CVE-2021-26855 Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.
Should You Worry?
- CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 3 Nov 2021.
- CISA lists it as known to be used in ransomware campaigns.
- Rated Critical severity (CVSS 9.1, scored by Vendor).
- This product type is often exposed to the internet, which is where attackers scan for it first.
- It has been in the catalogue for 1,829 days. Systems still unpatched now have almost certainly been scanned for it.
What It Is
Microsoft Exchange Server Remote Code Execution Vulnerability
Affected Versions
| Product | Versions |
|---|---|
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 21 | 15.00.0 to before 15.00.1395.012 |
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 22 | 15.00.0 to before 15.00.1473.006 |
| Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 | 15.00.0 to before 15.00.1497.012 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 10 | 15.01.0 to before 15.01.1531.012 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 11 | 15.01.0 to before 15.01.1591.018 |
| Microsoft Microsoft Exchange Server 2016 Cumulative Update 12 | 15.01.0 to before 15.01.1713.010 |
From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.
What To Do Now
- 1
Find it
Search your asset inventory and external attack surface for Microsoft Exchange Server. Include cloud images, appliances and anything a supplier runs for you.
Attack Surface Management - 2
- 3
Hunt for compromise
If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.
Incident Response - 4
Prove it is fixed
Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.
Penetration Testing
The Compliance Clock
General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.
Not Sure If You Are Exposed?
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