Patch tracker / CVE reference / CVE-2026-21258

CVE-2026-21258

CVE-2026-21258 is a medium severity Microsoft vulnerability with a CVSS score of 5.5. Microsoft has shipped the fix: deploy one of the 2 updates below from the Microsoft Update Catalog, and see where this CVE sits against the rest on the patch tracker.

Risk summary

Medium

5.5

CVSS v3

<1%

EPSS, 30 days

Medium

Severity

Not in the CISA KEV catalog. EPSS exploit probability <1%. CVSS 5.5 (Medium).

This CVE is among the lower half of tracked Microsoft CVEs by EPSS exploit probability.

Senserva AI Opinion for CVE-2026-21258

This Excel information disclosure bug, affecting Office Online Server and Office 2019, carries a High 7.8 severity but the lowest EPSS score of any item in this batch at just 1 percent. A vulnerability reachable through a malicious spreadsheet is still worth closing given how commonly Excel files move between organizations, even though attackers do not currently appear to be prioritizing it. It has not been confirmed exploited or tied to ransomware.

Do this: Patch Office 2019 and Office Online Server in the regular update cycle; this one does not need to be expedited given its 1 percent EPSS score.

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

Senserva AI Opinion and rich prompt for CVE-2026-21258

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

I am an IT admin assessing a vulnerability. Facts for CVE-2026-21258 (source: https://senserva.com/cve/CVE-2026-21258.html, data from MSRC, CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2026-21258: Medium severity Microsoft vulnerability, CVSS 5.5
- EPSS 30-day exploitation probability: <1%
- Actively exploited (CISA KEV): no
- Fixed by: KB5002835, KB5002837
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) What to patch or mitigate first. 4) How to check whether my devices are exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Schedule normally

No exploitation observed: not in the CISA KEV catalog, EPSS under 1%.

Fixed by KB5002835 and 1 more (by build, listed below). Download from the Microsoft Update Catalog, or deploy via Windows Update, Intune, or WSUS.

Do this now: Deploy with your normal update cycle. This page refreshes daily; the verdict changes the day the data does.

Official patch(es) for CVE-2026-21258

Lower priority: patch in your normal cycle. In Intune or Defender, confirm whether the fixing updates KB5002835, KB5002837 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-2026-21258. 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.

KB5002835 KB5002837

Scope and sources

Affected products

Fixed by the same updates

CVE-2026-21259 CVE-2026-21261

Authoritative references

Official CVE record (cve.org)NVD detail Microsoft MSRC FIRST.org EPSS

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