### Impact

All versions of Argo CD starting with v1.8.2 are vulnerable to an improper authorization bug causing the API to accept certain invalid tokens.

OIDC providers include an `aud` (audience) claim in signed tokens. The value of that claim specifies the intended audience(s) of the token (i.e. the service or services which are meant to accept the token). Argo CD _does_ validate that the token was signed by Argo CD’s configured OIDC provider. But Argo CD _does not_ validate the audience claim, so it will accept tokens that are not intended for Argo CD.

If Argo CD’s configured OIDC provider also serves other audiences (for example, a file storage service), then Argo CD will accept a token intended for one of those other audiences. Argo CD will grant the user privileges based on the token’s `groups` claim, even though those groups were not intended to be used by Argo CD.

This bug also increases the blast radius of a stolen token. If an attacker steals a valid token for a different audience, they can use it to access Argo CD.

### Patches

A patch for this vulnerability has been released in the following Argo CD versions:

* v2.6.0-rc5
* v2.5.8
* v2.4.20
* v2.3.14

The patch introduces a new `allowedAudiences` to the OIDC config block. By default, the client ID is the only allowed audience. Users who _want_ Argo CD to accept tokens intended for a different audience may use `allowedAudiences` to specify those audiences.

“`yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: argocd-cm
data:
oidc.config: |
name: Example
allowedAudiences:
– audience-1
– audience-2
– argocd-client-id # If `allowedAudiences` is non-empty, Argo CD’s client ID must be explicitly added if you want to allow it.
“`

Even though [the OIDC spec requires the audience claim](https://openid.net/specs/openid-connect-core-1_0.html#IDToken), some tokens may not include it. To avoid a breaking change in a patch release, versions = 2.6.0, Argo CD will reject all tokens which do not have an audience claim. Users can opt into the old behavior by setting an option:

“`yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: argocd-cm
data:
oidc.config: |
name: Example
skipAudienceCheckWhenTokenHasNoAudience: true
“`

### Workarounds

There is no workaround besides upgrading.

### Credits

The Argo CD team would like to express their gratitude to Vladimir Pouzanov (@farcaller) from Indeed, who discovered the issue, reported it confidentially according to our [guidelines](https://github.com/argoproj/argo-cd/blob/master/SECURITY.md#reporting-a-vulnerability), and actively worked with the project to provide a remedy. Many thanks to Vladimir!

### References

* [How to configure OIDC in Argo CD](https://argo-cd.readthedocs.io/en/latest/operator-manual/user-management/#existing-oidc-provider)
* [OIDC spec section discussing the audience claim](https://openid.net/specs/openid-connect-core-1_0.html#IDToken)
* [JWT spec section discussing the audience claim](https://www.rfc-editor.org/rfc/rfc7519#section-4.1.3)

### For more information

* Open an issue in [the Argo CD issue tracker](https://github.com/argoproj/argo-cd/issues) or [discussions](https://github.com/argoproj/argo-cd/discussions)
* Join us on [Slack](https://argoproj.github.io/community/join-slack) in channel #argo-cdRead More