an attack
   on the path between the proxy and the server recombines the blocks to
   a semantically different request).  Attacks of that shape can easily
   result in reassembled bodies chosen by the attacker, but no services
   are currently known that operate in this way.

   Summarizing, it is unlikely that an attacker can perform any of the
   fragment rearrangement attacks on any given system -- but given the
   diversity of applications built on CoAP, it is easily to imagine that
   single applications would be vulnerable.  As block-wise transfer is a
   basic feature of CoAP and its details are sometimes hidden behind
   abstractions or proxies, application authors can not be expected to
   design their applications with these attacks in mind, and mitigation
   on the protocol level is prudent.

2.5.  The Relay Attack

   Yet another type of attack can be performed in deployments where
   actuator actions are triggered automatically based on proximity and
   without any user interaction, e.g., a car (the client) constantly
   polling for the car key (the server) and unlocking both doors and
   engine as soon as the car key responds.  An attacker (or pair of
   attackers) may simply relay the CoAP messages out-of-band, using for



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   examples some other radio technology.  By doing this, the actuator
   (i.e., the car) believes that the client is close by and performs
   actions based on that false assumption.  The attack is illustrated in
   Figure 10.  In this example the car is using an application specific
   challenge-response mechanism transferred as CoAP payloads.

   Client   Foe         Foe   Server
      |      |           |      |
      +----->| ......... +----->|      Code: 0.02 (POST)
      | POST |           | POST |     Token: 0x3a
      |      |           |      |  Uri-Path: lock
      |      |           |      |   Payload: JwePR2iCe8b0ux (Challenge)
      |      |           |      |
      |<-----+ ......... |<-----+      Code: 2.04 (Changed)
      | 2.04 |           | 2.04 |     Token: 0x3a
      |      |           |      |   Payload: RM8i13G8D5vfXK (Response)
      |      |           |      |

            Figure 10: Relay attack (the client is the actuator)

   The consequences may be severe, and in the case of a car, lead to the
   attacker unlocking and driving away with the car, an attack that
   unfortunately is happening in practice.

   Remedy: Getting a response over a short-range radio cannot be taken
   as proof of proximity and can therefore not be used to take actions
   based on such proximity.  Any automatically triggered mechanisms
   relying on proximity need to use other stronger mechanisms to
   establish proximity.  Mechanisms that can be used are: measuring the
   round-trip time and calculating the maximum possible distance based
   on the speed of light, or using radio with an extremely short range
   like NFC (centimeters instead of meters).  Another option is to
   include geographical coordinates (from e.g., GPS) in the messages and
   calculate proximity based on these, but in this case the location
   measurements need to be very precise and the system need to make sure
   that an attacker cannot influence the location estimation.  Some
   types of global navigation satellite systems (GNSS) receivers are
   vulnerable to spoofing attacks.

3.  Security Considerations

   The whole document can be seen as security considerations for CoAP.

4.  IANA Considerations

   This document has no actions for IANA.

5.  Informative References



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   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-13, 18
              April 2022, <https://www.ietf.org/archive/id/draft-ietf-
              lake-edhoc-13.txt>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.







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   [RFC9175]  Amsüss, C., Preuß Mattsson, J., and G. Selander,
              "Constrained Application Protocol (CoAP): Echo, Request-
              Tag, and Token Processing", RFC 9175,
              DOI 10.17487/RFC9175, February 2022,
              <https://www.rfc-editor.org/info/rfc9175>.

Acknowledgements

   The authors would like to thank Carsten Bormann, Klaus Hartke, Jaime
   Jiménez, Ari Keränen, Matthias Kovatsch, Achim Kraus, Sandeep Kumar,
   and András Méhes for their valuable comments and feedback.

Authors' Addresses

   John Preuß Mattsson
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: john.mattsson@ericsson.com


   John Fornehed
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: john.fornehed@ericsson.com


   Göran Selander
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: goran.selander@ericsson.com


   Francesca Palombini
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: francesca.palombini@ericsson.com


   Christian Amsüss
   Energy Harvesting Solutions
   Email: c.amsuess@energyharvesting.at






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