- Is it exploited?
Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 3 Nov 2021, 246 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
- Which versions are affected?
Affected: 15.00.0 up to, not including, 15.00.1395.012; 15.00.0 up to, not including, 15.00.1473.006; 15.00.0 up to, not including, 15.00.1497.012; 15.01.0 up to, not including, 15.01.1531.012; 15.01.0 up to, not including, 15.01.1591.018; 15.01.0 up to, not including, 15.01.1713.010; 15.01.0 up to, not including, 15.01.1779.008; 15.01.0 up to, not including, 15.01.1847.012; 15.01.0 up to, not including, 15.01.1913.012; 15.01.0 up to, not including, 15.01.1979.008; 15.01.0 up to, not including, 15.01.2044.013; 15.01.0 up to, not including, 15.01.2106.013; 15.01.0 up to, not including, 15.01.2176.009; 15.01.0 up to, not including, 15.01.1415.010; 15.01.0 up to, not including, 15.01.1466.016; 15.02.0 up to, not including, 15.02.0221.018; 15.02.0 up to, not including, 15.01.1979.006; 15.02.0 up to, not including, 15.02.0397.011; 15.02.0 up to, not including, 15.02.0464.015; 15.02.0 up to, not including, 15.02.0659.012
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
Required action, set by CISA
Apply updates per vendor instructions.
CISA due date for this action: 3 May 2022.
Advisories and patches
Linked from CISA's catalog entry
Affected products and versions
As listed in the CVE record.
Microsoft Microsoft Exchange Server 2013 Cumulative Update 21| Status | Versions |
|---|
| Affected | 15.00.0 up to, not including, 15.00.1395.012 |
Microsoft Microsoft Exchange Server 2013 Cumulative Update 22| Status | Versions |
|---|
| Affected | 15.00.0 up to, not including, 15.00.1473.006 |
Microsoft Microsoft Exchange Server 2013 Cumulative Update 23| Status | Versions |
|---|
| Affected | 15.00.0 up to, not including, 15.00.1497.012 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 10| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1531.012 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 11| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1591.018 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 12| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1713.010 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 13| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1779.008 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 14| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1847.012 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 15| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1913.012 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 16| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1979.008 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 17| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.2044.013 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 18| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.2106.013 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 19| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.2176.009 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 8| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1415.010 |
Microsoft Microsoft Exchange Server 2016 Cumulative Update 9| Status | Versions |
|---|
| Affected | 15.01.0 up to, not including, 15.01.1466.016 |
Microsoft Microsoft Exchange Server 2019| Status | Versions |
|---|
| Affected | 15.02.0 up to, not including, 15.02.0221.018 |
Microsoft Microsoft Exchange Server 2019 Cumulative Update 1| Status | Versions |
|---|
| Affected | 15.02.0 up to, not including, 15.01.1979.006 |
Microsoft Microsoft Exchange Server 2019 Cumulative Update 2| Status | Versions |
|---|
| Affected | 15.02.0 up to, not including, 15.02.0397.011 |
Microsoft Microsoft Exchange Server 2019 Cumulative Update 3| Status | Versions |
|---|
| Affected | 15.02.0 up to, not including, 15.02.0464.015 |
Microsoft Microsoft Exchange Server 2019 Cumulative Update 4| Status | Versions |
|---|
| Affected | 15.02.0 up to, not including, 15.02.0659.012 |
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. This vulnerability is part of the ProxyLogon exploit chain.
Summary: CISA's KEV catalog, as of 28 Oct 2026.
- CISA added CVE-2021-26855 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 3 May 2022.
- The CVE record was published on 2 Mar 2021, 246 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
| Date | Event |
|---|
| 2 Mar 2021 | CVE record published |
| 3 Nov 2021 | Added to CISA KEV |
| 3 May 2022 | CISA due date for the required action |
| 19 Aug 2026 | CVE record last updated |
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