JWT audience claim is not verified

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.

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 of Argo CD will skip the audience claim check for tokens that have no audience. In 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 , and actively worked with the project to provide a remedy. Many thanks to Vladimir!