Known exploited vulnerability
CVE-2023-21529
The short answer
CVE-2023-21529 affects Microsoft Microsoft Exchange Server 2013 Cumulative Update 23 and is rated high (CVSS 8.8). CISA added it to the Known Exploited Vulnerabilities catalog on 2026-04-13: attackers are using it, so patch it first. The vendor has linked a patch.
What it is
Microsoft Exchange Server Remote Code Execution Vulnerability
Weakness: CWE-502.
Severity
8.8 high
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Exploited (CISA KEV)
Yes, since Apr 13, 2026
Federal due date Apr 27, 2026
EPSS, next 30 days
59%
Higher than 99% of scored CVEs
How it can be exploited
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- High
- Integrity impact
- High
- Availability impact
- High
What CISA says 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.
Affected products
Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Exchange Server 2016 Cumulative Update 23, Microsoft Exchange Server 2019 Cumulative Update 11, Microsoft Exchange Server 2019 Cumulative Update 12. NVD's record lists the exact affected versions.
References
- msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529 PatchVendor Advisory
- msrc.microsoft.com/update-guide/vulnerability/CVE-2023-21529 PatchVendor Advisory
- www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-21529 US Government Resource
- www.microsoft.com/en-us/security/blog/2026/04/06/storm-1175-focuses-gaze-on-vulnerable-web-facing-assets-in-high-tempo-medusa-ransomware-operations/ Technical Description
Questions
Is CVE-2023-21529 being exploited?
Yes. CISA added CVE-2023-21529 to its Known Exploited Vulnerabilities catalog on 2026-04-13, with a federal remediation due date of 2026-04-27. CISA lists it as used in ransomware campaigns.
How do I fix CVE-2023-21529?
CISA's 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.
From NVD, CISA KEV, FIRST EPSS and OSV.dev, checked . Status on NVD: Analyzed. The description is NVD's text, quoted as published.
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