# KB5001779: April 2021 security update

KB5001779, released April 13, 2021, fixes 7 CVEs across 5 products. 3 of them are being exploited (CVE-2021-34473); 3 on CISA KEV. Patch Radar priority: Patch now. The product is no longer supported.

- Web page: https://patchblox.com/security/fix/microsoft/kb5001779
- Platform: Microsoft
- Released: Apr 13, 2021
- Priority: Patch now (Closes CVE-2021-34473, exploited (CISA KEV, ransomware))
- CVEs: 7, exploited: 3, on CISA KEV: 3
- Vendor advisory: https://support.microsoft.com/help/5001779

## Applies to

| Product | Release | Fixed in |
|---|---|---|
| [Microsoft Exchange Server 2013 Cumulative Update 23](https://patchblox.com/security/data/product/microsoft/microsoft-exchange-server-2013-cumulative-update-23.md) |  |  |
| [Microsoft Exchange Server 2016 Cumulative Update 19](https://patchblox.com/security/data/product/microsoft/microsoft-exchange-server-2016-cumulative-update-19.md) |  |  |
| [Microsoft Exchange Server 2016 Cumulative Update 20](https://patchblox.com/security/data/product/microsoft/microsoft-exchange-server-2016-cumulative-update-20.md) |  |  |
| [Microsoft Exchange Server 2019 Cumulative Update 8](https://patchblox.com/security/data/product/microsoft/microsoft-exchange-server-2019-cumulative-update-8.md) |  |  |
| [Microsoft Exchange Server 2019 Cumulative Update 9](https://patchblox.com/security/data/product/microsoft/microsoft-exchange-server-2019-cumulative-update-9.md) |  |  |

## CVEs this fix closes

| CVE | Priority | Component | CVSS | EPSS | Exploitation |
|---|---|---|---:|---:|---|
| [CVE-2021-34473](https://patchblox.com/security/data/cve/CVE-2021-34473.md) | Patch now | Microsoft Exchange Server | 9.1 | 1.0000 | CISA KEV, Ransomware, Disclosed |
| [CVE-2021-34523](https://patchblox.com/security/data/cve/CVE-2021-34523.md) | Patch now | Microsoft Exchange Server | 9.0 | 0.9999 | CISA KEV, Ransomware, Disclosed |
| [CVE-2021-33766](https://patchblox.com/security/data/cve/CVE-2021-33766.md) | Patch now | Microsoft Exchange Server | 7.3 | 0.9818 | CISA KEV |
| [CVE-2021-28482](https://patchblox.com/security/data/cve/CVE-2021-28482.md) | Urgent | Microsoft Exchange Server | 8.8 | 0.8321 | More likely |
| [CVE-2021-28480](https://patchblox.com/security/data/cve/CVE-2021-28480.md) | Urgent | Microsoft Exchange Server | 9.8 | 0.7123 | More likely |
| [CVE-2021-28481](https://patchblox.com/security/data/cve/CVE-2021-28481.md) | Urgent | Microsoft Exchange Server | 9.8 | 0.3624 | More likely |
| [CVE-2021-28483](https://patchblox.com/security/data/cve/CVE-2021-28483.md) | Urgent | Microsoft Exchange Server | 9.0 | 0.0120 | More likely |

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Data as of 2026-11-12T13:55:20Z 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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