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Actively exploited

CVE-2026-42897 Microsoft Exchange Server Cross-Site Scripting Vulnerability

Microsoft Exchange Server contains a cross-site scripting vulnerability during web page generation in Outlook Web Access and when certain interaction conditions are met, arbitrary JavaScript can be executed in the browser context.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 15 May 2026.
  • Rated High severity (CVSS 8.1, scored by Vendor).
  • This product type is often exposed to the internet, which is where attackers scan for it first.
  • It has been in the catalogue for 175 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to perform spoofing over a network.

Affected Versions

ProductVersions
Microsoft Microsoft Exchange Server 2016 Cumulative Update 2315.01.0.0 to before 15.01.2507.069
Microsoft Microsoft Exchange Server 2019 Cumulative Update 1415.02.0.0 to before 15.02.1544.041
Microsoft Microsoft Exchange Server 2019 Cumulative Update 1515.02.0.0 to before 15.02.1748.046
Microsoft Microsoft Exchange Server Subscription Edition RTM15.02.0.0 to before 15.02.2562.043

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Microsoft. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 29 May 2026 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2026-42897
  3. CVE record: CVE-2026-42897
  4. FIRST EPSS
  5. msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2026-42897
  6. learn.microsoft.com/en-us/exchange/plan-and-deploy/post-installation-tasks/security-best-practices/e
  7. msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42897