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CVE-2026-21511

Microsoft Outlook Spoofing Vulnerability

No known exploitation. EPSS puts it in the 90th percentile. Fixed versions are out.

CVSS 3.17.5HighEPSS percentile90th4.0% chance in 30 daysCISA KEVNot listedNo confirmed exploitation
No public exploit knownFix5 products4 fixed builds listed below

Published Updated Sources: CVE.org, NVD, BDU

NVD CVE.org

What to do

Find your product and version, then upgrade to the build on the right.

  • Microsoft SharePoint Server Subscription EditionAffected: ≥ 16.0.0, < 16.0.19127.20518Upgrade to: 16.0.19127.20518Stated by: CVE record
  • Microsoft SharePoint Server 2019Affected: ≥ 16.0.0, < 16.0.10417.20097Upgrade to: 16.0.10417.20097Stated by: CVE record
  • Microsoft SharePoint Enterprise Server 2016Affected: ≥ 16.0.0, < 16.0.5539.1002Upgrade to: 16.0.5539.1002Stated by: CVE record
  • Microsoft Word 2016Affected: ≥ 16.0.1, < 16.0.5539.1002Upgrade to: 16.0.5539.1002Stated by: CVE record
  • Microsoft 365 Apps for EnterpriseAffected: ≥ 16.0.1, < https://aka.ms/OfficeSecurityReleasesUpgrade to: https://aka.ms/OfficeSecurityReleasesStated by: CVE record

Versions as the sources state them. 4 more products have version data. Full version matrix after sign-in

Check it yourselfA free OpenVAS check exists for this CVE. Run it against your hosts.Is your organisation exposed?Request an assessment to establish which checks fit your systems.

What it is

From the CVE record

Deserialization of untrusted data in Microsoft Office Outlook allows an unauthorized attacker to perform spoofing over a network.

In plain language

No plain-language summary for this CVE yet.

Exploitation

Where each signal puts this CVE on the scale from published to confirmed exploited.

EPSS90th
CISA KEV

Not in the catalog. CISA has not confirmed exploitation.

Public exploits

No public exploit or proof of concept found in the sources we track.

EPSS

4.0% chance of exploitation activity in the next 30 days, which ranks it in the 90th percentile of scored CVEs.

Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.

Lifecycle

6 events over 135 days, from the signal feeds we watch.

  1. OpenVAS check added
  2. Patch availablerecord updated
  3. Publishedweakness classified, att&ck mapped

Affected products

And 15 more affected products. See all after sign-in

Technical detail

CVSS 3.1 vector

Open in the CVSS calculator

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Scored 7.5 by NVD.

How it is reached

  • Attack Vector NetworkExploitable remotely over the network without any special conditions
  • Attack Complexity LowNo special conditions — the attack can be reliably reproduced
  • Privileges Required NoneNo authentication required — anyone can exploit this
  • User Interaction NoneNo user interaction needed — fully automated exploitation

Scope

  • Scope UnchangedImpact is limited to the vulnerable component itself

Impact if exploited

  • Confidentiality HighTotal information disclosure — all data in the component is compromised
  • Integrity NoneNo integrity impact
  • Availability NoneNo availability impact

Weaknesses

ATT&CK techniques

Mapped from the weaknesses above (CWE to ATT&CK), not observed in attacks.

Sources

References in the record

In the news

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We'll flag the next CVE, public exploit or patch for Microsoft Corp, not every advisory.