Hi all,
here are some comments as part of my shepherd review of the OAuth
Security BCP.
First, I want to send a big "Thanks" to everyone in the group for the
work on this document and to the authors in particular. It has taken
us a while to come up with such an impressive list of security
recommendations for OAuth 2.0.
At this point in time my review comments can only be minor given the
amount of feedback this documents has already received.
Here are a few remarks.
I believe we should indicate that the specification updates other
OAuth RFCs. The obvious documents it updates are RFC 6749, RFC 6750
and RFC 6819.
You can set these "updates" in the template you are using.
In Section 1 you say:
"
It does not supplant the security advice given in
[RFC6749], [RFC6750], and [RFC6819], but complements those documents.
"
In the subsequent paragraph you state that you "depreciate some modes
of operation".
I believe you are need to be clear about what you are doing in
relationship to these prior documents. It might also be useful to say
something about OAuth 2.1.
Expand abbreviations on first use. Example: "AS" and "PKCE" in Section
2.1. The AS abbreviation is only expanded later in Section 3. Decide
whether you want to use abbreviations or not. You mix them throughout
the document without no reasons.
Listing the abbreviations in Section 1.2 may also be useful. There are
not that many abbreviations anyway.
I have wording suggestions for this paragraph:
FROM:
"
Authorization servers SHOULD use client authentication if possible.
It is RECOMMENDED to use asymmetric (public-key based) methods for
client authentication such as mTLS [RFC8705] or using signed JWTs
("Private Key JWT") in accordance with [RFC7521] and [RFC7523] (in
[OpenID.Core] defined as the client authentication method
private_key_jwt). When such methods for client authentication are
used, authorization servers do not need to store sensitive symmetric
keys, making these methods more robust against a number of attacks.
"
TO:
"
Authorization servers SHOULD enforce client authentication, if
possible.
It is RECOMMENDED to use asymmetric cryptography for
client authentication, such as mTLS [RFC8705] or using signed JWTs
("Private Key JWT"), in accordance with [RFC7521] and [RFC7523] (in
[OpenID.Core] defined as the client authentication method
private_key_jwt). When asymmetric cryptography for client
authentication is
used, authorization servers do not need to store sensitive symmetric
keys, making client authentication more robust against leakage of keys.
"
(Note: For the reader it is always better if they are told what attacks
are prevented rather than saying "a number of attacks". You don't want
the reader
to guess what you mean.)
Section 2 is a summary of what follows in Section 4. Maybe you can
make this explicit
either in the title of Section 2 or in the first paragraph of Section 2.
Section 3.
You write:
"
These attackers conform to the attacker model that was used in formal
analysis efforts for OAuth [arXiv.1601.01229]. This is a minimal
attacker model. Implementers MUST take into account all possible
types of attackers in the environment in which their OAuth
implementations are expected to run.
"
When you say "these attackers" please clarify which attackers you are
talking about.
Prior to this paragraph you have just spoken about various forms of
network attackers.
Just before that you talked about network and web attackers.
Then, you introduce more attackers and you keep talking about "this
attacker model" and
"these attackers". Make it easier for the reader by referring
explictly which attackers
you are talking about in a specific paragraph.
Then, you conclude the section with a hint that there is an even
stronger attacker model.
As a reader I might want to know what this stronger attacker model
looks like and why you
do not consider it in this document.
Section 4.1.1:
You write:
"
Note: Vulnerabilities of this kind can also exist if the
authorization server handles wildcards properly.
"
I believe you are saying that the vulnerabilities caused by incorrect
redirect URI validation parsing when you refer to "this kind".
I would also remove the "note"
Section 4.1.3:
You write:
"
* Servers on which callbacks are hosted MUST NOT expose open
redirectors (see Section 4.11).
"
Are you talking about authorization servers (which is what was
referenced in the paragraph before)?
Section 4.10.1: Sender-constrained Access Tokens
The text gives the reader the impression that the token binding would
be an option for developers to use.
I don't think that this is the case. I am particularly referring to
this sentence:
"
* *DPoP* ([I-D.ietf-oauth-dpop]): DPoP (Demonstration of Proof-of-
Possession at the Application Layer) outlines an application-level
sender-constraining for access and refresh tokens that can be used
in cases where neither mTLS nor OAuth Token Binding (see below)
are available.
"
I would change it to:
"
* *DPoP* ([I-D.ietf-oauth-dpop]): DPoP (Demonstration of Proof-of-
Possession at the Application Layer) outlines an application-level
sender-constraining for access and refresh tokens that can be used
in cases where mTLS is not available.
"
I would then remove the subsequent text talking about old, expired drafts.
Alternatively, you could move the text to the appendix.
Section 4.10.2: Audience Restricted Access Tokens
In the text you say:
"
Audience restriction essentially restricts access tokens to a
particular resource server. The authorization server associates the
access token with the particular resource server and the resource
server SHOULD verify the intended audience.
"
You have to put a MUST here. If the resource server does not check the
audience
restriction when using audience restricted access tokens then you
obviously do not
get the value from it. It is like using DPOP and not using the
proof-of-possession.
Likewise the SHOULD language in this sentence is also questionable:
"
The client SHOULD tell the authorization server the intended resource
server. The proposed mechanism [RFC8707] could be used or by
encoding the information in the scope value.
"
If the client does not tell the authorization server what the intended
resource server
is then how should the authorization server know (unless in a very
limited setup).
Also the reference to RFC 8707 is a bit weak. We standardized resource
indicators: why not
recommend using it?
Section 4.10.3: The section heading is "Discussion: Preventing Leakage
via Metadata".
The content of the section is not really a discussion but rather a
description of why
this path has not been taken. I wonder whether it would be better to
move this section
to the appendix and then start the text by explaining why other
solutions have been used instead of this approach.
Section 4.11: I would put the definition about what an "open
redirector" is into the terminology section since you
are using the term already in earlier sections. Here is the definition:
"
An open redirector is an endpoint that forwards a user’s
browser to an arbitrary URI obtained from a query parameter.
"
Typos/Wording:
FROM:
"
Afterwards, the updated the OAuth attacker model is presented.
"
TO:
"
Afterwards, the updated OAuth attacker model is presented.
"
Section 4.1:
"... wild ."
^
Consider using the guidelines for inclusive language:
https://www.rfc-editor.org/part2/#inclusive_language
<https://www.rfc-editor.org/part2/#inclusive_language>
For example, "If the attacker is able to ... , **he** will directly
get access to ..."
Another example is "whitelisted".
Section 4.1.2: a wording suggestion.
FROM:
"
The attack
described here combines this behavior with the client as an open
redirector (see Section 4.11.1) in order to get access to access
tokens.
"
TO:
"
The attack
described here combines this behavior with the client as an open
redirector (see Section 4.11.1) to obtain access tokens.
"
Section 4.7.1: word missing
FROM:
"
PKCE provides robust protection against CSRF attacks even in presence
of an that can read the authorization response (see Attacker A3 in
Section 3).
"
TO:
"
PKCE provides robust protection against CSRF attacks even in presence
of an attacker that can read the authorization response (see
Attacker A3 in
Section 3).
"
Section 4.18.2: capitalization
FROM:
"
Wildcard origins like "*" in postMessage MUST not be used as
attackers can use them to leak a victim's in-browser message to
malicious origins.
"
TO:
"
Wildcard origins like "*" in postMessage MUST NOT be used as
attackers can use them to leak a victim's in-browser message to
malicious origins.
"
You might also want to replace the short title "oauth-security-topics"
(which can be found on each page) with something like "OAuth 2.0
Security BCP".
Ciao
Hanns
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