CVE record · CVE-2021-26857

Microsoft Exchange Server Remote Code Execution Vulnerability

CVSS 7.8 HighCriticalIn CISA KEV · ransomwareEPSS 0.96 · 99th percentile

Critical, because:

In CISA’s KEV catalogue since 3 Nov 2021: it is known to be exploited.
CVSS score 7.8 (7.0 or more) and EPSS 0.96 (0.1 or more): a high score and a real chance of exploitation in the next 30 days.

Published by the CVE Program (CNA: microsoft) at 2 Mar 2021, 23:55 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

Affected products, as the CNA wrote them
Vendor	Product	Versions
Microsoft	Microsoft Exchange Server 2010 Service Pack 3	affected 14.0.0.0 to before 14.03.0513.000 custom
Microsoft	Microsoft Exchange Server 2013 Cumulative Update 21	affected 15.00.0 to before 15.00.1395.012 custom
Microsoft	Microsoft Exchange Server 2013 Cumulative Update 22	affected 15.00.0 to before 15.00.1473.006 custom
Microsoft	Microsoft Exchange Server 2013 Cumulative Update 23	affected 15.00.0 to before 15.00.1497.012 custom
Microsoft	Microsoft Exchange Server 2013 Service Pack 1	affected 15.00.0 to before 15.00.0847.064 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 10	affected 15.01.0 to before 15.01.1531.012 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 11	affected 15.01.0 to before 15.01.1591.018 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 12	affected 15.01.0 to before 15.01.1713.010 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 13	affected 15.01.0 to before 15.01.1779.008 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 14	affected 15.01.0 to before 15.01.1847.012 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 15	affected 15.01.0 to before 15.01.1913.012 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 16	affected 15.01.0 to before 15.01.1979.008 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 17	affected 15.01.0 to before 15.01.2044.013 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 18	affected 15.01.0 to before 15.01.2106.013 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 19	affected 15.01.0 to before 15.01.2176.009 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 8	affected 15.01.0 to before 15.01.1415.010 custom
Microsoft	Microsoft Exchange Server 2016 Cumulative Update 9	affected 15.01.0 to before 15.01.1466.016 custom
Microsoft	Microsoft Exchange Server 2019	affected 15.02.0 to before 15.02.0221.018 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 1	affected 15.02.0 to before 15.01.1979.006 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 2	affected 15.02.0 to before 15.02.0397.011 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 3	affected 15.02.0 to before 15.02.0464.015 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 4	affected 15.02.0 to before 15.02.0659.012 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 5	affected 15.02.0 to before 15.02.0595.008 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 6	affected 15.02.0 to before 15.02.0529.013 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 7	affected 15.02.0 to before 15.02.0721.013 custom
Microsoft	Microsoft Exchange Server 2019 Cumulative Update 8	affected 15.02.0 to before 15.02.0792.010 custom

Identifiers: CVE-2021-26857
CNA: microsoft
Weakness (CWE): CWE-502
Tags: Microsoft Exchange Server 2010 Service Pack 3
Microsoft Exchange Server 2013 Cumulative Update 21
Microsoft Exchange Server 2013 Cumulative Update 22
Microsoft Exchange Server 2013 Cumulative Update 23
Microsoft Exchange Server 2013 Service Pack 1
Microsoft Exchange Server 2016 Cumulative Update 10
Microsoft Exchange Server 2016 Cumulative Update 11
Microsoft Exchange Server 2016 Cumulative Update 12
Insecure deserialization
Published: 2 Mar 2021
Updated: 19 Aug 2026

Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
7.8	High	3.1	CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C	CNA (microsoft)

CISA KEV Listed since 3 Nov 2021: known to be exploited. CISA reports use in ransomware campaigns.
EPSS 0.96, the 99th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

This record came from the full history of the CVE Program, published 2 Mar 2021. New records show when each source confirmed them.

Sources

References from the CNA

msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26857 vendor-advisorypatch

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.