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CVE-2021-26858: Microsoft Exchange Server Remote Code Execution Vulnerability

criticalknown exploitedpublic exploitCVE-2021-26858
Actively exploited. At least one CVE in this advisory is listed in the CISA Known Exploited Vulnerabilities catalog — exploitation has been observed in the wild. Treat remediation as urgent.
Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.

CSIRTS triage

vendor: Microsoftproduct: Exchange ServerRemote code execution
What
Exchange Server contains a vulnerability that allows for remote code execution.
Who is affected
Deployments of Microsoft Exchange Server.
Urgency
Remediation is critical as this vulnerability is part of the ProxyLogon exploit chain and is actively exploited.
Action
Apply the latest security patches for Exchange Server.

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Details

Source
CISA Known Exploited Vulnerabilities (US · database · site)
Severity
critical
Published
2021-11-03
Exploitation
Observed in the wild (CISA KEV)

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2021-26858

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Referenced CVEs

CVECSIRTS overviewExternal
CVE-2021-26858coverage & exploitation statusNVD · CVE.org

Recent advisories for Microsoft Exchange Server

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

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