CVE-2021-26855
Microsoft Exchange Server Remote Code Execution Vulnerability
CRITICAL
9.1
CVSS Score
Published: March 2, 2021 at 11:55 PM UTC
Updated: August 19, 2026 at 07:35 PM UTC
1 product affected across 1 vendorExchange Server
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Severity
CRITICAL9.1
Exploitation
CISA KEV
Exploit prediction
EPSS 100%
SSVC
Active
Actively Exploited
CISA Known Exploited Vulnerabilities (KEV) Catalog
EPSS Score100.0%
EPSS (Exploit Prediction Scoring System) estimates a 100.0% chance this CVE is exploited in the wild within the next 30 days — higher than 100% of all CVEs. It complements CVSS: a high EPSS on a mid-severity CVE can still be urgent.
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.
Required Action
Apply updates per vendor instructions.
Ransomware Use
Known
Added to KEV
Nov 3, 2021
Remediation Due
May 3, 2022
1642 days overdue
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Affected
Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.
15.00.0 ≤ v < 15.00.1395.01215.00.0 ≤ v < 15.00.1473.00615.00.0 ≤ v < 15.00.1497.01215.01.0 ≤ v < 15.01.1415.01015.01.0 ≤ v < 15.01.1466.01615.01.0 ≤ v < 15.01.1531.01215.01.0 ≤ v < 15.01.1591.01815.01.0 ≤ v < 15.01.1713.01015.01.0 ≤ v < 15.01.1779.00815.01.0 ≤ v < 15.01.1847.01215.01.0 ≤ v < 15.01.1913.01215.01.0 ≤ v < 15.01.1979.00815.01.0 ≤ v < 15.01.2044.01315.01.0 ≤ v < 15.01.2106.01315.01.0 ≤ v < 15.01.2176.00915.02.0 ≤ v < 15.01.1979.00615.02.0 ≤ v < 15.02.0221.01815.02.0 ≤ v < 15.02.0397.01115.02.0 ≤ v < 15.02.0464.01515.02.0 ≤ v < 15.02.0529.01315.02.0 ≤ v < 15.02.0595.00815.02.0 ≤ v < 15.02.0659.01215.02.0 ≤ v < 15.02.0721.01315.02.0 ≤ v < 15.02.0792.01015.00.015.01.015.02.02013 cumulative_update_212013 cumulative_update_222013 cumulative_update_232016 cumulative_update_102016 cumulative_update_112016 cumulative_update_122016 cumulative_update_132016 cumulative_update_142016 cumulative_update_152016 cumulative_update_162016 cumulative_update_172016 cumulative_update_182016 cumulative_update_192016 cumulative_update_82016 cumulative_update_920192019 cumulative_update_12019 cumulative_update_22019 cumulative_update_32019 cumulative_update_42019 cumulative_update_52019 cumulative_update_62019 cumulative_update_72019 cumulative_update_8
Affected range stops below 15.00.1395.012, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.00.1473.006, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.00.1497.012, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1415.010, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1466.016, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1531.012, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1591.018, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1713.010, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1779.008, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1847.012, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1913.012, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1979.006, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.1979.008, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.2044.013, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.2106.013, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.01.2176.009, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0221.018, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0397.011, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0464.015, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0529.013, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0595.008, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0659.012, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0721.013, so it looks safe — our inference, not a vendor statement
Affected range stops below 15.02.0792.010, so it looks safe — our inference, not a vendor statement
Remediation
No guide yetNo remediation guidance has been written for this CVE yet. What the record does show:
References (7)
ExploitThird Party AdvisoryVDB Entry
Source: [email protected]
http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.htmlExploitThird Party AdvisoryVDB Entry
Source: [email protected]
http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.htmlExploitThird Party AdvisoryVDB Entry
Source: [email protected]
http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.htmlExploitThird Party AdvisoryVDB Entry
Source: [email protected]
http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.htmlPatchVendor Advisory
Source: [email protected]
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855US Government Resource
Source: 134c704f-9b21-4f2e-91b3-4a467353bcc0
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855Vendor AdvisoryPatch
Source: cverecord
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855SSVC — Action Prioritization
SSVCActiveAutomatableTotal impact
SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.
CVSS
9.1CRITICALmicrosoft
Vector?
Quick Info
StatusANALYZED
CWE
CWE-918
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