ACE Working Group C. Sengul
Internet-Draft Brunel University
Intended status: Standards Track A. Kirby
Expires: January 14, 2021 Oxbotica
P. Fremantle
University of Portsmouth
July 13, 2020

MQTT-TLS profile of ACE


This document specifies a profile for the ACE (Authentication and Authorization for Constrained Environments) framework to enable authorization in an Message Queuing Telemetry Transport (MQTT)-based publish-subscribe messaging system. Proof-of-possession keys, bound to OAuth2.0 access tokens, are used to authenticate and authorize MQTT Clients. The protocol relies on TLS for confidentiality and MQTT server (broker) authentication.

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

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 January 14, 2021.

Copyright Notice

Copyright (c) 2020 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 ( in effect on the date of publication of this document. Please review these documents 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

This document specifies a profile for the ACE framework [I-D.ietf-ace-oauth-authz]. In this profile, Clients and Servers (Brokers) use MQTT to exchange Application Messages. The protocol relies on TLS for communication security between entities. The MQTT protocol interactions are described based on the MQTT v5.0 - the OASIS Standard. Since it is expected that MQTT deployments will continue to support MQTT v3.1.1 clients, this document also describes a reduced set of protocol interactions for MQTT v3.1.1 - the OASIS Standard. However, MQTT v5.0 is the RECOMMENDED version as it works more naturally with ACE-style authentication and authorization.

MQTT is a publish-subscribe protocol and after connecting to the MQTT Server (Broker), a Client can publish and subscribe to multiple topics. The Broker, which acts as the Resource Server (RS), is responsible for distributing messages published by the publishers to their subscribers. In the rest of the document the terms "RS", "MQTT Server" and "Broker" are used interchangeably.

Messages are published under a Topic Name, and subscribers must subscribe to the Topic Names to receive the corresponding messages. The Broker uses the Topic Name in a published message to determine which subscribers to relay the messages. In this document, topics, more specifically, Topic Names, are treated as resources. The Clients are assumed to have identified the publish/subscribe topics of interest out-of-band (topic discovery is not a feature of the MQTT protocol). A Resource Owner can pre-configure policies at the Authorisation Server (AS) that give Clients publish or subscribe permissions to different topics.

Clients prove their permission to publish/subscribe to topics hosted on an MQTT broker using an access token, bound to a proof-of-possession (PoP) key. This document describes how to authorize the following exchanges between the Clients and the Broker. CoAP Pub-Sub Profile.

Clients use MQTT PUBLISH message to publish to a topic. This document does not protect the payload of the PUBLISH message from the Broker. Hence, the payload is not signed or encrypted specifically for the subscribers. This functionality may be implemented using the proposal outlined in the

To provide communication confidentiality and RS authentication, TLS is used, and TLS 1.3 is RECOMMENDED. This document makes the same assumptions as Section 4 of the ACE framework regarding Client and RS registration with the AS and setting up keying material. While the Client-Broker exchanges are only over MQTT, the required Client-AS and RS-AS interactions are described for HTTPS-based communication, using 'application/ace+json' content type, and unless otherwise specified, using JSON encoding. The token may be a reference or JSON Web Token (JWT). For JWTs, this document follows RFC 7800 for PoP semantics for JWTs. The Client-AS and RS-AS MAY also use protocols other than HTTP, e.g. Constrained Application Protocol (CoAP) or MQTT. Implementations MAY also use 'application/ace+cbor' content type, and CBOR encoding, and CBOR Web Token (CWT) and associated PoP semantics to reduce the protocol memory and bandwidth requirements. For more information, see Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs).

1.1. Requirements Language

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174], when, and only when, they appear in all capitals, as shown here.

1.2. ACE-Related Terminology

The terminology for entities in the architecture is defined in OAuth 2.0 RFC 6749 such as "Client" (C), "Resource Server" (RS) and "Authorization Server" (AS).

The term "resource" is used to refer to an MQTT Topic Name, which is defined in Section 1.3. Hence, the "Resource Owner" is any entity that can authoritatively speak for the topic.

Certain security-related terms such as "authentication", "authorization", "confidentiality", "(data) integrity", "message authentication code", and "verify" are taken from RFC 4949.

1.3. MQTT-Related Terminology

The document describes message exchanges as MQTT protocol interactions. The Clients are MQTT Clients, which connect to the Broker to publish and subscribe to Application Messages, labelled with their topics. For additional information, please refer to the MQTT v5.0 - the OASIS Standard or the MQTT v3.1.1 - the OASIS Standard.


Secured transport profile of MQTT. MQTTS runs over TLS.

The Server in MQTT. It acts as an intermediary between the Clients that publish Application Messages, and the Clients that made Subscriptions. The Broker acts as the Resource Server for the Clients.

A device or program that uses MQTT.

A stateful interaction between a Client and a Broker. Some Sessions last only as long as the network connection, others can span multiple network connections.
Application Message

The data carried by the MQTT protocol. The data has an associated Quality-of-Service (QoS) level and a Topic Name.
QoS level

The level of assurance for the delivery of an Application Message. The QoS level can be 0-2, where "0" indicates "At most once delivery", "1" "At least once delivery", and "2" "Exactly once delivery".
Topic Name

The label attached to an Application Message, which is matched to a Subscription.

A Subscription comprises a Topic Filter and a maximum QoS. A Subscription is associated with a single session.
Topic Filter

An expression that indicates interest in one or more Topic Names. Topic Filters may include wildcards.

MQTT sends various control messages across a network connection. The following is not an exhaustive list and the control packets that are not relevant for authorization are not explained. These include, for instance, the PUBREL and PUBCOMP packets used in the 4-step handshake required for QoS level 2.


Client request to connect to the Broker. This is the first packet sent by a Client.

The Broker connection acknowledgment. CONNACK packets contain return codes indicating either a success or an error state to in response to a Client's CONNECT packet.

Authentication Exchange. An AUTH packet is sent from the Client to the Broker or from the Broker to the Client as part of an extended authentication exchange. AUTH Properties include Authentication Method and Authentication Data. The Authentication Method is set in the CONNECT packet, and consequent AUTH packets follow the same Authentication Method. The contents of the Authentication Data are defined by the Authentication Method.

Publish request sent from a publishing Client to the Broker, or from the Broker to a subscribing Client.

Response to PUBLISH request with QoS level 1. PUBACK can be sent from the Broker to a Client or a Client to the Broker.

Response to PUBLISH request with QoS level 2. PUBREC can be sent from the Broker to a Client or a Client to the Broker.

Subscribe request sent from a Client.

Subscribe acknowledgment.

A ping request sent from a Client to the Broker. It signals to the Broker that the Client is alive, and is used to confirm that the Broker is also alive. The "Keep Alive" period is set in the CONNECT message.

Response sent by the Broker to the Client in response to PINGREQ. It indicates the Broker is alive.

If the network connection is not closed normally, the Broker sends a last Will message for the Client, if the Client provided one in its CONNECT message. If the Will Flag is set, then the payload of the CONNECT message includes information about the Will. The information consists of the Will Properties, Will Topic, and Will Payload fields.

2. Authorizing Connection Requests

This section specifies how Client connections are authorized by the MQTT Broker. Figure 1 shows the basic protocol flow during connection set-up. The token request and response use the token endpoint at the AS, specified in Section 5.6 of the ACE framework. Steps (D) and (E) are optional and use the introspection endpoint, specified in Section 5.7 of the ACE framework. The Client and the Broker use HTTPS to communicate to AS via these endpoints. The Client and the Broker use MQTT to communicate between them.

If the Client is resource-constrained, a Client Authorisation Server may carry out the token request on behalf of the Client, and later, onboard the Client with the token. Also, the C-AS and Broker-AS interfaces may be implemented using protocols other than HTTPS, e.g. CoAP or MQTT. The interactions between a Client and its Client Authorization Server for token onboarding, and support for MQTTS-based token requests at the AS are out of scope of this document.

                          | Client              |
                          |                     |
   +---(A) Token request--| Client -            |
   |                      | Authorization       |
   |   +-(B) Access token-> Server Interface    |
   |   |                  |       (HTTPS)       |
   |   |                  |_____________________|
   |   |                  |                     |
+--v-------------+        |  Pub/Sub Interface  |
|  Authorization |        |     (MQTTS)         |
|  Server        |        +-----------^---------+
|________________|            |       |
   |    ^             (C)Connection  (F)Connection
   |    |               request +    response
   |    |               access token  |
   |    |                     |       |
   |    |                 +---v--------------+
   |    |                 |   Broker (MQTTS) |
   |    |                 |__________________|
   |    +(D)Introspection-|                  |
   |   request (optional) | RS-AS interface  |
   |                      |     (HTTPS)      |
     response (optional)

Figure 1: Connection set-up

2.1. Client Token Request to the Authorization Server (AS)

The first step in the protocol flow (Figure 1 (A)) is the token acquisition by the Client from the AS. The Client and the AS MUST perform mutual authentication. The Client requests an access token from the AS as described in Section 5.6.1 of the ACE framework, however, it MUST set the profile parameter to 'mqtt_tls'. The media format is 'application/ace+json'. The AS uses JSON in the payload of its responses to both to the Client and the RS.

If the AS successfully verifies the access token request and authorizes the Client for the indicated audience (i.e. RS) and scopes (i.e. publish/subscribe permissions over topics), the AS issues an access token (Figure 1 (B)). The response includes the parameters described in Section 5.6.2 of the ACE framework. The returned token is assumed to be Proof-of-Possession (PoP) token by default. This document follows RFC 7800 for PoP semantics for JWTs. The PoP token includes a 'cnf' parameter with a symmetric or asymmetric PoP key. Note that the 'cnf' parameter in the web tokens are to be consumed by the RS and not the Client. For the asymmetric case, the PoP token may include the 'rs_cnf' parameter containing the information about the public key to be used by the RS to authenticate as described in [I-D.ietf-ace-oauth-params].

The AS returns error responses for JSON-based interactions following Section 5.2 of RFC 6749. When CBOR is used, the interactions must implement Section 5.6.3 of the ACE framework.

2.2. Client Connection Request to the Broker (C)

2.2.1. Client-Server Authentication over TLS and MQTT

The Client and the Broker MUST perform mutual authentication. The Client MUST authenticate to the Broker either over MQTT or TLS. For MQTT, the options are "None" and "ace". For TLS, the options are "Anon" for an anonymous client, and "Known(RPK/PSK)" for Raw Public Keys (RPK) and Pre-Shared Keys (PSK), respectively. Combined, client authentication has the following options:

It is RECOMMENDED that the Client follows TLS:Anon-MQTT:ace.

The Broker MUST be authenticated during the TLS handshake. If the Client authentication uses TLS:Known(RPK/PSK), then the Broker is authenticated using the respective method. Otherwise, to authenticate the Broker, the client MUST validate a public key from a X.509 certificate or an RPK from the Broker against the 'rs_cnf' parameter in the token response. The AS MAY include the thumbprint of the RS's X.509 certificate in the 'rs_cnf' (thumbprint as defined in [I-D.ietf-cose-x509]). In this case, the client MUST validate the RS certificate against this thumbprint.

2.2.2. authz-info: The Authorization Information Topic

In the cases when the Client MUST transport the token to the Broker first, the Client connects to the Broker to publish its token to the "authz-info" topic. The "authz-info" topic MUST be publish-only (i.e. the Clients are not allowed to subscribe to it). "authz-info" is not protected, and hence, the Client uses the "TLS:Anon-MQTT:None" option over a TLS connection. After publishing the token, the Client disconnects from the Broker and is expected to reconnect using client authentication over TLS (i.e. TLS:Known(RPK/PSK)-MQTT:none).

The Broker stores and indexes all tokens received to the "authz-info" topic in its key store (similar to DTLS profile for ACE). This profile follows the recommendation of Section 5.8.1 of the ACE framework, and expects that the Broker stores only one token per proof-of-possession key, and any other token linked to the same key overwrites an existing token.

The Broker MUST verify the validity of the token (i.e. through local validation or introspection) as described in Section 2.2.5. To validate the token, RS MAY need to introspect the token with the AS, e.g. if the token is a reference. If the token is not valid, the Broker MUST discard the token. Depending on the QoS level of the PUBLISH message, the Broker may return the error response as a PUBACK or a DISCONNECT message.

If the QoS level is equal to 0, and the token is invalid or the claims cannot be obtained in the case of an introspected token, the Broker MUST send a DISCONNECT message with the reason code '0x87 (Not authorized)'. If the PUBLISH payload does not parse to a token, the RS MUST send a DISCONNECT with the reason code '0x99 (Payload format invalid)'.

If the QoS level of the PUBLISH message is greater than or equal to 1, the Broker MUST return 'Not authorized' in PUBACK. If the PUBLISH payload does not parse to a token, the PUBACK reason code is '0x99 (Payload format invalid)'.

It must be noted that when the RS sends the 'Not authorized' response, this corresponds to the token being invalid, and not that the actual PUBLISH message was not authorized. Given that the "authz-info" is a public topic, this response is not expected to cause confusion.

2.2.3. Transporting Access Token Inside the MQTT CONNECT

This section describes how the Client transports the token to the Broker (RS) inside the CONNECT message. If this method is used, the Client TLS connection is expected to be anonymous, and the Broker is authenticated during the TLS connection set-up. The approach described in this section is similar to an earlier proposal by Fremantle et al [fremantle14].

Figure 2 shows the structure of the MQTT CONNECT message used in MQTT v5.0. A CONNECT message is composed of a fixed header, a variable header and a payload. The fixed header contains the Control Packet Type (CPT), Reserved, and Remaining Length fields. The Variable Header contains the Protocol Name, Protocol Level, Connect Flags, Keep Alive, and Properties fields. The Connect Flags in the variable header specify the properties of the MQTT session. It also indicates the presence or absence of some fields in the Payload. The payload contains one or more encoded fields, namely a unique Client identifier for the Client, a Will Topic, Will Payload, User Name and Password. All but the Client identifier can be omitted depending on flags in the Variable Header.

       0            8            16            24            32
       |CPT=1 | Rsvd.|Remaining len.| Protocol  name len. = 4 |
       |                      'M' 'Q' 'T' 'T'                 |
       | Proto.level=5|Connect flags|          Keep alive     |
       |                 Property length                      |
       |          Auth. Method (0x15) | 'ace'                 |
       |          Auth. Data (0x16)   | token or              |
       |                                token + PoP data      |
       |                     Payload                          |

Figure 2: MQTT v5 CONNECT control message with ACE authentication. (CPT=Control Packet Type)

The CONNECT message flags are Username, Password, Will retain, Will QoS, Will Flag, Clean Start, and Reserved. Figure 8 shows how the flags MUST be set to use AUTH packets for authentication and authorisation, i.e. the username and password flags MUST be set to 0. An MQTT v5.0 RS MAY also support token transport using Username and Password to provide a security option for MQTT v3.1.1 clients, as described in Section 6.

|User name|Pass.|Will retain|Will QoS|Will Flag|Clean| Rsvd.|
|   Flag  |Flag |           |        |         |Start|      |
| 0       | 0   |    X      |   X X  |   X     |  X  |  0   |

Figure 3: CONNECT flags for AUTH

The Will Flag indicates that a Will message needs to be sent if the network connection is not closed normally. The situations in which the Will message is published include disconnections due to I/O or network failures, and the server closing the network connection due to a protocol error. The Client may set the Will Flag as desired (marked as 'X' in Figure 3). If the Will Flag is set to 1 and the Broker accepts the connection request, the Broker must store the Will message and publish it when the network connection is closed according to Will QoS and Will retain parameters and MQTT Will management rules. To avoid publishing Will Messages in the case of temporary network disconnections, the Client may specify a Will Delay Interval in the Will Properties. Section 5 explains how the Broker deals with the retained messages in further detail.

In MQTT v5.0, the Client signals a clean session (i.e. the session does not continue an existing session), by setting the Clean Start Flag to 1 and, the Session Expiry Interval to 0 in the CONNECT message. In this profile, the Broker SHOULD always start with a clean session regardless of how these parameters are set. Starting a clean session helps the Broker avoid keeping unnecessary session state for unauthorised clients. If the Broker starts a clean session, the Broker MUST set the Session Present flag to 0 in the CONNACK packet to signal this to the Client.

If necessary, the Broker MAY support session continuation, and hence, maintain and use client state from the existing session. The client state MAY include token and its introspection result (for reference tokens) in addition to the MQTT session state. When reconnecting to the Broker, the Client MUST still provide a token, as well as setting the Clean Start to 0 and supplying a Session Expiry interval in the CONNECT message. The Broker MUST perform proof-of-possession validation on the provided token. If the token matches the stored state, the Broker MAY skip introspecting a token by reference, and use the stored introspection result. Continuing, both the Client and the Broker MUST resend any unacknowledged PUBLISH packets (where QoS > 0) and PUBREL packets. The Broker MUST still verify the Client is authorized to receive or send these packets. When a Client connects with a long Session Expiry Interval, the Broker may need to maintain Client's MQTT session state after it disconnects for an extended period. Brokers SHOULD implement administrative policies to limit misuse.

2.2.4. Authentication Using AUTH Property

To use AUTH, the Client MUST set the Authentication Method as a property of a CONNECT packet by using the property identifier 21 (0x15). This is followed by a UTF-8 Encoded String containing the name of the Authentication Method, which MUST be set to 'ace'. If the RS does not support this profile, it sends a CONNACK with a Reason Code of '0x8C (Bad authentication method)'.

The Authentication Method is followed by the Authentication Data, which has a property identifier 22 (0x16) and is binary data. The binary data in MQTT is represented by a two-byte integer length, which indicates the number of data bytes, followed by that number of bytes. Based on the Authentication Data, this profile allows: Proof-of-Possession Using a Challenge from the TLS session

For this option, the Authentication Data MUST contain the two-byte integer token length, the token, and the keyed message digest (MAC) or the Client signature. The content to calculate the keyed message digest (MAC) or the Client signature (for the proof-of-possession) is obtained using a TLS exporter ([RFC5705] for TLS 1.2 and for TLS 1.3, Section 7.5 of [RFC8446]). The content is exported from TLS using the exporter label 'EXPORTER-ACE-MQTT-Sign-Challenge', an empty context, and length of 32 bytes. The token is also validated as described in Section 2.2.5 and, the server responds with a CONNACK with the appropriate response code. The client cannot reauthenticate using this method during the same session ( see Section 4). ) Proof-of-Possession via Broker-generated Challenge/Response

For this option, the RS follows a Broker-generated challenge/response protocol. The success case is illustrated in Figure 4. If the Authentication Data contains only the two-byte integer token length and the token, the RS MUST respond with an AUTH packet, with the Authenticate Reason Code set to '0x18 (Continue Authentication)'. This packet includes the Authentication Method, which MUST be set to 'ace' and Authentication Data. The Authentication Data MUST NOT be empty and contains an 8-byte nonce as a challenge for the Client. The Client responds to this with an AUTH packet with a reason code '0x18 (Continue Authentication)'. Similarly, the Client packet sets the Authentication Method to 'ace'. The Authentication Data in the Client's response is formatted as the client nonce length, the client nonce, and the signature or MAC computed over the RS nonce concatenated with the client nonce. Next, the token is validated as described in Section 2.2.5.

The client MAY also re-authenticate using this flow.

                    Client      Server
                     |             |
                     |<===========>| TLS connection set-up
                     |             |
                     |             |
                     +------------>| CONNECT with Authentication Data
                     |             | contains only token
                     |             |
                     <-------------+ AUTH '0x18 (Cont. Authentication)'
                     |             | 8-byte nonce as RS challenge
                     |             |
                     |------------>| AUTH '0x18 (Cont. Authentication)'
                     |             | 8-byte client nonce + signature/MAC
                     |             |
                     |             |---+ Token validation
                     |             |   | (may involve introspection)
                     |             |<--+
                     |             |
                     |<------------+ CONNACK '0x00 (Success)'

Figure 4: PoP Challenge/Response Protocol Flow - Success

2.2.5. Token Validation

The RS MUST verify the validity of the token either locally (e.g. in the case of a self-contained token) or the RS MAY send an introspection request to the AS. RS MUST verify the claims according to the rules set in the Section of the ACE framework.

To authenticate the Client, the RS validates the signature or the MAC, depending on how the PoP protocol is implemented. HS256 and Ed25519 are mandatory to implement depending on the choice of symmetric or asymmetric validation. Validation of the signature or MAC MUST fail if the signature algorithm is set to "none", when the key used for the signature algorithm cannot be determined, or the computed and received signature/MAC do not match.

2.2.6. The Broker's Response to Client Connection Request

Based on the validation result (obtained either via local inspection or using the /introspection interface of the AS), the Broker MUST send a CONNACK message to the Client. Unauthorised Request: Authorisation Server Discovery

If the Client does not provide a valid token or omits the Authentication Data field, the Broker triggers AS discovery. The Broker MUST NOT process any data sent by the Client after the CONNECT packet including AUTH packets (Note that this is different in MQTT v5.0, the Broker is allowed to process AUTH packets even if the Broker rejects the CONNECT).

The Broker responds with the CONNACK reason code '0x87 (Not Authorized)' and includes a User Property (identified by 38 (0x26)) for the AS Request Creation Hints. The User Property is a UTF-8 string pair, composed of a name and a value. The name of the User Property MUST be set to "ace_as_hint". The value of the user property is a UTF-8 encoded JSON string containing the mandatory "AS" parameter, and the optional parameters "audience", "kid", "cnonce", and "scope" as defined in Section 5.1.2 of the ACE framework. Authorisation Success

On success, the reason code of the CONNACK is '0x00 (Success)'. If the Broker starts a new session, it MUST also set Session Present to 0 in the CONNACK packet to signal a clean session to the Client. Otherwise, it MUST set Session Present to 1. In case of an invalid PoP token, the CONNACK reason code is '0x87 (Not Authorized)'.

If the Broker accepts the connection, it MUST store the token until the end of the connection. On Client or Broker disconnection, the Client is expected to transport a token again on the next connection attempt.

If the token is not self-contained and the Broker uses token introspection, it MAY cache the validation result to authorize the subsequent PUBLISH and SUBSCRIBE messages. PUBLISH and SUBSCRIBE messages, which are sent after a connection set-up, do not contain access tokens. If the introspection result is not cached, then the RS needs to introspect the saved token for each request. The Broker SHOULD also use a cache time out to introspect tokens regularly.

3. Authorizing PUBLISH and SUBSCRIBE Messages

 AIF-MQTT = AIF-Generic<topic_filter, permissions>
 AIF-Generic<topic_filter, permissions> = [* [topic_filter,permissions]]
 topic_filter = tstr
 permissions = [+permission]
 permission = "pub"/"sub"

Figure 5: AIF-MQTT data model

To authorize a Client's PUBLISH and SUBSCRIBE messages, the Broker uses the scope field in the token (or in the introspection result). The scope field contains the publish and subscribe permissions for the Client. The scope is a JSON array, each item following the Authorization Information Format (AIF) for ACE. The specific data model for MQTT is: MQTT v5.0 - the OASIS Standard and includes special wildcard characters. The multi-level wildcard, '#', matches any number of levels within a topic, and the single-level wildcard, '+', matches one topic level.

 [["topic1", ["pub","sub"]], ["topic2/#",["pub"]], ["+/topic3",["sub"]]]

Figure 6: Example scope

An example scope field may contain:

3.1. PUBLISH Messages from the Publisher Client to the Broker

On receiving the PUBLISH message, the Broker MUST use the type of message (i.e. PUBLISH) and the Topic name in the message header to match against the scope string in the cached token or its introspection result. Following the example in the previous section, a client sending a PUBLISH message to 'a/topic3' would be allowed to publish, as the scope includes the string 'publish_+/topic3'.

If the Client is allowed to publish to the topic, the Broker must publish the message to all valid subscribers of the topic. In the case of an authorization failure, the Broker MUST return an error, if the Client has set the QoS level of the PUBLISH message to greater than or equal to 1. Depending on the QoS level, the Broker responds with either a PUBACK or PUBREC packet with reason code '0x87 (Not authorized)'. On receiving a PUBACK with '0x87 (Not authorized)', the Client MAY reauthenticate by providing a new token as described in Section 4.

For QoS level 0, the Broker sends a DISCONNECT with reason code '0x87 (Not authorized)' and closes the network connection. Note that the server-side DISCONNECT is a new feature of MQTT v5.0 (in MQTT v3.1.1, the server needs to drop the connection).

3.2. PUBLISH Messages from the Broker to the Subscriber Clients

To forward PUBLISH messages to the subscribing Clients, the Broker identifies all the subscribers that have valid matching topic subscriptions (i.e. the tokens are valid, and token scopes allow a subscription to the particular topic). The Broker sends a PUBLISH message with the Topic name to all the valid subscribers.

The Broker MUST NOT forward messages to the unauthorized subscribers. There is no way to inform the Clients with invalid tokens that an authorization error has occurred other than sending a DISCONNECT message. The Broker SHOULD send a DISCONNECT message with the reason code '0x87 (Not authorized)'.

3.3. Authorizing SUBSCRIBE Messages

In MQTT, a SUBSCRIBE message is sent from a Client to the Broker to create one or more subscriptions to one or more topics. The SUBSCRIBE message may contain multiple Topic Filters. The Topic Filters may include wildcard characters.

On receiving the SUBSCRIBE message, the Broker MUST use the type of message (i.e. SUBSCRIBE) and the Topic Filter in the message header to match against a scope string of the stored token or introspection result. The Topic Filters MUST be equal or a subset of the scopes associated with the Client's token.

As a response to the SUBSCRIBE message, the Broker issues a SUBACK message. For each Topic Filter, the SUBACK packet includes a return code matching the QoS level for the corresponding Topic Filter. In the case of failure, the return code is 0x87, indicating that the Client is 'Not authorized'. A reason code is returned for each Topic Filter. Therefore, the Client may receive success codes for a subset of its Topic Filters while being unauthorized for the rest.

4. Token Expiration and Reauthentication

The Broker MUST check for token expiration whenever a CONNECT, PUBLISH or SUBSCRIBE message is received or sent. The Broker SHOULD check for token expiration on receiving a PINGREQUEST message. The Broker MAY also check for token expiration periodically, e.g. every hour. This may allow for early detection of a token expiry.

The token expiration is checked by checking the 'exp' claim of a JWT or introspection response, or via performing an introspection request with the AS as described in Section 5.7 of the ACE framework. Token expirations may trigger the RS to send PUBACK, SUBACK and DISCONNECT messages with return code set to 'Not authorised'. After sending a DISCONNECT message, the network connection is closed, and no more messages can be sent. However, as a response to the PUBACK and SUBACK, the Client MAY reauthenticate. The Clients MAY also proactively update their tokens, i.e. before they receive a message with 'Not authorized' return code.

To start reauthentication, the Client MUST send an AUTH packet with reason code '0x19 (Re-authentication)'. The Client MUST set the Authentication Method as 'ace' and transport the new token in the Authentication Data. The Broker accepts reauthentication requests if the Client has already submitted a token (may be expired) and validated via the challenge-response PoP as defined in Section The Client MUST use the challenge-response PoP. Otherwise, the Broker MUST deny the request. If the reauthentication fails, the Broker MUST send a DISCONNECT with the reason code '0x87 (Not Authorized)'.

5. Handling Disconnections and Retained Messages

In the case of a Client DISCONNECT, the Broker deletes all the session state but MUST keep the retained messages. By setting a RETAIN flag in a PUBLISH message, the publisher indicates to the Broker that it should store the most recent message for the associated topic. Hence, the new subscribers can receive the last sent message from the publisher of that particular topic without waiting for the next PUBLISH message. The Broker MUST continue publishing the retained messages as long as the associated tokens are valid.

In case of disconnections due to network errors or server disconnection due to a protocol error (which includes authorization errors), the Will message must be sent if the Client supplied a Will in the CONNECT message. The Client's token scopes MUST include the Will Topic. The Will message MUST be published to the Will Topic regardless of whether the corresponding token has expired. In the case of a server-side DISCONNECT, the server returns the '0x87 Not Authorized' return code to the Client.

6. Reduced Protocol Interactions for MQTT v3.1.1

This section describes a reduced set of protocol interactions for the MQTT v3.1.1 Clients. An MQTT v5.0 Broker MAY implement these interactions for the MQTT v3.1.1 clients; MQTT v5.0 clients are NOT RECOMMENDED to use the flows described in this section. Brokers that do not support MQTT v3.1.1 clients return a CONNACK packet with Reason Code '0x84 (Unsupported Protocol Version)' in response to the connection requests.

6.1. Token Transport

As in MQTT v5.0, The Token MAY either be transported before by publishing to the "authz-info" topic, or inside the CONNECT message.

In MQTT v3.1.1, after the Client published to the "authz-info" topic, the Broker cannot communicate the result of the token validation as PUBACK reason codes or server-side DISCONNECT messages are not supported. In any case, an invalid token would fail the subsequent TLS handshake, which can prompt the Client to obtain a valid token.

To transport the token to the Broker inside the CONNECT message, the Client uses the username and password fields. Figure 7 shows the structure of the MQTT CONNECT message.

       0            8            16            24            32
       |CPT=1 | Rsvd.|Remaining len.| Protocol  name len. = 4 |
       |                      'M' 'Q' 'T' 'T'                 |
       | Proto.level=4|Connect flags|          Keep alive     |
       | Payload                                              |
       |     Client Identifier                                |
       |     Username as access token (UTF-8)                 |
       |     Password length (2 Bytes)                        |
       |     Password data as signature/MAC (binary)          |

Figure 7: MQTT CONNECT control message. (CPT=Control Packet Type, Rsvd=Reserved, len.=length, Proto.=Protocol)

Figure 8 shows how the MQTT connect flags MUST be set to initiate a connection with the Broker.

|User name|Pass.|Will retain|Will QoS|Will Flag|Clean| Rsvd.|
| flag    |flag |           |        |         |     |      |
| 1       | 1   |    X      |   X X  |   X     |  X   |  0  |

Figure 8: MQTT CONNECT flags. (Rsvd=Reserved)

The Broker SHOULD NOT accept session continuation. To this end, the Broker ignores how the Clean Session Flag is set, and on connection success, the Broker MUST set the Session Present flag to 0 in the CONNACK packet to indicate a clean session to the Client. If the Broker wishes to support session continuation, it MUST still perform proof-of-possession validation on the provided Client token. MQTT v3.1.1 does not use a Session Expiry Interval, and the Client expects that the Broker maintains the session state after it disconnects. However, stored Session state can be discarded as a result of administrator policies, and Brokers SHOULD implement the necessary policies to limit misuse.

The Client may set the Will Flag as desired (marked as 'X' in Figure 8). Username and Password flags MUST be set to 1 to ensure that the Payload of the CONNECT message includes both Username and Password fields.

The CONNECT in MQTT v3.1.1 does not have a field to indicate the authentication method. To signal that the Username field contains an ACE token, this field MUST be prefixed with 'ace' keyword, which is followed by the access token. The Password field MUST be set to the keyed message digest (MAC) or signature associated with the access token for proof-of-possession. The Client MUST apply the PoP key on the challenge derived from the TLS session as described in Section

In MQTT v3.1.1, the MQTT Username as a UTF-8 encoded string (i.e. is prefixed by a 2-byte length field followed by UTF-8 encoded character data) and may be up to 65535 bytes. Therefore, an access token that is not a valid UTF-8 MUST be Base64 [RFC4648] encoded. (The MQTT Password allows binary data up to 65535 bytes.)

6.2. Handling Authorization Errors

Handling errors are more primitive in MQTT v3.1.1 due to not having appropriate error fields, error codes, and server-side DISCONNECTs. Therefore, the broker will disconnect on almost any error and may not keep session state, necessitating clients to make a greater effort to ensure that tokens remain valid and not attempt to publish to topics that they do not have permissions for. The following lists how the broker responds to specific errors.

7. IANA Considerations

This document registers 'EXPORTER-ACE-MQTT-Sign-Challenge' from Section in the TLS Exporter Label Registry TLS-REGISTRIES.

In addition, the following registrations are done for the ACE OAuth Profile Registry following the procedure specified in [I-D.ietf-ace-oauth-authz].

Note to the RFC editor: Please replace all occurrences of "[RFC-XXXX]" with the RFC number of this specification and delete this paragraph.

Name: mqtt_tls

Description: Profile for delegating Client authentication and authorization using MQTT as the application protocol and TLS For transport layer security.

CBOR Value:

Reference: [RFC-XXXX]

8. Security Considerations

This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework [I-D.ietf-ace-oauth-authz]. Therefore, the security considerations outlined in [I-D.ietf-ace-oauth-authz] apply to this work.

In addition, the security considerations outlined in MQTT v5.0 - the OASIS Standard and MQTT v3.1.1 - the OASIS Standard apply. Mainly, this document provides an authorization solution for MQTT, the responsibility of which is left to the specific implementation in the MQTT standards. In the following, we comment on a few relevant issues based on the current MQTT specifications.

After the RS validates an access token and accepts a connection from a client, it caches the token to authorize a Client's publish and subscribe requests in an ongoing session. RS does not cache any invalid tokens. If a client's permissions get revoked but the access token has not expired, the RS may still grant publish/subscribe to revoked topics. If the RS caches the token introspection responses, then the RS should use a reasonable cache timeout to introspect tokens regularly. When permissions change dynamically, it is expected that AS also follows a reasonable expiration strategy for the access tokens.

The RS may monitor Client behaviour to detect potential security problems, especially those affecting availability. These include repeated token transfer attempts to the public "authz-info" topic, repeated connection attempts, abnormal terminations, and Clients that connect but do not send any data. If the RS supports the public "authz-info" topic, described in Section 2.2.2, then this may be vulnerable to a DDoS attack, where many Clients use the "authz-info" public topic to transport fictitious tokens, which RS may need to store indefinitely.

For MQTT v5.0, when a Client connects with a long Session Expiry Interval, the RS may need to maintain Client's MQTT session state after it disconnects for an extended period. For MQTT v3.1.1, the session state may need to be stored indefinitely, as it does not have a Session Expiry Interval feature. The RS SHOULD implement administrative policies to limit misuse of the session continuation by the Client.

9. Privacy Considerations

The privacy considerations outlined in [I-D.ietf-ace-oauth-authz] apply to this work.

In MQTT, the RS is a central trusted party and may forward potentially sensitive information between Clients. This document does not protect the contents of the PUBLISH message from the Broker, and hence, the content of the PUBLISH message is not signed or encrypted separately for the subscribers. This functionality may be implemented using the proposal outlined in the CoAP Pub-Sub Profile. However, this solution would still not provide privacy for other properties of the message such as Topic Name.

10. References

10.1. Normative References

[I-D.bormann-core-ace-aif] Bormann, C., "An Authorization Information Format (AIF) for ACE", Internet-Draft draft-bormann-core-ace-aif-09, June 2020.
[I-D.ietf-ace-oauth-authz] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S. and H. Tschofenig, "Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)", Internet-Draft draft-ietf-ace-oauth-authz-35, June 2020.
[I-D.ietf-ace-oauth-params] Seitz, L., "Additional OAuth Parameters for Authorization in Constrained Environments (ACE)", Internet-Draft draft-ietf-ace-oauth-params-13, April 2020.
[I-D.ietf-cose-x509] Schaad, J., "CBOR Object Signing and Encryption (COSE): Header parameters for carrying and referencing X.509 certificates", Internet-Draft draft-ietf-cose-x509-06, March 2020.
[MQTT-OASIS-Standard] Banks, A. and R. Gupta, "OASIS Standard MQTT Version 3.1.1 Plus Errata 01", 2015.
[MQTT-OASIS-Standard-v5] Banks, A., Briggs, E., Borgendale, K. and R. Gupta, "OASIS Standard MQTT Version 5.0", 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, March 2010.
[RFC6749] Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012.
[RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S. and T. Kivinen, "Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250, June 2014.
[RFC7800] Jones, M., Bradley, J. and H. Tschofenig, "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)", RFC 7800, DOI 10.17487/RFC7800, April 2016.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018.
[RFC8747] Jones, M., Seitz, L., Selander, G., Erdtman, S. and H. Tschofenig, "Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March 2020.

10.2. Informative References

[fremantle14] Fremantle, P., Aziz, B., Kopecky, J. and P. Scott, "Federated Identity and Access Management for the Internet of Things", research International Workshop on Secure Internet of Things, September 2014.
[I-D.ietf-ace-dtls-authorize] Gerdes, S., Bergmann, O., Bormann, C., Selander, G. and L. Seitz, "Datagram Transport Layer Security (DTLS) Profile for Authentication and Authorization for Constrained Environments (ACE)", Internet-Draft draft-ietf-ace-dtls-authorize-12, July 2020.
[I-D.ietf-ace-pubsub-profile] Palombini, F., "Pub-Sub Profile for Authentication and Authorization for Constrained Environments (ACE)", Internet-Draft draft-ietf-ace-pubsub-profile-01, July 2020.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007.

Appendix A. Checklist for profile requirements

Appendix B. Document Updates

Version 05 to 06:

Version 04 to 05:

Version 03 to 04:

Version 02 to 03:

Version 01 to 02:

Version 00 to 01:


The authors would like to thank Ludwig Seitz for his review and his input on the authorization information endpoint, presented in the appendix.

Authors' Addresses

Cigdem Sengul Brunel University Dept. of Computer Science Uxbridge, UB8 3PH UK EMail:
Anthony Kirby Oxbotica 1a Milford House, Mayfield Road, Summertown Oxford, OX2 7EL UK EMail:
Paul Fremantle University of Portsmouth School of Computing, Buckingham House Portsmouth, PO1 3HE UK EMail: