CVE-2021-34473: Microsoft Exchange Server Remote Code Execution Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 3 Nov 2021, 112 days after the CVE record was published.

How likely is exploitation in the next 30 days?

100%

EPSS score, percentile 100 of all scored records

How severe?

9.1 Critical

CVSS v3.1 base score, given by the CNA

What should I do?

CISA deadline 17 Nov 2021. Apply updates per vendor instructions.

Advisories and patches (1)

Which versions are affected?

Affected

• from 15.00.0 to before 15.00.1497.015
• from 15.01.0 to before 15.01.2176.012
• from 15.01.0 to before 15.01.2242.008
• from 15.02.0 to before 15.02.0792.013
• from 15.02.0 to before 15.02.0858.010

Versions by product

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

CISA due date for this action: 17 November 2021.

Advisories and patches

• Microsoft Exchange Server Remote Code Execution Vulnerability (Vendor advisory, Patch)

Linked from CISA's catalog entry

• https://nvd.nist.gov/vuln/detail/CVE-2021-34473

Affected products and versions

As listed in the CVE record.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

• CISA added CVE-2021-34473 to its Known Exploited Vulnerabilities (KEV) catalog on 3 Nov 2021.
• CISA marks it as known to be used in ransomware campaigns.
• The due date CISA sets for the required action is 17 Nov 2021.
• The CVE record was published on 14 Jul 2021, 112 days before it was added to KEV.
• The CVE record carries a CVSS v3.1 base score of 9.1 (Critical), given by its CNA.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 100% (percentile 100), as of 28 Oct 2026.
• CISA's SSVC assessment in the CVE record: exploitation active, automatable yes, 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-918: Server-Side Request Forgery (SSRF)

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

More exploited Microsoft vulnerabilities

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• CVE-2026-65660: Microsoft SharePoint Code Injection Vulnerability (added 25 Sep 2026)
• CVE-2026-81963: Microsoft Windows Link Following Vulnerability (added 8 Sep 2026)
• CVE-2026-85880: Microsoft Windows Heap-Based Buffer Overflow Vulnerability (added 8 Sep 2026)
• CVE-2019-1068: Microsoft SQL Server Remote Code Execution Vulnerability (added 26 Aug 2026)
• CVE-2026-33824: Microsoft Internet Key Exchange (IKE) Service Extensions Double Free Vulnerability (added 18 Aug 2026)