The CBOR library supports optional tags that enable CBOR objects to contain references to objects within them. Versions earlier than 4.0 resolved those references automatically. While this by itself doesn't cause much of a security problem, a denial of service can happen if those references are deeply nested and used multiple times (so that the same reference to the same object occurs multiple times), and if the decoded CBOR object is sent to a serialization method such as "EncodeToBytes", "ToString", or "ToJSONString", since the objects referred to are expanded in the process and take up orders of magnitude more memory than if the references weren't resolved. The impact of this problem on any particular system varies. In general, the risk is higher if the system allows users to send arbitrary CBOR objects without authentication, or exposes a remote endpoint in which arbitrary CBOR objects can be sent without authentication.