Known CVE

Microsoft SharePoint Server - WS-Federation BinaryFormatter Deserialization RCE

Microsoft SharePoint Server (2016, 2019, Subscription Edition <= 2026.3.1) contains a remote code execution vulnerability in the WS-Federation passive sign-in endpoint (/_trust/default.aspx). The SharePoint authentication module (SPFederationAuthenticationModuleV2) uses a deflate-only DeflateCookieTransform (no MAC/signature check) to decode and BinaryFormatter to deserialize the SecurityContextToken Cookie value in the wresult parameter without type restrictions. An unauthenticated attacker can send a crafted WS-Federation token containing a BinaryFormatter gadget chain (DataSet + LosFormatter + XamlAssemblyLoadFromFile) to execute arbitrary code in the context of the IIS application pool identity (w3wp.exe). The fix (KB5002882, July 2026 CU) replaces the DeflateCookieTransform with NotSupportedCookieTransform, making ReadToken() throw before deserialization.