How severe is CVE-2021-42321?

CVE-2021-42321 has a CVSS score of 8.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-42321?

Affected package information for CVE-2021-42321 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-42321?

No fixed version has been published for CVE-2021-42321 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-2021-42321 actively exploited in the wild?

Yes. There are 11 known exploit references for CVE-2021-42321, including 9 in-the-wild exploitations 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-2021-42321?

CVE-2021-42321 has an EPSS (Exploit Prediction Scoring System) score of 93.6%, placing it in the 100th 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. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

Is CVE-2021-42321 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-42321 (Microsoft Exchange Server Remote Code Execution Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2021-11-17. CISA required all US federal civilian agencies to remediate this vulnerability by 2021-12-01. The required action was: Apply updates per vendor instructions. 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.

Which products and versions are affected by CVE-2021-42321?

CVE-2021-42321 affects microsoft exchange server (2 versions, e.g. 2016, 2019). Check the affected-products list on this page against your deployed version to confirm exposure.

When was CVE-2021-42321 published, and has it been updated?

CVE-2021-42321 was published on November 10, 2021 and was last updated on October 30, 2025. 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 2021-11-17 — agencies required to remediate by 2021-12-01 · Ransomware

CVE-2021-42321

HIGH

Microsoft Exchange Server Remote Code Execution Vulnerability

Published

Nov 10, 2021

Updated

Oct 30, 2025

Affected

0 pkgs

Patched

None yet

Exploits

11 known

EPSS Exploitation Probability

via FIRST.org ↗

93.6%probability of exploitation in next 30 days

Very High Risk100th percentile0.00%

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

2 versions

20162019

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

msrc.microsoft.comGoogle-sheetNov 2021

Microsoft Exchange Server Remote Code Execution Vulnerability

portal.msrc.microsoft.comMSNov 2021

Microsoft Exchange Server Remote Code Execution Vulnerability

cisa.govCISANov 2021

Microsoft Exchange Server Remote Code Execution Vulnerability

github.comGithubNov 2021

Microsoft Exchange Server Remote Code Execution Vulnerability

github.comMetasploitFeb 2022

Microsoft Exchange Server Remote Code Execution Vulnerability

packetstormsecurity.comNVDMar 2022

Microsoft Exchange Server Remote Code Execution Vulnerability

github.comMetasploitAug 2022

Microsoft Exchange Server Remote Code Execution Vulnerability

packetstormsecurity.comNVDAug 2022

Microsoft Exchange Server Remote Code Execution Vulnerability

github.comGithubOct 2022

Frequently Asked Questions

Microsoft Exchange Server Remote Code Execution Vulnerability

O3 Security · Impact-Aware SCA

Is CVE-2021-42321 in your stack?

O3 detects CVE-2021-42321 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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