Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability (CVE-2023-21529)
Severity: high · Patch status: patched · Since 2026-04-13 · CVE-2023-21529
Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution. CISA added CVE-2023-21529 to its Known Exploited Vulnerabilities catalog on Apr 13, 2026, which means attackers are exploiting it in the wild. CISA reports known use in ransomware campaigns.
What to do
- Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- US federal agencies must remediate by Apr 27, 2026 (BOD 22-01); treat that as the outer limit.
Sources
All live incidents on Threat Watch
