Exploited vulnerability · CISA KEV

Is CVE-2021-26855 exploited? Yes: in CISA KEV since 3 Nov 2021

Gociux · Microsoft Exchange Server · data: CISA KEV, CVE record, FIRST EPSS

Yes. CVE-2021-26855, Microsoft Exchange Server Remote Code Execution Vulnerability, is being exploited in the wild. CISA added it to its Known Exploited Vulnerabilities catalog on 3 Nov 2021, 246 days after it was published. It is known to be used in ransomware campaigns. It affects Microsoft Exchange Server. If you run it, CISA's required action is: Apply updates per vendor instructions.

What it is

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

What to do

1. Check whether you run it. Look for Exchange Server in your asset inventory, including appliances and cloud images. Stack check lists every exploited Microsoft CVE at once.
2. Apply the fix. Apply updates per vendor instructions.
3. If you can't patch today, take the affected service off the internet or restrict who can reach it, and check its logs for signs of compromise since 2 Mar 2021.
4. Get told next time. Email alerts when CISA adds a CVE for Microsoft.

Other exploited CVEs in Exchange Server

• CVE-2023-21529: Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability (13 Apr 2026)
• CVE-2021-31196: Microsoft Exchange Server Information Disclosure Vulnerability (21 Aug 2024)
• CVE-2024-21410: Microsoft Exchange Server Privilege Escalation Vulnerability (15 Feb 2024)
• CVE-2022-41080: Microsoft Exchange Server Privilege Escalation Vulnerability (10 Jan 2023)
• CVE-2022-41082: Microsoft Exchange Server Remote Code Execution Vulnerability (30 Sep 2022)
• CVE-2022-41040: Microsoft Exchange Server Server-Side Request Forgery Vulnerability (30 Sep 2022)

Sources: CISA KEV (exploitation, ransomware use, required action, due date), the CVE record (publication date, CVSS, CNA), NVD and FIRST EPSS (live score). More: all KEV additions in November 2021 · Microsoft scorecard · act-now radar.