CVE-2021-26855

CRITICALCVSS 9.1Known Exploited

CISA Known Exploited Vulnerability

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-2021-26855

Remediation deadline: May 3, 2022

Ransomware use: Known

Description

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

Timeline

Published:March 3rd, 2021 12:15 AM

Last modified:August 19th, 2026 7:22 PM

Added to KEV:November 3rd, 2021

CVSS Scoring

CVSS v3: 9.1 (CRITICAL)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

CVSS v2: 7.5

Affected Software

From NVD CPE configuration data

Vendors

microsoft

Products

exchange_server

Weaknesses (CWE)

CWE-918

References & Reports

Advisories, patches, and third-party reports

Source: NIST NVD · Data may lag official sources by up to one minute