CVE-2023-21529
Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability
Microsoft Exchange Server
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 13 April 2026. US federal agencies were required to remediate it by 27 April 2026. CISA records this vulnerability as known to be used in ransomware campaigns.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 62.1%, which is higher than 99.1% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
Frequently asked questions
Is CVE-2023-21529 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2023-21529.
What does CVE-2023-21529 affect?
CVE-2023-21529 affects Microsoft Exchange Server.
How severe is CVE-2023-21529?
EPSS puts the probability of exploitation in the next 30 days at 62.1%, higher than 99.1% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 27 April 2026.
How should organisations respond to CVE-2023-21529?
Apply the vendor patch for Microsoft Exchange Server as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.