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

[Andrii Deinega]

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> 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> 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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