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CVE

CVE-2021-26858

Microsoft Exchange Server Remote Code Execution Vulnerability

Microsoft Exchange Server Remote Code Execution Vulnerability.

HIGH · CVSS 7.8 ⚠ CISA KEV EPSS 0.73244 Ransomware: known
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  • Listed on CISA KEV (known exploited in the wild)
  • Linked to known ransomware campaigns
  • SSVC exploitation status: active
  • EPSS ≥ 0.50 - high probability of exploitation in the next 30 days
  • EPSS percentile: top 1% 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

21
actorAPT10
actorAPT31
actorOilRig
actorAPT35
actorAPT40
actorBlackTech
▤

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
relatedcriticalBitbucket Unauthorized Access To A Resource
relatedcriticalAntivirus Exploitation Framework Detection
relatedcriticalAntivirus Password Dumper Detection
relatedcriticalAntivirus Ransomware Detection
relatedcriticalLinux Reverse Shell Indicator
▣

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-2021-26858
msrcCVE-2021-26858Remote Code Execution
elastic-detectionelastic-detection-rules-windows-initial_access_suspicious_ms_exchange_files
🔗

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-2021-26858PatchVendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26858US Government Resource
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