An authentication bypass vulnerability exists in Windows Communication Foundation (WCF) and Windows Identity Foundation (WIF), allowing signing of SAML tokens with arbitrary symmetric keys. This vulnerability allows an attacker to impersonate another user, which can lead to elevation of privileges. The vulnerability exists in WCF, WIF 3.5 and above in .NET Framework, WIF 1.0 component in Windows, WIF Nuget package, and WIF implementation in SharePoint.
An unauthenticated attacker can exploit this by signing a SAML token with any arbitrary symmetric key.
This security update addresses the issue by ensuring all versions of WCF and WIF validate the key used to sign SAML tokens correctly.