Risk Letters

Exploited vulnerability

CVE-2023-21529

TLP:CLEAR

Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability

Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.

Facts

  • Vendor: Microsoft
  • Product: Exchange Server
  • CVSS: 8.8
  • EPSS (probability of exploitation within 30 days): 59.3 %
  • On CISA's Known Exploited Vulnerabilities list since: 13 April 2026
  • First known exploitation: 13 April 2026
  • Used in ransomware campaigns (per CISA): Yes
  • CISA's deadline for US federal agencies: 27 April 2026
  • Weakness: CWE-502
  • First seen by Risk Letters: 13 April 2026

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Get the full brief

Data: NVD (public domain), CISA KEV, FIRST EPSS, MITRE ATT&CK, AlienVault OTX. NVD