On-Prem

Microsoft mitigation may bork inline images, calendar printing while admins wait for a proper patch

Microsoft has confirmed a vulnerability in on-premises Exchange Server that could result in surprise script execution in victims' browsers.

Tracked as CVE-2026-42897, the flaw affects Outlook Web Access (OWA) and can be triggered by a specially crafted email opened in OWA, assuming "certain interaction conditions are met." The prize for attackers is arbitrary JavaScript execution in the mark's browser context.

The advisory describes the flaw as a spoofing vulnerability stemming from cross-site scripting, which will set alarm bells ringing for administrators, and it appears the vulnerability is being exploited. The bug was assigned a CVSS score of 8.1.

Exchange Server 2016, 2019, and the latest version, Exchange Server Subscription Edition (SE), are all affected regardless of their update level. A mitigation has been released via the Exchange Emergency Mitigation (EM) Service. 

However, Microsoft warned the mitigation might break other things – inline images might stop working in the recipient's OWA reading pane (use attachments instead) and the OWA Print Calendar functionality might not work (use a screenshot or the Outlook Desktop client).

Finally, OWA Light might not work properly. Microsoft deprecated this in 2024, so affected users should consider an upgrade.

The mitigation can also be applied manually in scenarios where customers are not using the EM service. These might be disconnected or air-gapped environments – exactly the sort of environments where on-premises Exchange tends to linger.

Microsoft is working on a full security update, although only the Exchange SE version will be publicly available. Exchange 2016 and 2019 customers will receive it only if enrolled in Period 2 of the Exchange Server Extended Security Updates (ESU) program. The second period of Exchange Server ESU kicked off this month, with Microsoft sternly warning that there would be no extensions past its end. The vulnerability does not affect Exchange Online.

Microsoft has not given any details on how the exploit works, nor how widely it is being exploited. ®