How severe is CVE-2021-26855?

CVE-2021-26855 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2021-26855 being actively exploited?

Yes. CVE-2021-26855 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2021-11-03, which means active exploitation has been confirmed. It should be prioritised for remediation.

What products are affected by CVE-2021-26855?

CVE-2021-26855 primarily affects Microsoft Exchange Server. In total, 24 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-26855?

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-2021-26855 have an EU (EUVD) identifier?

Yes. CVE-2021-26855 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-13639. It is also flagged as exploited in the EUVD (since 2021-11-03).

When was CVE-2021-26855 published?

CVE-2021-26855 was published on 2021-03-03 and last updated on 2026-08-19.

CVE-2021-26855

CVE-2021-26855 is a critical-severity vulnerability in Microsoft Exchange Server with a CVSS 3.x base score of 9.1. It is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming it has been exploited in the wild (added 2021-11-03). The underlying weakness is classified as CWE-918.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
CVSS v2: 7.5
EPSS exploit prediction: 100% (100th percentile)
Actively exploited: Yes — listed in CISA KEV (added 2021-11-03)
EU (EUVD) id: EUVD-2021-13639
EU exploitation: Flagged exploited in the ENISA EU Vulnerability Database (since 2021-11-03)
Weakness: CWE-918
Affected product: Microsoft Exchange Server
Published: 2021-03-03
Last modified: 2026-08-19

Description

Microsoft Exchange Server Remote Code Execution Vulnerability

Frequently asked questions

What is CVE-2021-26855?: Microsoft Exchange Server Remote Code Execution Vulnerability
How severe is CVE-2021-26855?: CVE-2021-26855 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2021-26855 being actively exploited?: Yes. CVE-2021-26855 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2021-11-03, which means active exploitation has been confirmed. It should be prioritised for remediation.
What products are affected by CVE-2021-26855?: CVE-2021-26855 primarily affects Microsoft Exchange Server. In total, 24 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-26855?: 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-2021-26855 have an EU (EUVD) identifier?: Yes. CVE-2021-26855 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-13639. It is also flagged as exploited in the EUVD (since 2021-11-03).
When was CVE-2021-26855 published?: CVE-2021-26855 was published on 2021-03-03 and last updated on 2026-08-19.

References

Affected products (24)

cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_21:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_22:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_23:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_10:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_11:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_12:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_13:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_14:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_15:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_16:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_17:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_18:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_19:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_8:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_9:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:-:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_1:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_2:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_3:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_4:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_5:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_6:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_7:*:*:*:*:*:*
cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8:*:*:*:*:*:*

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Threat intelligence

Threat-intel indicators referencing this CVE:

85.50.130.150 (ipv4-addr)