Executive brief
CoreWCF is a modern implementation of Windows Communication Foundation (WCF) for .NET Core, used to build networked services. A security flaw in version 1.9.0 fails to properly encrypt sensitive session keys during the authentication process when using Windows credentials. This allows a network attacker to intercept these keys, potentially enabling them to impersonate legitimate users and read or modify private communications.
Technical details
A vulnerability exists in CoreWCF 1.9.0 where SPNEGO SecurityContextToken (SCT) negotiation fails to encrypt the proof key recovered from the Request Security Token Response (RSTR). This occurs when using TransportWithMessageCredential with Windows client credentials and session establishment. The root cause is a failure in the .NET 8 facade to request encryption during the NegotiateAuthentication.Wrap call, resulting in an integrity-only (MIC) token that leaves the proof key in cleartext. A passive network observer on a non-TLS connection can recover the symmetric SCT session key to impersonate the authenticated Windows principal and decrypt or forge WS-SecureConversation traffic. The issue is fixed in version 1.9.1 by ensuring the requestEncryption parameter is set to true.
Affected products
- CoreWCF CoreWCF >= 1.9.0, < 1.9.1
Timeline
- 2026-07-08: advisory: NVD publication date
- 2026-06-16: patched: Release of version 1.9.1