I reviewed this document as part of the security directorate's ongoing 
effort to review all IETF documents being processed by the IESG.These 
comments were written with the intent of improving security requirements 
and considerations in IETF drafts.Comments not addressed in last call 
may be included in AD reviews during the IESG review.Document editors 
and WG chairs should treat these comments just like any other last call 
comments.

This document describes an API for use with OAuth 2.0. The API enables 
the recipient of a token to query an authorization server to determine 
the set of metadata presented to the server (by the OAuth client) when 
the token was issued. This API is intended to overcome the lack of a 
standard way to convey token metadata in the OAuth 2.0 context.

I didn’t find any significant security problems with the document, but 
there are a number of places where the exposition needs improvement.

Specific comments

Section 2

The text says:

The introspection endpoint MUST be protected by a transport-layer 
security mechanism as described in Section 4.

It might be clearer to say here that the token endpoint MUST communicate 
security with the introspection endpoint, and that TLS 1.2 is the 
mandatory-to-implement mechanism for such security. Also, the text 
should be clearer about the relationship between an authorization server 
and an introspection endpoint. In some places the two terms seem to be 
used interchangeably.

Section 2.1

The texts says:

The endpoint MAY allow other parameters to provide further context to 
the query.

How about:

The endpoint MAY *accept* other parameters to provide further context to 
the query.

How does a protected resource know which other parameters an 
introspection endpoint requires/accepts?

This section mandates (MUST) protection against “unauthorized token 
scanning” but mandates no standard way to do so. [Also, when would a 
scanning “attack” be authorized ;-)]

The text says:

For example, the following example shows a protected resource calling 
the token introspection endpoint to query about an OAuth 2.0 bearer.

How about:

For example, the following example shows a protected resource calling 
the token introspection endpoint to query about an OAuth 2.0 bearer *token*.

Section 2.2

He text says:

The authorization server MAY respond differently to different

protected resources making the same request.For instance, an

authorization server MAY limit which scopes from a given token are

returned for each protected resource in order to prevent protected

resources from learning more about the larger network than is

necessary for their function.

How about:

*An*authorization server MAY respond differently to different

protected resources making the same request.For instance, an

authorization server MAY limit which scopes from a given token are

returned *to* each protected resource in order to prevent *a *protected

resources from learning more about the larger network than is

necessary for *its* *operation*.

Section 3.1

Experience suggests a longer review period is appropriate, e.g., to 
accommodate holidays, vacations, etc.

The text says:

IANA must only accept registry updates from the Designated Expert(s)

and should direct all requests for registration to the review mailing

list.

How about:

IANA *MUST* accept registry updates *only* from the Designated Expert(s)

and *SHOULD* direct all requests for registration to the review mailing

list.

Section 3.1.1

If a proposed Name matches one already registered as per 7519, ought it 
not have an “equivalent” (vs. “comparable”) definition if it is to be 
accepted?

Section 4

The text lists four checks to be performed by an authorization server, 
yet the introduction to this list says “For instance:” A more normative 
introduction would seem appropriate here. I realize that the text says 
that not all of these checks may be applicable in all OAuth 2.0 
contexts, but since each check begins with “If the token …” isn’t that a 
sufficient caveat?

The text says:

If an authorization server fails to perform any applicable check, the

resource server could make an errant security decision based on that

response.

How about:

If an authorization server fails to perform any applicable check, the

resource server could make an *erroneous* security decision based on 
that response.

The text says:

per RFC 6125 [RFC6125].

How about:

as specified in [RFC6125].

The discussion of introspection endpoint response caching seems to omit 
a simple, useful heuristic. Since the expiration time for a token is an 
optional parameter in the response, why not say that IF this parameter 
is present, the response MUST NOT be cached beyond that time?

Section 5

This section says: “One way to limit disclosure is to require

authorization to call the introspection endpoint and to limit calls

to only registered and trusted protected resource servers.” But

Section 4 says: “… the authorization server MUST require authentication 
of protected resources that need to access the introspection endpoint 
and SHOULD require protected resources to be specifically authorized to 
call the introspection endpoint.”It seems that the requirement imposed 
in Section 4 already mandate what is merely suggested in Section 5.