[KEV] Unsafe Deserialization in Microsoft Exchange Server (CVE-2021-26857)
Summary
Vulnerability in Microsoft Exchange Server (CVE-2021-26857). Successful exploitation can lead to full system takeover. Listed in CISA KEV — actively exploited.
Original description
The description as published by the source.
Microsoft Exchange Server Remote Code Execution Vulnerability This CVE ID is unique from CVE-2021-26412, CVE-2021-26854, CVE-2021-26855, CVE-2021-26858, CVE-2021-27065, CVE-2021-27078.
Source: CISA Known Exploited Vulnerabilities NVD GitHub Advisory CISA KEV
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2021-26857 has been found in Microsoft Exchange Server.
Successful exploitation can lead to full system takeover. CVSS score: 7.8/10.
Note: CISA has officially confirmed this is actively exploited in the wild. Treat it with elevated urgency.
What to do: apply the vendor's official patched release.
If unsure, ask your IT team or search "Microsoft Exchange Server CVE-2021-26857" on the vendor's site.
CVE-2021-26857 (Microsoft Exchange Server) — CWE-502 / CVSS v3 7.8
Attack vector: local / unauthenticated
Listed in CISA KEV — actively exploited, treat as top priority.
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
Unsafe deserialization (CWE-502) exists in Microsoft Exchange Server.
Check the vendor advisory for the affected versions of Microsoft Exchange Server.
Severity: High (CVSS 7.8/10). Successful exploitation can lead to full system takeover. Listed in CISA KEV — actively exploited in the wild, treat as top priority.
Update to the patched release per vendor advisory.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
Audit SBOM/dependencies for affected components.資産台帳・SBOM・依存マニフェストで Microsoft Exchange Server を使っているサービスとバージョンを洗い出す。
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2Consider incident declaration escalate
Notify SOC / on-callCISA KEV登録済 = 実環境で悪用が観測されている。侵入の兆候があればインシデント対応宣言、無くてもパッチ適用までWAF強化を最優先で。
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3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
References
- advisory NVD
- patch portal.msrc.microsoft.com
- patch secure@microsoft.com
- web cisa.gov
- web github.com