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CVE

CVE-2021-26857

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability.

HIGH · CVSS 7.8 ⚠ CISA KEV EPSS 0.39406 Ransomware: known
Act now
  • Listed on CISA KEV (known exploited in the wild)
  • Linked to known ransomware campaigns
  • SSVC exploitation status: active
  • EPSS ≥ 0.10 - elevated exploitation probability
  • EPSS percentile: top 3% of all CVEs by exploitation likelihood
  • CVSS base score ≥ 7.0
Sigma rules10 YARA rules1
⬢

Required Remediation

Apply updates per vendor instructions.

◆

Threat Actors Linked

14
actorBlackTech
actorDaggerfly
actorEarth Lusca
actorAPT27
actorGALLIUM
actorGhostEmperor
⬡

Weakness Classification

CWE-502Deserialization of Untrusted Data
CWE-502Deserialization of Untrusted Data
▤

Affected Products & Versions

1
microsoft exchange serverall versions
◈

Sigma Hunt Rules

10
Exact rules name this CVE ID. Product rules name an affected product in their title. Related rules cover techniques used by actors who exploited this CVE. Showing the most relevant matches; the complete related set is on the full drill-down.
relatedcriticalBitbucket Unauthorized Full Data Export Triggered
relatedcriticalAntivirus Exploitation Framework Detection
relatedcriticalAntivirus Password Dumper Detection
relatedcriticalAntivirus Ransomware Detection
relatedcriticalLinux Reverse Shell Indicator
relatedcriticalWebshell Remote Command Execution
▣

Scoring & Timeline

7.8
HIGH · CVSS v3.1 · secure@microsoft.com
View on NVD
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
Published to NVD03 Mar 2021 · 12:15 AM
CVSS VectorCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
SSVC triage · cisa-vulnrichment
Exploitation
active
Automatable
no
Technical impact
total
SSVC asks the questions that actually drive patch urgency: is it being exploited, can attacks be automated, and how total is the impact.
⚑

Vendor Advisories

3
cisaKEV-CVE-2021-26857
msrcCVE-2021-26857Remote Code Execution
elastic-detectionelastic-detection-rules-windows-initial_access_suspicious_ms_exchange_process
🔗

References & Sources

2
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26857PatchVendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26857US Government Resource
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