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CISA has given a new warning about a Microsoft Exchange Server flaw that attackers are already using, which worries organizations that depend on local email systems.
The issue CVE-2026-42897 is a cross-site scripting (XSS) flaw in Microsoft Exchange Server, mainly in Outlook Web Access (OWA).
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The official warning says the flaw happens when making a web page. It can occur when certain actions happen, letting attackers run any JavaScript in someone else’s browser.
The flaw was added to CISA’s list of known issues on May 15, 2026, showing that it is being actively used by attackers.
Federal agencies and groups that follow the Binding Operational Directive (BOD) 22-01 must fix the problem by May 29, 2026.
Microsoft Exchange Server Vulnerability Exploit
Security experts say that XSS flaws in business email systems like Exchange are very risky because they can be used to take over logged-in accounts.
An attacker could fool a user into clicking a special link that runs bad scripts in their browser.This can lead to credential theft, mailbox access, or further internal compromise.
Exchange servers have always been a prime target for hackers because they manage important messages and account information.
The flaw is marked as CWE-79, a common type of web security issue that happens when input is not properly cleaned up while making web pages.
CISA wants agencies to apply the security updates and fixes from vendors without delay.
If patches are not ready or cannot be used, agencies should follow other safety steps suggested by Microsoft or think about stopping the use of affected systems until they are safe.
Security teams need to check Exchange server logs for any strange activity. This includes odd login patterns, unexpected script running, or unusual user actions in Outlook Web Access sessions.
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