Controlling Actuators with CoAP
draft-mattsson-core-coap-actuators-00
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 | ||
| Last updated | 2015-10-19 | ||
| Replaced by | draft-mattsson-core-coap-attacks, draft-mattsson-core-coap-attacks | ||
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draft-mattsson-core-coap-actuators-00
Network Working Group J. Mattsson
Internet-Draft J. Fornehed
Intended status: Standards Track G. Selander
Expires: April 21, 2016 F. Palombini
Ericsson
October 19, 2015
Controlling Actuators with CoAP
draft-mattsson-core-coap-actuators-00
Abstract
Being able to trust information from sensors and to securely control
actuators is essential in a world of connected and networking things
interacting with the physical world. In this memo we show that just
using COAP with a security protocol like DTLS or OSCOAP is not
enough. We describe several serious attacks any on-path attacker can
do, and discuss tougher requirements and mechanisms to mitigate the
attacks. While this document is focused on actuators, one of the
attacks applies equally well to sensors using DTLS.
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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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 April 21, 2016.
Copyright Notice
Copyright (c) 2015 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. The Block Attack . . . . . . . . . . . . . . . . . . . . 3
2.2. The Request Delay Attack . . . . . . . . . . . . . . . . 4
2.3. The Response Delay and Mismatch Attack . . . . . . . . . 7
2.4. The Relay Attack . . . . . . . . . . . . . . . . . . . . 10
3. The Repeat Option . . . . . . . . . . . . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . 14
6.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Being able to trust information from sensors and to securely control
actuators is 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). Any Internet-of-Things (IoT) deployment valuing security and
privacy would use a security protocol such as DTLS [RFC6347] or
OSCOAP [I-D.selander-ace-object-security] to protect CoAP, but we
show that this is not enough. 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. The request delay attack (valid for both DTLS and OSCOAP
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 described in Section 2.3)
lets an attacker respond to a client with a response meant for an
older request. In Section 3, a new CoAP Option, the Repeat Option,
mitigating the delay attack in specified.
2. Attacks
Internet-of-Things (IoT) deployments valuing security and privacy,
MUST use a security protocol such as DTLS or OSCOAP to protect CoAP.
This is especially true for deployments of actuators where attacks
often (but not always) have serious consequences. The attacks
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described in this section are made under the assumption that CoAP is
already protected with a security protocol such as DTLS or OSCOAP, as
an attacker otherwise can easily forge false requests and responses.
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 or OSCOAP is used. Encryption makes
selective blocking of messages harder, but not impossible or even
infeasible. With DTLS, proxies have access to the complete CoAP
message, and with OSCOAP, the CoAP header and several CoAP options
are not encrypted. In both 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 Figure 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
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-
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band information) whether the lock is unlocked or locked, just like
the observer in the famous Schroedinger'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: In actuator deployments where confirmation is important, the
application MUST notify the user upon reception of the response, or
warn the user when a response is not received. The application
SHOULD also indicate to the user that the status of the actuator is
now uncertain.
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. This is true even if DTLS or OSCOAP is used, as long as the
delayed packet is delivered inside the replay window. The replay
window has a default length of 64 in DTLS and is application
dependent in OSCOAP. The attacker can control the replay window by
blocking some or all other 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. The server has in general, no way of knowing that
the request was delayed and will therefore happily process the
request.
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 spacetime).
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 in DTLS and
unspecified in OSCOAP), 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 OSCOAP 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. This can be
accomplished with either a challenge-response pattern or by
exchanging timestamps. Security solutions based on timestamps
require exactly synchronized time, and this is hard to control with
complications such as time zones and daylight saving. 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 expose
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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 is much more failure proof
and easy to analyze. One such mechanism, the CoAP Repeat Option, is
specified in Section 3.
Remedy: The CoAP Repeat Option specified in Section 3 SHALL be used
for controlling actuators 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 and IPsec, but not OSCOAP. The
attacker performs the attack by delaying delivery of a response until
the client sends a request with the same token. As long as the
response is inside the replay window (which the attacker can make
sure by blocking later responses), the response will be accepted by
the client as a valid response to the later request. CoAP [RFC7252]
does not give any guidelines for the use of token with DTLS, except
that the tokens currently "in use" SHOULD (not SHALL) be unique.
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. The response delay and mismatch
attack is illustrated in Figure 5.
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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.
The same attack may be performed on sensors, also this with serious
consequences. 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.
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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 DTLS
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 DTLS 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
OSCOAP is not susceptible to these attacks since it provides a secure
binding between request and response messages.
Remedy: If CoAP is protected with a security protocol not providing
bindings between requests and responses (e.g. DTLS) the client MUST
NOT reuse any tokens for a given source/destination which the client
has not received responses to. The easiest way to accomplish this is
to implement the token as a counter and never reuse any tokens at
all, this approach SHOULD be followed.
2.4. 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
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 8. In this example the car is using an application specific
challenge-response mechanism transferred as CoAP payloads.
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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 8: 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 MUST NOT be taken
as proof of proximity and therefore MUST NOT be used to take actions
based on such proximity. Any automatically triggered mechanisms
relying on proximity MUST use other stronger mechanisms to guarantee
proximity. Mechanisms that MAY be used are: measuring the round-trip
time and calculate 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 including
geographical coordinates (from e.g. GPS) in the messages and
calculate proximity based on these, but in this case the location
measurements MUST be very precise and the system MUST make sure that
an attacker cannot influence the location estimation, something that
is very hard in practice.
3. The Repeat Option
The Repeat Option is a challenge-response mechanism for CoAP, binding
a resent request to an earlier 4.03 forbidden response. The
challenge (for the client) is simply to echo the Repeat Option value
in a new request. The Repeat Option enables the server to verify the
freshness of a request, thus mitigating the Delay Attack described in
Section 2.2. An example message flow is illustrated in Figure 9.
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Client Server
| |
+----->| Code: 0.03 (PUT)
| PUT | Token: 0x41
| | Uri-Path: lock
| | Payload: 0 (Unlock)
| |
|<-----+ t0 Code: 4.03 (Forbidden)
| 4.03 | Token: 0x41
| | Repeat: 0x6c880d41167ba807
| |
+----->| t1 Code: 0.03 (PUT)
| PUT | Token: 0x42
| | Uri-Path: lock
| | Repeat: 0x6c880d41167ba807
| | Payload: 0 (Unlock)
| |
|<-----+ Code: 2.04 (Changed)
| 2.04 | Token: 0x42
| |
Figure 9: The Repeat Option
The Repeat Option may be used for all Methods and Response Codes. In
responses, the value MUST be a (pseudo-)random bit string with a
length of at least 64 bits. A new (pseudo-)random bit string MUST be
generated for each response. In requests, the Repeat Option MUST
echo the value from a previously received response.
The Repeat Option is critical, Safe-to-Forward, not part of the
Cache-Key, and not repeatable.
Upon receiving a request without the Repeat Option to a resource with
freshness requirements, the server sends a 4.03 Forbidden response
with a Repeat Option and stores the option value and the response
transmit time t0.
Upon receiving a 4.03 Forbidden response with the Repeat Option, the
client SHOULD resend the request, echoing the Repeat Option value.
Upon receiving a request with the Repeat Option, the server verifies
that the option value equals the previously sent value; otherwise the
request is not processed further. The server calculates the round-
trip time RTT = (t1 - t0), where t1 is the request receive time. The
server MUST only accept requests with a round-trip time below a
certain threshold T, i.e. RTT < T, otherwise the request is not
processed further, and an error message MAY be sent. The threshold T
is application specific.
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An attacker able to control the server's clock with high precision,
could still be able to perform a delay attack by moving the server's
clock back in time, thus making the measured round-trip time smaller
than the actual round-trip time. The times t0 and t1 MUST therefore
be measured with a steady clock (one that cannot be adjusted).
EDITORS NOTE: The mechanism described above gives the server
freshness guarantee independently of what the client does. The
disadvantages are that the mechanism always takes two round-trips and
that the server has to save the option value and the time t0. Other
solutions involving time may be discussed:
o The server may simply send the client the current time in its
timescale, i.e. a timestamp (option value = t0). The client may
then use this timestamp to estimate the current time in the
servers timescale when sending future requests (i.e. not echoing).
This approach has the benefit of reducing round-trips and server
state, but has the security problems discussed in Section 2.2.
o The server may instead of a pseudorandom value send an encrypted
timestamp (option value = E(k, t0)). CTR-mode would from a
security point be like sending (value = t0). ECB-mode or CCM-mode
would work, but would expand the value length. With CCM, the
server might also bind the option value to request (value =
AEAD(k, t0, parts of request)). This approach does not reduce the
number of round-trips but eliminates server state.
4. IANA Considerations
This document defines the following Option Number, whose value have
been assigned to the CoAP Option Numbers Registry defined by
[RFC7252].
+--------+------------------+
| Number | Name |
+--------+------------------+
| 29 | Repeat |
+--------+------------------+
5. Security Considerations
The whole document can be seen as security considerations for CoAP.
6. References
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6.1. Normative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
6.2. Informative References
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"June 29, 2015", draft-selander-ace-object-security-02
(work in progress), June 2015.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
Authors' Addresses
John 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
Goran Selander
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: goran.selander@ericsson.com
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Francesca Palombini
Ericsson AB
SE-164 80 Stockholm
Sweden
Email: francesca.palombini@ericsson.com
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