Microsoft Exchange Server Remote Code Execution Vulnerability
⚠️ Actively exploited — CISA KEV
This CVE was added to CISA's Known Exploited Vulnerabilities catalog on 2021-11-03. It is confirmed exploited in the wild and must be patched as a priority.
Remediation deadline (US federal agencies): 2022-05-03
Used in known ransomware campaigns.
EPSS · FIRST.org
100.0%probability of exploitation in the next 30 days
Higher than 99% of scored CVEs
EPSS (Exploit Prediction Scoring System) estimates, from observed exploitation activity, how likely a CVE is to be exploited in the next 30 days. It complements CVSS, which measures severity, not likelihood. Score of 2026-10-16, recomputed daily by FIRST.org.
In-depth analysis on cveo.tech
We've published a detailed analysis of this CVE — exploitation, IOCs and mitigation.
CVSS v3.1 Score
Information
- Published
- 3 mars 2021
- Updated
- 19 août 2026
- Status
- Analyzed
- Source
- secure@microsoft.com
Affected products
Weaknesses (CWE)
References (7)
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855PatchVendor Advisory
- http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.htmlExploitThird Party AdvisoryVDB Entry
- http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.htmlExploitThird Party AdvisoryVDB Entry
- http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.htmlExploitThird Party AdvisoryVDB Entry
- http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.htmlExploitThird Party AdvisoryVDB Entry
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855US Government Resource
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