Patch tracker / CVE reference / CVE-2026-21243

CVE-2026-21243

CVE-2026-21243 is a Microsoft vulnerability that Microsoft rates Important, with a CVSS score of 7.5 (High). Its EPSS score puts the probability of exploitation in the next 30 days at 25%. Microsoft has shipped the fix: deploy one of the 6 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Risk summary

High
7.5
CVSS v3
25%
EPSS, 30 days
High
Severity

Not in the CISA KEV catalog. EPSS exploit probability 25%. CVSS 7.5 (High).

This CVE is among the top 25% of tracked Microsoft CVEs by EPSS exploit probability.

Senserva AI Opinion for CVE-2026-21243

A denial of service flaw in the Windows LDAP implementation on Server 2019 and 2022, rated High Severity at CVSS 8.8 with a notable 26 percent exploit probability. LDAP on Windows servers means domain controllers: a repeatable crash there degrades logons, group policy, and every app that binds to the directory. DoS flaws in DC-facing protocols get weaponized for disruption and as cover for other activity.

Do this: Patch your domain controllers on Server 2019 and 2022 in the first maintenance window, taking them in pairs to preserve availability.

AI-generated from public data, 2026-07-07. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2026-21243

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

Patch this cycle
Not confirmed exploited yet, but the 30-day exploit probability is high (EPSS 25%).
Fixed by KB5075897 and 5 more (by build, listed below). Download from the Microsoft Update Catalog, or deploy via Windows Update, Intune, or WSUS.
Do this now: Patch in this maintenance window; do not let it slip to the next one.

Official patch(es) for CVE-2026-21243

Elevated priority: high severity or a meaningful exploitation probability, sooner if an affected product is internet-facing. In Intune or Defender, confirm whether the fixing updates KB5075897, KB5075899, KB5075904, KB5075906, KB5075942, KB5075943 are installed across the affected products. Senserva does this automatically across your tenant and flags every missing patch.

Apply the Microsoft security updates below to fix CVE-2026-21243. Each KB page carries the Microsoft Update Catalog download, Microsoft's known issues for the update, a copy-paste PowerShell check for whether it is installed, and the Senserva AI read.

KB5075897KB5075899KB5075904KB5075906KB5075942KB5075943

Scope and sources

Affected products
Fixed by the same updates
CVE-2026-21510CVE-2026-21513CVE-2026-21519CVE-2026-21533CVE-2026-21525CVE-2026-21255CVE-2026-21231CVE-2026-21232
Authoritative references
Official CVE record (cve.org)NVD detailMicrosoft MSRCFIRST.org EPSS

Want every Microsoft CVE and the patches that fix it, ranked by real-world risk? See the Microsoft Patch Tracker.

New to a term on this page? CVE, KB, KEV, CVSS, and EPSS are each explained once, on the Senserva security lexicon. Reference: Microsoft CVE and vulnerability management and the Microsoft patching guide.
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