CVE-2026-42897 Exchange OWA — EM Service auto-mitigation depends on outbound connectivity to officemitigations.microsoft.com

UPDATE (originally covered 2026-05-15 / deep-dive 2026-05-16): The Microsoft Exchange Team Blog post addressing CVE-2026-42897 was last modified 2026-05-17 to clarify an operational dependency that defenders must verify on every Exchange Mailbox host: the Exchange Emergency Mitigation Service (EM Service / EEMS) — which auto-applies the URL-Rewrite mitigation labelled M2.1.x — only delivers that mitigation when it can reach officemitigations.microsoft.com over outbound HTTPS. Segmented on-premises Exchange 2016 / 2019 / Subscription-Edition deployments that block direct outbound HTTPS from the Mailbox role will therefore not have received the automatic mitigation and remain exposed to the actively-exploited OWA stored-XSS chain.

The CVE remains CISA KEV-listed (added 2026-05-15) with no permanent cumulative-update fix as of 2026-05-18; Microsoft states verbatim "We are working on developing and testing a more permanent fix which we will provide when it meets our quality standards." Exchange Online is unaffected. Operational verification per server: Get-ExchangeDiagnosticInfo -Server <server> -Process EdgeTransport -Component EmergencyMitigation returns Status: Active and rule M2.1.x applied; manual application on hosts that cannot reach the mitigation service: .\EOMT.ps1 -CVE "CVE-2026-42897" from an elevated Exchange Management Shell, or apply the documented URL Rewrite rule by hand.