CoreWCF, a .NET Core port of Windows Communication Foundation, has a flaw in its SPNEGO SecurityContextToken handling. In version 1.9.0, the proof key recovered from the RSTR can be exposed when TransportWithMessageCredential and Windows client credentials are used together with session establishment. An eavesdropper who observes the negotiation can recover this key, enabling them to impersonate the authenticated Windows principal and decrypt or forge subsequent WS‑SecureConversation traffic. The issue was resolved in version 1.9.1.

CoreWCF users running version 1.9.0 are affected by this flaw; version 1.9.1 and later contain the fix. The vendor CoreWCF explicitly identifies 1.9.0 as the vulnerable release, and no other listed versions are affected.

The CVSS score of 7.4 indicates a high severity. The EPSS score very low, but non-zero, likelihood of exploitation, and the vulnerability is not listed in the CISA KEV catalog. Any party that can observe the transport when the client uses Windows credentials and session establishment can potentially be at risk. Based on the description, it is inferred that an attacker could passively capture traffic during the SPNEGO negotiation to recover the proof key, without needing active credentials. Once the proof key is captured, it can be reused to impersonate the user for subsequent secure conversations.