CVE-2023-21529
Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability
Does this matter?
Known to be exploited in the wild (CISA KEV, CISA remediation deadline 27 April 2026). Treat as an emergency change: patch or isolate now, then hunt for prior compromise.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
- CVSS 3.1
- 8.8 HIGHCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- EPSS
- 59.29% probability · 99th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Listed 13 April 2026 · due 27 April 2026 · used in ransomware campaigns
- Weakness
- CWE-502
- Affected
- microsoft/exchange server
- Source
- secure@microsoft.com
EPSS moved from 59.3% on 24 September 2026 to 59.3% on 30 October 2026. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”.
CISA notes
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529 ; https://nvd.nist.gov/vuln/detail/CVE-2023-21529
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529Patch, Vendor Advisory
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529Patch, Vendor Advisory
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-21529US Government Resource
- 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/Technical Description
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.