CVEs / CVE-2021-26855
CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability
- Known exploited in the CISA KEV catalog since 3 Nov 2021
- high 7.5 CVSS 2.0 (NVD)
- EPSS 99.9% chance of exploitation in the next 30 days, higher than 99% of CVEs (30 Oct 2026)
- Last changed at its sources
Are you affected? Check your servers with Stackurity: it lists the software on each server and the vulnerabilities to fix first.
Check your servers with Stackurity Already using Stackurity? Search for CVE-2021-26855
Summary
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Affected and fixed versions
| Product | Affected | Fixed in |
|---|---|---|
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 21 | from 15.00.0, before 15.00.1395.012 | 15.00.1395.012 |
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 22 | from 15.00.0, before 15.00.1473.006 | 15.00.1473.006 |
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 | from 15.00.0, before 15.00.1497.012 | 15.00.1497.012 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 10 | from 15.01.0, before 15.01.1531.012 | 15.01.1531.012 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 11 | from 15.01.0, before 15.01.1591.018 | 15.01.1591.018 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 12 | from 15.01.0, before 15.01.1713.010 | 15.01.1713.010 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 13 | from 15.01.0, before 15.01.1779.008 | 15.01.1779.008 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 14 | from 15.01.0, before 15.01.1847.012 | 15.01.1847.012 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 15 | from 15.01.0, before 15.01.1913.012 | 15.01.1913.012 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 16 | from 15.01.0, before 15.01.1979.008 | 15.01.1979.008 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 17 | from 15.01.0, before 15.01.2044.013 | 15.01.2044.013 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 18 | from 15.01.0, before 15.01.2106.013 | 15.01.2106.013 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 19 | from 15.01.0, before 15.01.2176.009 | 15.01.2176.009 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 8 | from 15.01.0, before 15.01.1415.010 | 15.01.1415.010 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 9 | from 15.01.0, before 15.01.1466.016 | 15.01.1466.016 |
| Microsoft Microsoft Exchange Server 2019 | from 15.02.0, before 15.02.0221.018 | 15.02.0221.018 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 1 | from 15.02.0, before 15.01.1979.006 | 15.01.1979.006 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 2 | from 15.02.0, before 15.02.0397.011 | 15.02.0397.011 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 3 | from 15.02.0, before 15.02.0464.015 | 15.02.0464.015 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 4 | from 15.02.0, before 15.02.0659.012 | 15.02.0659.012 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 5 | from 15.02.0, before 15.02.0595.008 | 15.02.0595.008 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 6 | from 15.02.0, before 15.02.0529.013 | 15.02.0529.013 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 7 | from 15.02.0, before 15.02.0721.013 | 15.02.0721.013 |
| Microsoft Microsoft Exchange Server 2019 Cumulative Update 8 | from 15.02.0, before 15.02.0792.010 | 15.02.0792.010 |
Exploitation
| In CISA KEV since | 3 Nov 2021 |
|---|---|
| Due date for US federal agencies | 3 May 2022 |
| Used in ransomware campaigns | Known |
| Required action | Apply updates per vendor instructions. |
Severity
| CVSS 2.0 | 7.5 highAV:N/AC:L/Au:N/C:P/I:P/A:PScored by NVD |
|---|---|
| Weakness | CWE-918 |
References
- CVE-2021-26855 on NVD
- CVE-2021-26855 in the CISA KEV catalog
- https://www.cisa.gov/news-events/directives/ed-21-02-mitigate-microsoft-exchange-premises-product-vulnerabilities (cited by CISA)
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855 Patch, Vendor Advisory
- http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html Exploit, Third Party Advisory, VDB Entry
- http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html Exploit, Third Party Advisory, VDB Entry
- http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html Exploit, Third Party Advisory, VDB Entry
- http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html Exploit, Third Party Advisory, VDB Entry
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855 Patch, Vendor Advisory
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855 US Government Resource
Sources
Exploitation status from the CISA Known Exploited Vulnerabilities catalog. EPSS scores from FIRST. Descriptions, severity and references from the National Vulnerability Database. Open-source package versions from OSV (CC-BY 4.0). Text is shown as each source states it; Stackurity does not edit it. Check the vendor's advisory before acting.
This product uses data from the NVD API but is not endorsed or certified by the NVD.