CVE-2021-42321
CVE-2021-42321 is a high-severity vulnerability in Microsoft Exchange Server with a CVSS 3.x base score of 8.8. It is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming it has been exploited in the wild (added 2021-11-17).
Key facts
- Severity: High (CVSS 3.x base score 8.8)
- CVSS v2: 6.5
- EPSS exploit prediction: 92% (100th percentile)
- Actively exploited: Yes — listed in CISA KEV (added 2021-11-17)
- EU (EUVD) id: EUVD-2021-29296
- EU exploitation: Flagged exploited in the ENISA EU Vulnerability Database (since 2021-11-17)
- Affected product: Microsoft Exchange Server
- Published:
- Last modified:
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Frequently asked questions
- What is CVE-2021-42321?
- Microsoft Exchange Server Remote Code Execution Vulnerability
- How severe is CVE-2021-42321?
- CVE-2021-42321 has a CVSS 3.x base score of 8.8, rated high severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
- Is CVE-2021-42321 being actively exploited?
- Yes. CVE-2021-42321 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2021-11-17, which means active exploitation has been confirmed. It should be prioritised for remediation.
- What products are affected by CVE-2021-42321?
- CVE-2021-42321 primarily affects Microsoft Exchange Server. In total, 4 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
- How do I fix CVE-2021-42321?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.
- Does CVE-2021-42321 have an EU (EUVD) identifier?
- Yes. CVE-2021-42321 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-29296. It is also flagged as exploited in the EUVD (since 2021-11-17).
- When was CVE-2021-42321 published?
- CVE-2021-42321 was published on 2021-11-10 and last updated on 2026-08-19.
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-42321
- http://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html
- http://packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Execution.html
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42321
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42321
Affected products (4)
- cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_21:*:*:*:*:*:*
- cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_22:*:*:*:*:*:*
- cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_10:*:*:*:*:*:*
- cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_11:*:*:*:*:*:*
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