CVE-2023-21529: in the CISA KEV catalogue
Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability. CISA added CVE-2023-21529 to the Known Exploited Vulnerabilities catalogue on 2026-04-13 with a default due date of 2026-04-27. Known ransomware campaign use: Known. Forensic triage flag: No.
Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution. (CISA description)
- Vendor / product
- Microsoft Exchange Server
- CWE
- CWE-502
- CISA 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.
- EPSS
- 59%, percentile 99 (scored 2026-11-03)
EPSS puts about 59 in 100 odds (59%) on exploitation activity being observed in the next 30 days. Its score is higher than or equal to that of 99% of scored CVEs. This is a model estimate about exploitation anywhere, not about your environment.
Vendor and reference links (from CISA)
Your own deadline under CISA BOD 26-04 depends on exposure, automatability and technical impact: work it out with the BOD 26-04 calculator.
Frequently asked questions
Is CVE-2023-21529 in the CISA KEV catalogue?
Yes. CISA added CVE-2023-21529 (Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability) to the Known Exploited Vulnerabilities catalogue on 2026-04-13. CISA's default due date for it is 2026-04-27.
Is CVE-2023-21529 used in ransomware attacks?
CISA marks known ransomware campaign use as Known for CVE-2023-21529.
What should I do about CVE-2023-21529?
CISA's 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.
Sources
- CISA, Known Exploited Vulnerabilities Catalog (accessed 2026-11-03)
- FIRST, Exploit Prediction Scoring System (EPSS) (EPSS by FIRST; scores generated by Empirical Security)
- NVD record for CVE-2023-21529