Microsoft Exchange Server Cross-Site Scripting Vulnerability
CVE-2026-42897 is a security vulnerability in Microsoft Exchange Server, added to CISA KEV on May 15, 2026. Patch order: third of four.
What it is
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.
VulnCheck tracked exploitation 1 day before CISA's KEV listing. VulnCheck's broader catalog of in-the-wild exploitation evidence picked this up before it landed on CISA's formal list. Per-CVE fact about this entry; not a population claim about lead time in general.
Who's affected
Affects anyone running on-premises Microsoft Exchange Server. If you have Exchange in your office (as opposed to Microsoft 365 hosted email), it's the mail server holding all internal email — full compromise reads every conversation it stores.
What to do
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
The action above is CISA's wording. The directive it names, BOD 22-01, was revoked by BOD 26-04 on June 10, 2026; CISA's catalog still carries the older text for this entry.
If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Book a free 20-minute triage call.
Insurance readiness
Your insurer will ask if this is patched.
Unpatched entries on CISA's Known Exploited Vulnerabilities list are exactly what cyber-insurance carrier questionnaires probe for, and knowing which controls they check is how you keep a renewal from stalling.
Get the free carrier questionnaire →Severity
CVSS base score: 8.1 — HIGH
Weakness classification: CWE-79
Microsoft fix
- KB5094144 (Security Update)
- KB5094142 (Security Update)
- KB5094140 (Security Update)
- KB5094139 (Security Update)
Microsoft Exploitability Index (assessed at update release): Exploitation Detected
Microsoft has confirmed in-the-wild exploitation in the latest software release. This aligns with CISA's listing of this CVE.
Vendor workaround
No vendor workaround published. Patching is the only fix.
CISA due date
CISA's listed due date: May 29, 2026.
Federal civilian agencies work under CISA's Binding Operational Directive 26-04, which replaced BOD 22-01 on June 10, 2026. It sets the deadline for each affected system by:
- How exposed that system is to the internet
- Whether the attack can be automated
- How much control it gives an attacker
Its table gives catalogued vulnerabilities 3 or 14 days.
None of this binds a private business, but treat the date as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).
EPSS
EPSS score: 0.5% — higher than 42% of all scored CVEs.
EPSS estimates the probability of exploitation activity in the next 30 days. Every entry on this site is already confirmed exploited, so read it as relative urgency among the things you still have open, not as permission to wait.
Source
This entry comes from CISA's Known Exploited Vulnerabilities catalog, public US-Government data we pull daily. The plain-language framing and the what-to-do guidance above are ours. View the original entry on cisa.gov.