CVE-2023-21529
Actively exploited. CVE-2023-21529 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-04-13) — exploitation has been observed in the wild, and US federal agencies are required to remediate it under BOD 22-01. Treat patching as urgent.
Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.
CSIRTS triage
- What
- Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.
- Who is affected
- Deployments of Microsoft Exchange Server.
- Urgency
- Immediate action is necessary due to active exploitation and critical severity.
- Action
- Patch Microsoft Exchange Server to the latest version.
AI-assisted analysis generated from the source advisory — verify against the original.
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Exploitation outlook
- Exploitation confirmedAlready exploited in the wild (CISA KEV) — the prediction phase is over. Patch now. Riskier than 99% of all EPSS-scored CVEs.
Advisory coverage (1)
- criticalexploitedCVE-2023-21529: Microsoft Exchange Server Deserialization of Untrusted Data Vulnerabilitycisa-kev · 2026-04-13
External references
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[](https://www.csirts.com/cve/CVE-2023-21529)