CVE-2023-21529
Published 2023-02-14 · Updated 2026-08-19 · Source secure@microsoft.com
Summary
CVE-2023-21529 is a high-severity (CVSS 8.8) vulnerability in Microsoft Exchange Server. CISA added it to the Known Exploited Vulnerabilities catalog on April 13, 2026, which means exploitation in the wild has been confirmed; US federal agencies had to remediate it by April 27, 2026. CISA also lists it as known to be used in ransomware campaigns, so it should be prioritised accordingly. Microsoft has 390 vulnerabilities in the KEV catalog, 117 of them linked to ransomware.
All 390 known-exploited Microsoft vulnerabilities →Description
Microsoft Exchange Server Remote Code Execution Vulnerability
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CISA Known-Exploited Vulnerability
Product: Microsoft — Exchange Server
Name: Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability
Date added: 2026-04-13 · Due: 2026-04-27
Required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-21529
- https://www.microsoft.com/en-us/security/blog/2026/04/06/storm-1175-focuses-gaze-on-vulnerable-web-facing-assets-in-high-tempo-medusa-ransomware-operations/