CVE-2021-42321: Microsoft Exchange Server Remote Code Execution Vulnerability
- Is it exploited?
Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 17 Nov 2021, 7 days after the CVE record was published.
- How likely is exploitation in the next 30 days?
- 91.7%
EPSS score, percentile 99.8 of all scored records
- How severe?
- 8.8 High
CVSS v3.1 base score, given by the CNA
- What should I do?
CISA deadline 1 Dec 2021. Apply updates per vendor instructions.
- Which versions are affected?
Affected
- from 15.01.0 to before 15.01.2308.020
- from 15.0.0 to before 15.01.2375.017
- from 15.02.0 to before 15.02.0792.019
- from 15.02.0 to before 15.02.0986.014
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.
What to do
Required action, set by CISA
Apply updates per vendor instructions.
CISA due date for this action: 1 December 2021.
Advisories and patches
Linked from CISA's catalog entry
Affected products and versions
As listed in the CVE record.
| Status | Versions |
|---|---|
| Affected | from 15.01.0 to before 15.01.2308.020 |
| Status | Versions |
|---|---|
| Affected | from 15.0.0 to before 15.01.2375.017 |
| Status | Versions |
|---|---|
| Affected | from 15.02.0 to before 15.02.0792.019 |
| Status | Versions |
|---|---|
| Affected | from 15.02.0 to before 15.02.0986.014 |
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.
About this vulnerability
An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.
Summary: CISA's KEV catalog, as of 28 Oct 2026.
- CISA added CVE-2021-42321 to its Known Exploited Vulnerabilities (KEV) catalog on 17 Nov 2021.
- CISA marks it as known to be used in ransomware campaigns.
- The due date CISA sets for the required action is 1 Dec 2021.
- The CVE record was published on 10 Nov 2021, 7 days before it was added to KEV.
- The CVE record carries a CVSS v3.1 base score of 8.8 (High), given by its CNA.
- FIRST's EPSS model puts the probability of exploitation in the next 30 days at 91.7% (percentile 99.8), as of 28 Oct 2026.
- CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.
Description
Microsoft Exchange Server Remote Code Execution Vulnerability
Microsoft Exchange Server Remote Code Execution Vulnerability
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.
Weakness
CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.
Timeline
| Date | Event |
|---|---|
| 10 Nov 2021 | CVE record published |
| 17 Nov 2021 | Added to CISA KEV |
| 1 Dec 2021 | CISA due date for the required action |
| 19 Aug 2026 | CVE record last updated |
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