# CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability

> CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability. CVSS 9.1 (CVSS 3.1), in CISA KEV.

Canonical: https://rewindex.com/cve/CVE-2021-26855
Last updated: 2026-08-19T16:30:15Z

- Published: 2021-03-02 23:55 UTC · Updated: 2026-08-19 16:30 UTC · Assigner: microsoft
- CVSS: 9.1 (CVSS 3.1) · Severity: Critical · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C
- EPSS: 100.0%, 100th percentile (scored 2026-10-25)
- CISA KEV: yes, added 2021-11-03, federal due date 2022-05-03, ransomware use: Known

## Description

Microsoft Exchange Server Remote Code Execution Vulnerability

## CISA required action

Apply updates per vendor instructions.

## Affected vendors and products

- Microsoft: Microsoft Exchange Server 2013 Cumulative Update 21
- Microsoft: Microsoft Exchange Server 2013 Cumulative Update 22
- Microsoft: Microsoft Exchange Server 2013 Cumulative Update 23
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 10
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 11
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 12
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 13
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 14
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 15
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 16
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 17
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 18
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 19
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 8
- Microsoft: Microsoft Exchange Server 2016 Cumulative Update 9
- Microsoft: Microsoft Exchange Server 2019
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 1
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 2
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 3
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 4
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 5
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 6
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 7
- Microsoft: Microsoft Exchange Server 2019 Cumulative Update 8

## References

- [CVE record on cve.org](https://www.cve.org/CVERecord?id=CVE-2021-26855)
- [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2021-26855)
- [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855)

---

_Sources: CVE records from the CVE Program (cve.org). CVE® is a registered trademark of The MITRE Corporation. (https://www.cve.org/) This product uses data from the NVD API but is not endorsed or certified by the NVD. (https://nvd.nist.gov/) Known Exploited Vulnerabilities catalog from CISA. (https://www.cisa.gov/known-exploited-vulnerabilities-catalog) EPSS scores from FIRST (Exploit Prediction Scoring System). (https://www.first.org/epss/)_


---
Canonical: https://rewindex.com/cve/CVE-2021-26855 · Markdown: https://rewindex.com/cve/CVE-2021-26855.md · JSON: https://rewindex.com/api/v1/cves/CVE-2021-26855 · Data licensed CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) · Generated 2026-10-26T02:17:01.692Z