CVE-2021-26858
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability is confirmed as actively exploited and is listed in the CISA KEV catalog.
Required action
Apply updates per vendor instructions.
CISA notes
Reference CISA's ED 21-02 (https://www.cisa.gov/news-events/directives/ed-21-02-mitigate-microsoft-exchange-premises-product-vulnerabilities) for further guidance and requirements. Note: The due date for addressing this vulnerability aligns with the requirements outlined in ED 21-02. https://nvd.nist.gov/vuln/detail/CVE-2021-26858
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Severity
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:CExploitation Context
as of October 17, 2026- CISA added CVE-2021-26858 to the KEV catalog 246 days after its CVE record was published.
- It is one of 391 Microsoft entries in the CISA KEV catalog, and one of 83 added in 2021.
- Its current EPSS score (93.7%) is higher than that of 76% of CISA KEV entries.
Affected Products
| Vendor | Product | Affected versions |
|---|---|---|
| Microsoft | Microsoft Microsoft Exchange Server 2013 Cumulative Update 21 | 15.00.0 to <15.00.1395.012 |
| Microsoft | Microsoft Microsoft Exchange Server 2013 Cumulative Update 22 | 15.00.0 to <15.00.1473.006 |
| Microsoft | Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 | 15.00.0 to <15.00.1497.012 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 10 | 15.01.0 to <15.01.1531.012 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 11 | 15.01.0 to <15.01.1591.018 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 12 | 15.01.0 to <15.01.1713.010 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 13 | 15.01.0 to <15.01.1779.008 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 14 | 15.01.0 to <15.01.1847.012 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 15 | 15.01.0 to <15.01.1913.012 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 16 | 15.01.0 to <15.01.1979.008 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 17 | 15.01.0 to <15.01.2044.013 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 18 | 15.01.0 to <15.01.2106.013 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 | 15.01.0 to <15.01.2176.009 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 8 | 15.01.0 to <15.01.1415.010 |
| Microsoft | Microsoft Microsoft Exchange Server 2016 Cumulative Update 9 | 15.01.0 to <15.01.1466.016 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 | 15.02.0 to <15.02.0221.018 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 1 | 15.02.0 to <15.01.1979.006 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 2 | 15.02.0 to <15.02.0397.011 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 3 | 15.02.0 to <15.02.0464.015 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 4 | 15.02.0 to <15.02.0659.012 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 5 | 15.02.0 to <15.02.0595.008 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 6 | 15.02.0 to <15.02.0529.013 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 7 | 15.02.0 to <15.02.0721.013 |
| Microsoft | Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 | 15.02.0 to <15.02.0792.010 |
References
2 LinksRecord Details
More from Microsoft
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Elevation of Privilege Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
CVE record data © The MITRE Corporation, used under the CVE Terms of Use. Exploited-in-the-wild data from the CISA Known Exploited Vulnerabilities catalog (CC0). Exploit prediction scores by FIRST EPSS (first.org/epss). This site is not endorsed or certified by MITRE, NVD, CISA, or FIRST.