CVE-2021-26858: Microsoft Exchange Server Remote Code Execution Vulnerability
Reference page built from the source records below, each shown as its source publishes it. Dataset version ref-e87acbcaef9c, updated 2026-11-02T23:39:54.475Z.
CISA Known Exploited Vulnerabilities
- Name
- Microsoft Exchange Server Remote Code Execution Vulnerability
- Vendor
- Microsoft
- Product
- Exchange Server
- Added to the catalog
- 2021-11-03
- Due date for US federal agencies
- 2022-05-03
- Catalog version
- 2026.10.30
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
Required action (CISA)
Apply updates per vendor instructions.
Ransomware use: CISA marks this vulnerability as known to be used in ransomware campaigns. CISA does not say which group used it.
Links in CISA's notes
- https://www.cisa.gov/news-events/directives/ed-21-02-mitigate-microsoft-exchange-premises-product-vulnerabilities)
- https://nvd.nist.gov/vuln/detail/CVE-2021-26858
Read on 2026-11-02T23:39:54.475Z under licence cisa-kev-cc0@b0067899 from https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json
NVD analysis
- Status
- Analyzed
- Published
- 2021-03-03T00:15:12.227
- Last modified
- 2026-10-01T21:17:16.777
- Weaknesses
- NVD-CWE-noinfo
Microsoft Exchange Server Remote Code Execution Vulnerability
| Metric | Score | Severity | Vector | Scored by |
|---|---|---|---|---|
| CVSS 3.1 | 7.8 | HIGH | CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H | [email protected] (Secondary) |
| CVSS 3.1 | 7.8 | HIGH | CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H | [email protected] (Secondary) |
| CVSS 2.0 | 6.8 | MEDIUM | AV:N/AC:M/Au:N/C:P/I:P/A:P | [email protected] (Primary) |
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26858 (Patch, Vendor Advisory)
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26858 (Patch, Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26858 (US Government Resource)
This product uses the NVD API but is not endorsed or certified by the NVD.
Read on 2026-11-02T23:41:51.476Z under licence nvd-api-terms-of-use@ff2a5288 from https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2021-26858
CVE record
- Title (CNA)
- Microsoft Exchange Server Remote Code Execution Vulnerability
- Assigner
- microsoft
- State
- PUBLISHED
- Published
- 2021-03-02T23:55:27.000Z
- Updated
- 2026-10-01T20:12:19.501Z
- Problem types
- Remote Code Execution
References
Read on 2026-11-03T01:06:56.102Z under licence cve-terms-of-use@81bfa2f3 from https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2021/26xxx/CVE-2021-26858.json
Detection rules (SigmaHQ)
SigmaHQ rules whose own tags name this CVE. Each rule page shows the rule as its author published it.
- CVE-2021-26858 Exchange Exploitation (status test, level high)
- ProxyLogon Reset Virtual Directories Based On IIS Log (status test, level critical)
Sources and licences
CISA
Source: CISA Catalog of Known Exploited Vulnerabilities, version 2026.10.30, CC0 1.0. Use of this data does not imply endorsement by CISA or DHS.
National Vulnerability Database (NIST)
This product uses the NVD API but is not endorsed or certified by the NVD.
NVD API Terms of Use (NIST publications in the public domain, 17 U.S.C. 105)
CVE Program, The MITRE Corporation
Copyright © 1999-2026, The MITRE Corporation. CVE is a trademark and the CVE logo is a registered trademark of The MITRE Corporation. CVE Usage: MITRE hereby grants you a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable copyright license to reproduce, prepare derivative works of, publicly display, publicly perform, sublicense, and distribute Common Vulnerabilities and Exposures (CVE™). Any copy you make for such purposes is authorized provided that you reproduce MITRE's copyright designation and this license in any such copy.
SigmaHQ
"CVE-2021-26858 Exchange Exploitation" by Bhabesh Raj (SigmaHQ). Rule: https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules-emerging-threats/2021/Exploits/CVE-2021-26858/file_event_win_cve_2021_26858_msexchange.yml.
Licensed under the Detection Rule License (DRL) 1.1: https://github.com/SigmaHQ/Detection-Rule-LicenseSigmaHQ
"ProxyLogon Reset Virtual Directories Based On IIS Log" by frack113 (SigmaHQ). Rule: https://github.com/SigmaHQ/sigma/blob/ccbb0369507f796db0b0ae6854c8a2be471b9d84/rules-emerging-threats/2021/Exploits/CVE-2021-26858/web_cve_2021_26858_iis_rce.yml.
Licensed under the Detection Rule License (DRL) 1.1: https://github.com/SigmaHQ/Detection-Rule-License
Cite this page
Dragons Eye Ransomware Tracker, "CVE-2021-26858: Microsoft Exchange Server Remote Code Execution Vulnerability",
entity page, dataset version ref-e87acbcaef9c, updated 2026-11-02T23:39:54.475Z,
https://ransomware-tracker.com/cve/CVE-2021-26858 (accessed 2026-11-03).
Underlying source: CISA Catalog of Known Exploited Vulnerabilities, version 2026.10.30.
Underlying source: National Vulnerability Database (NIST), CVE API 2.0, record last modified 2026-10-01T21:17:16.777.
Underlying source: CVE List (CVE Program, The MITRE Corporation), record updated 2026-10-01T20:12:19.501Z.
Underlying source: SigmaHQ rule b06335b3-55ac-4b41-937e-16b7f5d57dfd, commit ccbb0369507f796db0b0ae6854c8a2be471b9d84.
Underlying source: SigmaHQ rule effee1f6-a932-4297-a81f-acb44064fa3a, commit ccbb0369507f796db0b0ae6854c8a2be471b9d84.
Pages change. The dataset version identifies the data, but no archive of old versions is kept, so this citation cannot be replayed later.