CVE-2023-21529: Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability

In short

Microsoft Exchange Server has a weakness classified as deserialization of untrusted data (CWE-502). CISA added it to its Known Exploited Vulnerabilities catalog on 2026-04-13. NVD lists CVSS 3.1 at 8.8 HIGH. CISA reports known ransomware campaign use.

What it is

“Microsoft Exchange Server contains a deserialization of untrusted data that allows an authenticated attacker to achieve remote code execution.” — CISA KEV, retrieved 2026-11-01

“Microsoft Exchange Server Remote Code Execution Vulnerability” — NVD, retrieved 2026-11-01

Who is affected

Microsoft Exchange Server. NVD lists the following versions as affected:

• Versions version 2013
• Versions version 2016
• Versions version 2019

Exploitation status

CISA KEV added: 2026-04-13. Known ransomware campaign use: Known (reported by CISA). Due date CISA set for US federal civilian agencies: 2026-04-27.

Scores

NVD CVSS 3.1: 8.8 HIGH; vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H; source [email protected].

EPSS by FIRST: 59.29%; percentile 99.12; score date 2026-10-31. EPSS estimates the probability of exploitation in the next 30 days.

CWE: CWE-502.

Timeline

CISA required action

“Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.” — CISA KEV

What to do

• Confirm whether you run Microsoft Exchange Server and which version.
• Apply the vendor update or mitigation in the advisory.
• Follow CISA’s required action.
• Review the vendor guidance on signs of compromise.
• Track the fix through completion.

References

• Patch, Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529
• NVD detail
• CVE.org record
• CISA KEV catalog

Sources and dates

CISA Known Exploited Vulnerabilities Catalog, version 2026.10.30; retrieved 2026-11-01. CISA KEV data: CC0 1.0. No endorsement by CISA.

NVD retrieved: 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.

EPSS scores by FIRST; score date 2026-10-31.