How severe is CVE-2023-21529?

CVE-2023-21529 has a CVSS score of 8.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2023-21529?

Affected package information for CVE-2023-21529 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2023-21529?

No fixed version has been published for CVE-2023-21529 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2023-21529 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2023-21529, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

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

CVE-2023-21529 has an EPSS (Exploit Prediction Scoring System) score of 27.0%, placing it in the 96th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score warrants monitoring and prompt remediation.

Is CVE-2023-21529 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2023-21529 (Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2026-04-13. CISA required all US federal civilian agencies to remediate this vulnerability by 2026-04-27. The required action was: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. This CVE has been used in known ransomware campaigns. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

What type of vulnerability is CVE-2023-21529?

CVE-2023-21529 is classified as Deserialization of Untrusted Data (CWE-502). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

Which products and versions are affected by CVE-2023-21529?

CVE-2023-21529 affects microsoft exchange server (3 versions, e.g. 2013, 2016, 2019). Check the affected-products list on this page against your deployed version to confirm exposure.

When was CVE-2023-21529 published, and has it been updated?

CVE-2023-21529 was published on February 14, 2023 and was last updated on April 14, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2026-04-13 — agencies required to remediate by 2026-04-27 · Ransomware

CVE-2023-21529

HIGH

Microsoft Exchange Server Remote Code Execution Vulnerability

Published

Feb 14, 2023

Updated

Apr 14, 2026

Affected

0 pkgs

Patched

None yet

Exploits

1 known

EPSS Exploitation Probability

via FIRST.org ↗

27.0%probability of exploitation in next 30 days

Moderate Risk96th percentile-0.51%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

Affected Products

1 product · 4 configurations

Application

exchange servermicrosoft

3 versions

201320162019

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Microsoft Exchange Server Remote Code Execution Vulnerability

github.comGithubMar 2024

Frequently Asked Questions

Microsoft Exchange Server Remote Code Execution Vulnerability

O3 Security · Impact-Aware SCA

Is CVE-2023-21529 in your stack?

O3 detects CVE-2023-21529 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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