Hi Axel,
I would be happy to see OAuth move away from state as a CSRF protection
mechanism in the future, but there is not too much to be gained from
relying solely on PKCE right now. The main advantage is that relying on
PKCE incentivizes clients to properly manage the session state in a
cookie instead of relying on a parameter. Beyond that, there's a small
reduction in effort required by the client, messages will be smaller
messages, etc.. The disadvantage is that a client puts CSRF protection
in the hands of the AS. Therefore, we chose the wording "ensure" to say
that the client has be sure that the AS actually implements PKCE
correctly before relying on it. What that means in the concrete instance
is up to the client.
Likewise, to your second point, I do not see enough of an advantage to
RECOMMEND relying solely on PKCE for CSRF protection.
The main intention here is to open the door to rely on PKCE, e.g., in
closed ecosystems, ecosystems with in-depth conformance testing,
first-party applications and similar. This also helps to avoid a lot of
convoluted language telling client developers how to properly choose,
track, and check state values in profiles such as FAPI (and the pitfalls
when interpreting that language).
-Daniel
Am 02.01.24 um 10:31 schrieb axel.nenn...@telekom.de:
Hi,
sorry for being late in the game.
I am not too happy with this section:
"Clients that have ensured that the authorization server supports
Proof Key for Code Exchange (PKCE, [RFC7636
<https://oauthstuff.github.io/draft-ietf-oauth-security-topics/draft-ietf-oauth-security-topics.html#RFC7636>])
MAY rely on the CSRF protection provided by PKCE."
1. Maybe a minor point that is due to not being a native speaker, but
the verb "ensure" seems too strong.
If the AZ states in its metadata, that it supports PKCE than this
is "ensurance" enough, right? The client does not have to "ensure"
the support by actually testing compliance, right?
I suggest rephrasing that to "*If the authorization server
**states in its meta-data support for Proof Key for Code Exchange*
(PKCE, [RFC7636
<https://oauthstuff.github.io/draft-ietf-oauth-security-topics/draft-ietf-oauth-security-topics.html#RFC7636>])
the client MAY rely on the CSRF protection provided by PKCE."
2. I suggest changing the "MAY" into a recommendation for all
OAuth2-based protocols. OIDC flows can easily support PKCE, and
new clients SHOULD use PKCE, I think.
Suggestion:
"If the authorization server states in its meta-data support for
Proof Key for Code Exchange (PKCE, [RFC7636
<https://oauthstuff.github.io/draft-ietf-oauth-security-topics/draft-ietf-oauth-security-topics.html#RFC7636>]),
it is RECOMMENDED the clientrelies on the CSRF protection provided
by PKCE."
Kind regards
Axel
*From: *OAuth <oauth-boun...@ietf.org> on behalf of Daniel Fett
<fett=40danielfett...@dmarc.ietf.org>
*Date: *Thursday, 28. December 2023 at 14:38
*To: *oauth@ietf.org <oauth@ietf.org>
*Subject: *Re: [OAUTH-WG] Shepherd Review of
draft-ietf-oauth-security-topics-23
Hi Hannes,
thanks again for your feedback! It is incorporated in the editor's
copy now.
-
https://oauthstuff.github.io/draft-ietf-oauth-security-topics/draft-ietf-oauth-security-topics.html
- Diff to published version:
https://author-tools.ietf.org/api/iddiff?doc_1=draft-ietf-oauth-security-topics&url_2=https://oauthstuff.github.io/draft-ietf-oauth-security-topics/draft-ietf-oauth-security-topics.txt
<https://author-tools.ietf.org/api/iddiff?doc_1=draft-ietf-oauth-security-topics&url_2=https://oauthstuff.github.io/draft-ietf-oauth-security-topics/draft-ietf-oauth-security-topics.txt>
I plan to publish the next version once we have resolved the
discussion points from Roman's AD review.
-Daniel
Am 04.10.23 um 15:41 schrieb Tschofenig, Hannes:
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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