# CVE-2026-21511: Microsoft Outlook Spoofing Vulnerability

CVE-2026-21511 is fixed in 5 products on 1 platform, first by KB5002840, released February 10, 2026. It is not reported as exploited, but the vendor rates exploitation more likely. Patch Radar priority: Urgent.

- Web page: https://patchblox.com/security/cve/CVE-2026-21511
- Priority: Urgent
- CVSS: 7.5 (Important)
- EPSS: 0.0397
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Vendor exploitability assessment: Exploitation More Likely
- Published: Feb 10, 2026
- Weakness: Deserialization of Untrusted Data
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21511

## Description

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

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Microsoft | Microsoft SharePoint Enterprise Server 2016 |  | [KB5002840](https://patchblox.com/security/data/fix/microsoft/kb5002840.md) | Feb 10, 2026 |
| Microsoft | Microsoft SharePoint Enterprise Server 2016 |  | [KB5002841](https://patchblox.com/security/data/fix/microsoft/kb5002841.md) | Feb 10, 2026 |
| Microsoft | Microsoft SharePoint Server 2019 |  | [KB5002836](https://patchblox.com/security/data/fix/microsoft/kb5002836.md) | Feb 10, 2026 |
| Microsoft | Microsoft SharePoint Server 2019 |  | [KB5002834](https://patchblox.com/security/data/fix/microsoft/kb5002834.md) | Feb 10, 2026 |
| Microsoft | Microsoft SharePoint Server Subscription Edition |  | [KB5002833](https://patchblox.com/security/data/fix/microsoft/kb5002833.md) | Feb 10, 2026 |
| Microsoft | Microsoft Word 2016 (32-bit edition) |  | [KB5002839](https://patchblox.com/security/data/fix/microsoft/kb5002839.md) | Feb 10, 2026 |
| Microsoft | Microsoft Word 2016 (64-bit edition) |  | [KB5002839](https://patchblox.com/security/data/fix/microsoft/kb5002839.md) | Feb 10, 2026 |

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Data as of 2026-11-09T07:25:28Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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