Attacks on the Constrained Application Protocol (CoAP)
draft-ietf-core-attacks-on-coap-01
The information below is for an old version of the document.
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|---|---|---|---|
| Authors | John Preuß Mattsson , John Fornehed , Göran Selander , Francesca Palombini , Christian Amsüss | ||
| Last updated | 2022-11-10 | ||
| Replaces | draft-mattsson-core-coap-attacks | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-core-attacks-on-coap-01
Network Working Group J. Preuß Mattsson
Internet-Draft J. Fornehed
Intended status: Informational G. Selander
Expires: 14 May 2023 F. Palombini
Ericsson
C. Amsüss
Energy Harvesting Solutions
10 November 2022
Attacks on the Constrained Application Protocol (CoAP)
draft-ietf-core-attacks-on-coap-01
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. Using a security protocol such
as DTLS, TLS, or OSCORE to protect CoAP is a requirement for secure
operation and protects against many attacks. This document
summarizes a number of known attacks on CoAP deployments 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 RFC 9175.
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/. Subscribe at
https://www.ietf.org/mailman/listinfo/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/attacks-on-coap.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 14 May 2023.
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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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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Attacks on CoAP . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. The Blocking 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
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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
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. Using a
security protocol to protect CoAP is a requirement for secure
operation and protects against many attacks. The CoAP NoSec mode
does not provide any security as an attacker can easily eavesdrop on
all messages and forge false requests and responses.
The four properties traditionally provided by security protocols are:
* Data confidentiality
* Data origin authentication
* Data integrity protection
* Data replay protection
In this document we show that protecting CoAP with a security
protocol is not 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
also requires these four properties:
* Availability
* Data-to-data binding
* Data-to-space binding
* Data-to-time binding
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Availability means that services and information must be available
when needed. "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".
Freshness (as defined in [RFC9175]) 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.
Some of the presented attacks such as the blocking attack, the
request delay attack, and the relay attack are not specific to CoAP.
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.
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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.
2.1. The Blocking 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 blocking 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
| | |
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Figure 2: Blocking a response
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 Blocking
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-17, 12
October 2022, <https://www.ietf.org/archive/id/draft-ietf-
lake-edhoc-17.txt>.
[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>.
[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>.
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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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