Exploited

CVE-2026-42897

Microsoft Exchange Server Cross-Site Scripting Vulnerability

At a glance

Exploited. Added to CISA’s catalog of exploited vulnerabilities on May 15, 2026.

Affected product: Microsoft Exchange Server (category Email, indicative classification). Name in CISA’s catalog: Microsoft Microsoft.

What should I do?

Apply the patches or mitigations from Microsoft; if none are available, stop using the product. For cloud services, follow the guidance from Microsoft.

Deadline set by CISA for US federal agencies: 14 days (due May 29, 2026). It only binds those agencies, but it is a useful measure of urgency.

CVE-2026-42897 in Microsoft’s Security Update Guide
Microsoft security advisories page (general page, not the advisory for this vulnerability)
The vendor’s specific advisory is listed in the NVD references: CVE-2026-42897 on the NVD website.

Original text of the required action (CISA)

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

Timeline

May 14, 2026 Vulnerability published Publication date (NVD)
May 15, 2026 Exploitation confirmed Added to CISA’s KEV catalog
May 29, 2026 CISA deadline Remediation deadline set for US federal agencies

The facts, with their sources

Exploitation confirmed: Yes, on May 15, 2026 Date added to the catalog of exploited vulnerabilities. Exploitation itself may have started earlier. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Ransomware: No known use to date That does not guarantee it is not being used: the information is not public. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Hunt for compromise: Not requested by CISA Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA required action: Apply the patches or mitigations from Microsoft; if none are available, stop using the product. For cloud services, follow the guidance from Microsoft. Rewritten by this site from CISA’s original text. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CISA deadline: May 29, 2026, 14 days Set for US federal agencies: a measure of urgency, not an obligation elsewhere. Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
Microsoft patch: Not available Source: Security Update Guide, collected September 29, 2026 at 14:57 UTC
Vulnerability published: May 14, 2026 Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC
Product according to CISA: Microsoft Microsoft Filed by this site under “Email” (indicative classification). Source: KEV catalog, collected September 29, 2026 at 14:57 UTC
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N Technical detail of the score: attack vector, complexity, impact. Source: National Vulnerability Database (NVD), collected September 29, 2026 at 15:00 UTC

Severity and activity

6.1 / 10 CVSS severity: medium

Score from the NVD. Collected Sep 29, 2026.

— Probability of exploitation in the next 30 days (EPSS)

FIRST’s EPSS model. Since exploitation is already confirmed, this figure is mostly useful to compare activity. Collected Sep 29, 2026.

Description

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.

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.

Summary from CISA (KEV catalog).

CERT-FR advisories

CERTFR-2026-ALE-005: [Màj] Vulnérabilité dans Microsoft Exchange Server (updated July 13, 2026)

Information reused under the Open Licence 2.0 (Etalab); the date of last update is shown for each advisory. This site is neither affiliated with nor endorsed by ANSSI.

Sources for this page

CISA KEV catalog: exploitation, date added, required action, deadline, ransomware.
Microsoft Security Update Guide: product, patch.
NVD (NIST): CVSS severity, publication date, description, vendor references.
EPSS (FIRST): probability of exploitation.
CERT-FR: advisories and alerts (in French).
Brand and category: indicative classification by this site (method).