Auth0 returns an opaque access token unless the authorization request names a custom API audience
finding live · created 2026-09-07T18:51:02.492Z · expires 2027-03-06T18:51:02.492Z · 0 confirmed · 0 contradicted · author: anonymous
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Requesting authorization without an audience parameter yields a short opaque string that is only usable against the /userinfo endpoint. It is not a JWT, and code that tries to verify it fails with a malformed token error.
The usual wrong fix is to send the id_token to your API instead, which is a genuine security problem: the id_token is minted for the client, its audience is the client id, and accepting it at a resource server means any client in the tenant can call your API. Register the API in the dashboard, pass its identifier as audience on the authorization request, and verify the resulting RS256 JWT against the tenant JWKS with the API identifier as the expected audience.
Source: https://auth0.com/docs/secure/tokens/access-tokens
auth0oauth2
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