[Openid-specs-ab] Call for Adoption for the OpenID Connect Key Binding Specification

[Ralph Bragg]

Likewise, I do not support utilisation the id token In this way for the reasons already raised. We’ve just had issues with multiple aud values being identified as a significant source of vulnerability with PKJ so extending / modifying an id tokens aud raises too many concerns for me as a potential mitigation. I don’t believe that this token should be relied on by any actor other than the intended audience.


Ralph Bragg

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________________________________
From: Openid-specs-ab <openid-specs-ab-bounces at lists.openid.net> on behalf of Andrii Deinega via Openid-specs-ab <openid-specs-ab at lists.openid.net>
Sent: Saturday, September 27, 2025 4:26:07 AM
To: Artifact Binding/Connect Working Group <openid-specs-ab at lists.openid.net>
Cc: Andrii Deinega <andrii.deinega at gmail.com>
Subject: Re: [Openid-specs-ab] Call for Adoption for the OpenID Connect Key Binding Specification

I believe that any recipient of a JWT (in this case, an ID Token) should immediately reject it if it isn't the intended audience (which is indicated by the aud claim), regardless of whether cryptographic binding is present or not. This alone makes the statement below too problematic for me.

When an RP wants to prove to another system that it has authenticated a user, it may present the ID Token as a bearer token. However, bearer tokens are vulnerable to theft and replay attacks - if an attacker intercepts the ID Token, they can impersonate the authenticated user to downstream systems that accept a ID Token as a bearer token.

It's difficult to imagine multiple systems (recipients) sharing the same value in the aud claim (this value must be a client_id of the RP per the Core spec). It's fair to add the aud claim may contain an array with more than one element, but it's also fair to say this practice is discouraged (1) and comes with additional complexity and concerns (2).

At the end of the day... I see a lot of value, and I see the reason why people want to have the standard around "proving to another system that it has authenticated a user," but I don't think that repurposing existing ID Tokens for it is the right way to go.... I’d suggest, and actually love to see - the use of SD JWT VCs (or other VCs) for this purpose instead.

I haven’t reached the point where I need to "touch" Justin’s concerns... I fully agree with him on them.

All the best,
Andrii

On Fri, Sep 26, 2025 at 9:05 AM Justin Richer via Openid-specs-ab <openid-specs-ab at lists.openid.net<mailto:openid-specs-ab at lists.openid.net>> wrote:
I do not support adoption of this work. The ID Token is not intended to be a conveyable artifact, and using it as such is a security layer boundary. It’s hard enough to get people to not use ID Tokens as Access Tokens today, since a lot of developers see all JWTs as equivalent, really. This work would make this problem significantly worse.

 — Justin

On Sep 15, 2025, at 6:57 PM, Michael Jones via Openid-specs-ab <openid-specs-ab at lists.openid.net<mailto:openid-specs-ab at lists.openid.net>> wrote:

This starts a two-week call for feedback on whether to adopt the OpenID Connect OpenID Connect Key Binding specification contributed to the working group by Dick Hardt and Ethan Heilman as an OpenID Connect Working Group specification.  Please reply-all by Monday, September 29, 2025 saying whether you are favor of adoption or not, also saying why.

The specification was contributed at https://lists.openid.net/pipermail/openid-specs-ab/2025-August/010890.html.  It has been extensively discussed by the working group both on calls and on the mailing list.  From my observations of the discussion as a working group chair, I believe that there is consensus that it would be useful to have a standard solving the problem addressed by this specification.

                                                Writing as a working group chair,
                                                                -- Mike

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