CVE-2023-21529
Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability
Microsoft Exchange Server
Microsoft Exchange Server Remote Code Execution Vulnerability
Ransomware groups exploiting CVE-2023-21529
Rhysida is a ransomware-as-a-service (RAAS) group that emerged in May 2023. The group utilizes a namesake ransomware through phishing attacks and Cobalt Strike to breach the targets' networks and deploy their payloads. The group threatens to publicly distribut
Space Bears is a double-extortion ransomware group that emerged in April 2024, distinguished by a professional "corporate" aesthetic on its leak site, leveraging Phobos RaaS infrastructure and targeting small-to-medium organizations in manufacturing, technolog
Dire Wolf is a sophisticated human-operated ransomware group first documented in May 2025, written in Golang using Curve25519/ChaCha20 encryption, targeting manufacturing and technology sectors across 13+ countries with ransoms up to $500,000, operated by a ti
Exploitation is attributed to these groups across their recorded activity. It does not follow that every organisation they listed was reached through CVE-2023-21529 — the source data does not record an entry point per incident.
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 59.3%, 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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Recent listings by these groups
Disclosures from the groups above, newest first.
Frequently asked questions
Is CVE-2023-21529 being exploited by ransomware groups?
Yes. CVE-2023-21529 is recorded as exploited by Rhysida, Spacebears, Direwolf, Panzer, Emperador. Between them these groups account for 571 victim disclosures in this database. CISA also records this vulnerability as used in ransomware campaigns.
What does CVE-2023-21529 affect?
CVE-2023-21529 affects Microsoft Exchange Server. Microsoft Exchange Server Remote Code Execution Vulnerability
How severe is CVE-2023-21529?
CVE-2023-21529 is rated HIGH with a CVSS base score of 8.8. EPSS puts the probability of exploitation in the next 30 days at 59.3%, 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.