CVE-2021-26855 ProxyLogon

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon…

Browse: All 2021 CVEs · All vendors

CVSS score
9.1 Critical
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
CISA KEV
Yes — actively exploited (added 2021-11-03, remediation due 2022-05-03)
Ransomware
Known ransomware campaign use
Weakness (CWE)
CWE-918
Published
2021-03-03
Last modified
2026-08-19
NVD status
Analyzed

Description

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

Affected products

Affected ranges per NVD CPE data; confirm exact versions in the vendor advisory.

Required action (CISA)

Apply updates per vendor instructions. Federal remediation deadline: 2022-05-03.

How to prioritize this

This vulnerability is on the CISA KEV list, meaning it is being actively exploited — treat it as urgent regardless of the raw CVSS number. Use the live CVE lookup for the latest NVD data, and the CVSS 3.1 calculator to model your own environmental score.

References

FAQ

Is CVE-2021-26855 actively exploited?

Yes. CISA added CVE-2021-26855 to its Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03, with a federal remediation deadline of 2022-05-03. A KEV listing means real-world exploitation is confirmed, so patching this should be prioritized above many higher-CVSS bugs that aren't being exploited.

What is the CVSS score of CVE-2021-26855?

CVE-2021-26855 has a CVSS base score of 9.1 (Critical). Its vector string is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N.

Related vulnerabilities

Data source: NVD & CISA KEV. Shown as-is; nothing added. Last generated from NVD data — re-check the live tool for updates.