CVE-2021-27065: Microsoft Exchange Arbitrary File Write (ProxyLogon)
Summary
Microsoft Exchange Server allows arbitrary file write via crafted requests enabling web shell deployment (ProxyLogon chain).
Why It Matters
HIGH severity — known to be exploited in the wild（CISA KEV）；action required: Apply updates per vendor instructions. ；affects Microsoft Exchange Server 2013, Exchange Server 2016, Exchange Server 2019.
Current Status
- Vulnerability status: Known exploited (CISA KEV)
- Last intelligence update: 2026-10-27 · source: NVD
- CISA KEV added: 2021-11-03 · remediation deadline: 2022-05-03
Entity Activity Timeline
- 2021-11-03 — Added to CISA KEV as known-exploited · action: Apply updates per vendor instructions.
- 2021-03-02 — NVD publication / disclosure
- 2026-10-27 — Intelligence record updated（NVD sync）
Severity
HIGH
CVSS Score: 7.8
AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
Known Exploited (CISA KEV)
Known Exploited Vulnerability
Added: 2021-11-03 · Remediation deadline: 2022-05-03
Apply updates per vendor instructions.
Source Attribution
Source: NVD · Updated: 2026-10-27T00:00:00.000 · Confidence: high
Impact
Based on CVSS vector: AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H — assess confidentiality, integrity, and availability impact via the official vector documentation.
Affected Software
- Microsoft Exchange Server 2013 (Unknown)
- Exchange Server 2016 (Unknown)
- Exchange Server 2019 (Unknown)
Fix
Upgrade affected software to the fixed version identified above. Apply vendor patches and monitor advisories before exposed systems go unpatched.
Sources & Evidence
Export threat intelligence
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