Actively Exploited — CISA KEV

CVE-2023-21529: Microsoft Exchange Server High-Severity Vulnerability

Severity: High | CVSS 3.1: 8.8 | Published: 2023-02-14 | KEV Added: 2026-04-13

What is CVE-2023-21529?

Microsoft Exchange Server Remote Code Execution Vulnerability

Technical Analysis

Weakness Classification

This vulnerability is classified as CWE-502 — Deserialization of Untrusted Data. The application deserializes untrusted data without sufficient validation, which can lead to remote code execution, denial of service, or other attacks.

Attack Vector Breakdown

  • Attack Vector: Network
  • Attack Complexity: Low
  • Privileges Required: Low
  • User Interaction: None
  • Scope: Unchanged
  • Confidentiality: High
  • Integrity: High
  • Availability: High

Impact Analysis

This vulnerability is exploitable over the network, meaning any system reachable via the internet or internal network is potentially at risk without physical access. The attack complexity is low, meaning exploitation does not depend on specialized conditions or race windows. Successful exploitation can result in full confidentiality breach (complete data disclosure), complete integrity compromise (data modification or code execution), total availability loss (denial of service). The application deserializes untrusted data without sufficient validation, which can lead to remote code execution, denial of service, or other attacks.

Exploitation Probability (EPSS): 27.0% chance of exploitation in the next 30 days (96th percentile). This is a significantly elevated exploitation probability — prioritize patching.

Is CVE-2023-21529 being exploited?

Yes. CVE-2023-21529 is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, restricted to CVEs with confirmed active exploitation in the wild. CISA added this vulnerability on 2026-04-13. Inclusion in KEV triggers the patching mandate under BOD 22-01 for U.S. federal civilian agencies, with a remediation deadline of 2026-04-27.

How to fix CVE-2023-21529

Apply the latest security update via Windows Update, WSUS, or Microsoft Update Catalog. For enterprise environments, use SCCM/Intune for managed deployment. If the patch cannot be applied immediately, check the MSRC advisory for workarounds such as registry-based mitigations or disabling affected features. Check the Microsoft Security Response Center page for the specific patch or update. CISA's Known Exploited Vulnerabilities catalog requires U.S. federal agencies to remediate this vulnerability by the due date shown below under Binding Operational Directive 22-01. Private organizations should treat KEV entries as priority-one patches because active exploitation has been confirmed in the wild.

Related Vulnerabilities

Other actively exploited vulnerabilities affecting Exchange Server or sharing the same weakness (CWE-502):

  • CVE-2026-20963 (Critical 9.8) [same vendor + same CWE] — Deserialization of untrusted data in Microsoft Office SharePoint allows an…
  • CVE-2026-45247 (Critical 9.8) [same CWE] — Mirasvit Full Page Cache Warmer for Magento 2 before version 1.11.12 contains…
  • CVE-2008-4250 (Critical 9.8) [same vendor] — The Server service in Microsoft Windows 2000 SP4, XP SP2 and SP3, Server 2003…
  • CVE-2009-1537 (High 8.8) [same vendor] — Unspecified vulnerability in the QuickTime Movie Parser Filter in quartz.dll…
  • CVE-2010-0249 (High 8.8) [same vendor] — Use-after-free vulnerability in Microsoft Internet Explorer 6, 6 SP1, 7, and 8…

Frequently asked questions

Is CVE-2023-21529 critical?

CVE-2023-21529 has a CVSS 3.1 base score of 8.8 — High severity. It should be treated as a priority patch, especially given the active exploitation confirmed by CISA.

Is CVE-2023-21529 being actively exploited?

Yes — CISA added CVE-2023-21529 to the Known Exploited Vulnerabilities catalog on 2026-04-13. Inclusion in KEV means CISA has confirmed in-the-wild exploitation.

How do I patch CVE-2023-21529?

Apply the security update from Microsoft for Exchange Server. Federal agencies must remediate by 2026-04-27 under BOD 22-01. Refer to the Microsoft Security Response Center for the official patch.

What type of vulnerability is CVE-2023-21529?

CVE-2023-21529 is classified under CWE-502 (Deserialization of Untrusted Data). The application deserializes untrusted data without sufficient validation, which can lead to remote code execution, denial of service, or other attacks.

What is the CVSS score for CVE-2023-21529?

CVE-2023-21529 has a CVSS 3.1 base score of 8.8, classifying it as High severity. The attack vector is Network, attack complexity is Low, and non-zero privileges are required.