Duende IdentityServer is an OpenID Connect and OAuth 2.x framework for ASP.NET Core. IdentityServers local API authentication handler performs insufficient validation of the cnf claim in DPoP access tokens. This allows an attacker to use leaked DPoP access tokens at local api endpoints even without possessing the private key for signing proof tokens. Note that this only impacts custom endpoints within an IdentityServer implementation that have explicitly used the LocalApiAuthenticationHandler for authentication. This vulnerability is patched in IdentityServer 7.0.8. Version 6.3 and below are unaffected, as they do not support DPoP in Local APIs.
When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.