Push-Based Security Event Token (SET) Delivery Using HTTP

Push-Based Security Event Token (SET) Delivery Using HTTP Amazon

richanna@amazon.com

Microsoft

mbj@microsoft.com https://self-issued.info/

Coinbase

marius.scurtescu@coinbase.com

Independent

morteza@sharppics.com

Independent

nadalin@prodigy.net

Security Security Events Working Group JSON Web Token JWT Security Event Token SET Delivery JavaScript Object Notation JSON This specification defines how a Security Event Token (SET) can be delivered to an intended recipient using HTTP POST over TLS. The SET is transmitted in the body of an HTTP POST request to an endpoint operated by the recipient, and the recipient indicates successful or failed transmission via the HTTP response.

Introduction and Overview This specification defines a mechanism by which a transmitter of a Security Event Token (SET) can deliver the SET to an intended SET Recipient via HTTP POST over TLS. This is an alternative SET delivery method to the one defined in . Push-based SET delivery over HTTP POST is intended for scenarios where all of the following apply:

The transmitter of the SET is capable of making outbound HTTP requests.
The recipient is capable of hosting a TLS-enabled HTTP endpoint that is accessible to the transmitter.
The transmitter and recipient are willing to exchange data with one another.

In some scenarios, either push-based or poll-based delivery could be used, and in others, only one of them would be applicable. A mechanism for exchanging configuration metadata such as endpoint URLs, cryptographic keys, and possible implementation constraints such as buffer size limitations between the transmitter and recipient is out of scope for this specification. How SETs are defined and the process by which security events are identified for SET Recipients are specified in .

Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. Throughout this document, all figures may contain spaces and extra line wrapping for readability and due to space limitations.

Definitions This specification utilizes the following terms defined in : "Security Event Token (SET)", "SET Issuer", "SET Recipient", and "Event Payload", as well as the term defined below:

SET Transmitter:: An entity that delivers SETs in its possession to one or more SET Recipients.

SET Delivery To deliver a SET to a given SET Recipient, the SET Transmitter makes a SET Transmission Request to the SET Recipient, with the SET itself contained within the request. The SET Recipient replies to this request with a response either acknowledging successful transmission of the SET or indicating that an error occurred while receiving, parsing, and/or validating the SET. Upon receipt of a SET, the SET Recipient SHALL validate that all of the following are true:

The SET Recipient can parse the SET.
The SET is authentic (i.e., it was issued by the issuer specified within the SET, and if signed, was signed by a key belonging to the issuer).
The SET Recipient is identified as an intended audience of the SET.
The SET Issuer is recognized as an issuer that the SET Recipient is willing to receive SETs from (e.g., the issuer is listed as allowed by the SET Recipient).
The SET Recipient is willing to accept this SET from this SET Transmitter (e.g., the SET Transmitter is expected to send SETs with the issuer and subject of the SET in question).

The mechanisms by which the SET Recipient performs this validation are out of scope for this document. SET parsing, issuer identification, and audience identification are defined in . The mechanism for validating the authenticity of a SET is deployment specific and may vary depending on the authentication mechanisms in use and whether the SET is signed and/or encrypted (See ). SET Transmitters MAY transmit SETs issued by another entity. The SET Recipient may accept or reject (i.e., return an error response such as access_denied) a SET at its own discretion. The SET Recipient persists the SET in a way that is sufficient to meet the SET Recipient's own reliability requirements. The level and method of retention of SETs by SET Recipients is out of scope of this specification. Once the SET has been validated and persisted, the SET Recipient SHOULD immediately return a response indicating that the SET was successfully delivered. The SET Recipient SHOULD NOT perform further processing of the SET beyond the required validation steps prior to sending this response. Any additional steps SHOULD be executed asynchronously from delivery to minimize the time the SET Transmitter is waiting for a response. The SET Transmitter MAY transmit the same SET to the SET Recipient multiple times, regardless of the response from the SET Recipient. The SET Recipient MUST respond as it would if the SET had not been previously received by the SET Recipient. The SET Recipient MUST NOT expect or depend on a SET Transmitter to retransmit a SET or otherwise make a SET available to the SET Recipient once the SET Recipient acknowledges that it was received successfully. The SET Transmitter should not retransmit a SET unless the SET Transmitter suspects that previous transmissions may have failed due to potentially recoverable errors (such as network outage or temporary service interruption at either the SET Transmitter or SET Recipient). In all other cases, the SET Transmitter SHOULD NOT retransmit a SET. The SET Transmitter SHOULD delay retransmission for an appropriate amount of time to avoid overwhelming the SET Recipient (see ).

Transmitting a SET To transmit a SET to a SET Recipient, the SET Transmitter makes an HTTP POST request to a TLS-enabled HTTP endpoint provided by the SET Recipient. The Content-Type header field of this request MUST be application/secevent+jwt as defined in Sections and of , and the Accept header field MUST be application/json. The request body MUST consist of the SET itself, represented as a JSON Web Token (JWT). The SET Transmitter MAY include in the request an Accept-Language header field to indicate to the SET Recipient the preferred language(s) in which to receive error messages. The mechanisms by which the SET Transmitter determines the HTTP endpoint to use when transmitting a SET to a given SET Recipient are not defined by this specification and are deployment specific. The following is a non-normative example of a SET Transmission Request:

Example SET Transmission Request

Success Response If the SET is determined to be valid, the SET Recipient SHALL acknowledge successful transmission by responding with HTTP Response Status Code 202 (Accepted) (see ). The body of the response MUST be empty. The following is a non-normative example of a successful receipt of a SET.

Example Successful Delivery Response

Failure Response In the event of a general HTTP error condition, the SET Recipient responds with the applicable HTTP Status Code, as defined in . When the SET Recipient detects an error parsing, validating, or authenticating a SET transmitted in a SET Transmission Request, the SET Recipient SHALL respond with an HTTP Response Status Code of 400 (Bad Request). The Content-Type header field of this response MUST be application/json, and the body MUST be a UTF-8 encoded JSON object containing the following name/value pairs:

err:: A Security Event Token Error Code (see ).
description:: A UTF-8 string containing a human-readable description of the error that may provide additional diagnostic information. The exact content of this field is implementation specific.

The response MUST include a Content-Language header field whose value indicates the language of the error descriptions included in the response body. If the SET Recipient can provide error descriptions in multiple languages, they SHOULD choose the language to use according to the value of the Accept-Language header field sent by the SET Transmitter in the transmission request, as described in . If the SET Transmitter did not send an Accept-Language header field, or if the SET Recipient does not support any of the languages included in the header field, the SET Recipient MUST respond with messages that are understandable by an English-speaking person, as described in . The following is a non-normative example error response indicating that the key used to encrypt the SET has been revoked.

Example Error Response (invalid_key) The following is a non-normative example error response indicating that the access token included in the request is expired.

Example Error Response (authentication_failed) The following is a non-normative example error response indicating that the SET Receiver is not willing to accept SETs issued by the specified issuer from this particular SET Transmitter.

Example Error Response (access_denied)

Security Event Token Error Codes Security Event Token Error Codes are strings that identify a specific category of error that may occur when parsing or validating a SET. Every Security Event Token Error Code MUST have a unique name registered in the IANA "Security Event Token Error Codes" registry established by . The following table presents the initial set of Error Codes that are registered in the IANA "Security Event Token Error Codes" registry: SET Error Codes

Error Code	Description
invalid_request	The request body cannot be parsed as a SET, or the Event Payload within the SET does not conform to the event's definition.
invalid_key	One or more keys used to encrypt or sign the SET is invalid or otherwise unacceptable to the SET Recipient (expired, revoked, failed certificate validation, etc.).
invalid_issuer	The SET Issuer is invalid for the SET Recipient.
invalid_audience	The SET Audience does not correspond to the SET Recipient.
authentication_failed	The SET Recipient could not authenticate the SET Transmitter.
access_denied	The SET Transmitter is not authorized to transmit the SET to the SET Recipient.

Other Error Codes may also be received, as the set of Error Codes is extensible via the IANA "Security Event Token Error Codes" registry established in .

Authentication and Authorization The SET delivery method described in this specification is based upon HTTP over TLS and standard HTTP authentication and authorization schemes, as per . The TLS server certificate MUST be validated using DNS-ID and/or DNS-Based Authentication of Named Entities (DANE) . Authorization for the eligibility to provide actionable SETs can be determined by using the identity of the SET Issuer, the identity of the SET Transmitter, perhaps using mutual TLS, or via other employed authentication methods. Because SETs are not commands, SET Recipients are free to ignore SETs that are not of interest.

Delivery Reliability Delivery reliability requirements may vary depending upon the use cases. This specification defines the response from the SET Recipient in such a way as to provide the SET Transmitter with the information necessary to determine what further action is required, if any, in order to meet their requirements. SET Transmitters with high reliability requirements may be tempted to always retry failed transmissions. However, it should be noted that for many types of SET delivery errors, a retry is extremely unlikely to be successful. For example, invalid_request indicates a structural error in the content of the request body that is likely to remain when retransmitting the same SET. Others such as access_denied may be transient, for example, if the SET Transmitter refreshes expired credentials prior to retransmission. The SET Transmitter may be unaware of whether or not a SET has been delivered to a SET Recipient. For example, a network interruption could prevent the SET Transmitter from receiving the success response, or a service outage could prevent the SET Transmitter from recording the fact that the SET was delivered. It is left to the implementer to decide how to handle such cases, based on their requirements. For example, it may be appropriate for the SET Transmitter to retransmit the SET to the SET Recipient, erring on the side of guaranteeing delivery, or it may be appropriate to assume delivery was successful, erring on the side of not spending resources retransmitting previously delivered SETs. Other options, such as sending the SET to a "dead letter queue" for manual examination may also be appropriate. Implementers SHOULD evaluate the reliability requirements of their use cases and the impact of various retry mechanisms and retransmission policies on the performance of their systems to determine an appropriate strategy for handling various error conditions.

Security Considerations

Authentication Using Signed SETs JWS signed SETs can be used (see and ) to enable the SET Recipient to validate that the SET Issuer is authorized to provide actionable SETs.

HTTP Considerations SET delivery depends on the use of Hypertext Transfer Protocol and is thus subject to the security considerations of HTTP () and its related specifications.

Confidentiality of SETs SETs may contain sensitive information, including Personally Identifiable Information (PII), or be distributed through third parties. In such cases, SET Transmitters and SET Recipients MUST protect the confidentiality of the SET contents. TLS MUST be used to secure the transmitted SETs. In some use cases, encrypting the SET as described in JWE will also be required. The Event delivery endpoint MUST support at least TLS version 1.2 and SHOULD support the newest version of TLS that meets its security requirements, which as of the time of this publication is TLS 1.3 . The client MUST perform a TLS/SSL server certificate check using DNS-ID and/or DANE . How a SET Transmitter determines the expected service identity to match the SET Recipient's server certificate against is out of scope for this document. The implementation security considerations for TLS in "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)" MUST be followed.

Denial of Service The SET Recipient may be vulnerable to a denial-of-service attack where a malicious party makes a high volume of requests containing invalid SETs, causing the endpoint to expend significant resources on cryptographic operations that are bound to fail. This may be mitigated by authenticating SET Transmitters with a mechanism such as mutual TLS. Rate-limiting problematic transmitters is also a possible means of mitigation.

Authenticating Persisted SETs At the time of receipt, the SET Recipient can rely upon TLS mechanisms, HTTP authentication methods, and/or other context from the transmission request to authenticate the SET Transmitter and validate the authenticity of the SET. However, this context is typically unavailable to systems to which the SET Recipient forwards the SET, or to systems that retrieve the SET from storage. If the SET Recipient requires the ability to validate SET authenticity outside of the context of the transmission request, then the SET Recipient SHOULD ensure that such SETs have been signed in accordance with . Needed context could also be stored with the SET and retrieved with it.

Privacy Considerations SET Transmitters should attempt to deliver SETs that are targeted to the specific business and protocol needs of subscribers. When sharing personally identifiable information or information that is otherwise considered confidential to affected users, SET Transmitters and Recipients MUST have the appropriate legal agreements and user consent or terms of service in place. Furthermore, data that needs confidentiality protection MUST be encrypted, at least with TLS and sometimes also using JSON Web Encryption (JWE) . In some cases, subject identifiers themselves may be considered sensitive information, such that their inclusion within a SET may be considered a violation of privacy. SET Issuers and SET Transmitters should consider the ramifications of sharing a particular subject identifier with a SET Recipient (e.g., whether doing so could enable correlation and/or de-anonymization of data) and choose appropriate subject identifiers for their use cases.

IANA Considerations

Security Event Token Error Codes This document defines Security Event Token Error Codes, for which IANA has created and now maintains a new registry titled "Security Event Token Error Codes". Initial values for the "Security Event Token Error Codes" registry are defined in and registered below. Future assignments are to be made through the Specification Required registration policy and shall follow the template below. Error Codes are intended to be interpreted by automated systems; therefore, they SHOULD identify classes of errors to which an automated system could respond in a meaningfully distinct way (e.g., by refreshing authentication credentials and retrying the request). Error Code names are case sensitive. Names may not match other registered names in a case-insensitive manner unless the Designated Experts state that there is a compelling reason to allow an exception. Criteria that should be applied by the Designated Experts includes determining whether the proposed registration duplicates existing functionality, whether it is likely to be of general applicability or whether it is useful only for a single application, and whether the registration description is clear. It is suggested that multiple Designated Experts be appointed who are able to represent the perspectives of different applications using this specification in order to enable broadly informed review of registration decisions. In cases where a registration decision could be perceived as creating a conflict of interest for a particular expert, that expert should defer to the judgment of the other experts.

Registration Template

Error Code: The name of the Security Event Token Error Code, as described in . The name MUST be a case-sensitive ASCII string consisting only of letters, digits, and underscore; these are the characters whose codes fall within the inclusive ranges 0x30-39, 0x41-5A, 0x5F, and 0x61-7A.
Description: A brief human-readable description of the Security Event Token Error Code.
Change Controller: For error codes registered by the IETF or its working groups, list "IETF". For all other error codes, list the name of the party responsible for the registration. Contact information such as mailing address, email address, or phone number may also be provided.
Reference: A reference to the document or documents that define the Security Event Token Error Code. The definition MUST specify the name and description of the error code and explain under what circumstances the error code may be used. URIs that can be used to retrieve copies of each document at no cost SHOULD be included.

Initial Registry Contents

Error Code:: invalid_request
Description:: The request body cannot be parsed as a SET or the Event Payload within the SET does not conform to the event's definition.
Change Controller:: IETF
Reference:: of RFC 8935

Error Code:: invalid_key
Description:: One or more keys used to encrypt or sign the SET is invalid or otherwise unacceptable to the SET Recipient (expired, revoked, failed certificate validation, etc.).
Change Controller:: IETF
Reference:: of RFC 8935

Error Code:: invalid_issuer
Description:: The SET Issuer is invalid for the SET Recipient.
Change Controller:: IETF
Reference:: of RFC 8935

Error Code:: invalid_audience
Description:: The SET Audience does not correspond to the SET Recipient.
Change Controller:: IETF
Reference:: of RFC 8935

Error Code:: authentication_failed
Description:: The SET Recipient could not authenticate the SET Transmitter.
Change Controller:: IETF
Reference:: of RFC 8935

Error Code:: access_denied
Description:: The SET Transmitter is not authorized to transmit the SET to the SET Recipient.
Change Controller:: IETF
Reference:: of RFC 8935

References Normative References Informative References Poll-Based Security Event Token (SET) Delivery Using HTTP

Unencrypted Transport Considerations Earlier versions of this specification made the use of TLS optional and described security and privacy considerations resulting from use of unencrypted HTTP as the underlying transport. When the working group decided to mandate usage of HTTP over TLS, it also decided to preserve the description of these considerations in this non-normative appendix. SETs may contain sensitive information that is considered Personally Identifiable Information (PII). In such cases, SET Transmitters and SET Recipients MUST protect the confidentiality of the SET contents. When TLS is not used, this means that the SET MUST be encrypted as described in JWE. If SETs were allowed to be transmitted over unencrypted channels, some privacy-sensitive information about them might leak, even though the SETs themselves are encrypted. For instance, an attacker may be able to determine whether or not a SET was accepted and the reason for its rejection or may be able to derive information from being able to observe the size of the encrypted SET. (Note that even when TLS is utilized, some information leakage is still possible; message padding algorithms to prevent side channels remain an open research topic.)

Acknowledgments The editors would like to thank the members of the SCIM Working Group, which began discussions of provisioning events starting with draft-hunt-scim-notify-00 in 2015. We would like to thank and the other authors of draft-ietf-secevent-delivery-02, upon which this specification is based. We would like to thank the participants in the SecEvents Working Group for their contributions to this specification. Additionally, we would like to thank the following individuals for their reviews of the specification: , , , , , , , , , , , and .