Exploitation evidence
Known exploited · CISA KEV listed
Added 2021-11-03. Ransomware campaign use: Known.
CISA required action: Apply updates per vendor instructions.
CISA due date: 2022-05-03. This is the catalog’s deadline, not an assessment of your organization’s exposure.
KEV checked 13 Nov 2026, 00:20 Jerusalem · Original catalog ↗Reported products and versions
Microsoft / Microsoft Exchange Server 2013 Cumulative Update 21
Default status: Not specified
- affected: 15.00.0 to < 15.00.1395.012 (custom)
Microsoft / Microsoft Exchange Server 2013 Cumulative Update 22
Default status: Not specified
- affected: 15.00.0 to < 15.00.1473.006 (custom)
Microsoft / Microsoft Exchange Server 2013 Cumulative Update 23
Default status: Not specified
- affected: 15.00.0 to < 15.00.1497.012 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 10
Default status: Not specified
- affected: 15.01.0 to < 15.01.1531.012 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 11
Default status: Not specified
- affected: 15.01.0 to < 15.01.1591.018 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 12
Default status: Not specified
- affected: 15.01.0 to < 15.01.1713.010 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 13
Default status: Not specified
- affected: 15.01.0 to < 15.01.1779.008 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 14
Default status: Not specified
- affected: 15.01.0 to < 15.01.1847.012 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 15
Default status: Not specified
- affected: 15.01.0 to < 15.01.1913.012 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 16
Default status: Not specified
- affected: 15.01.0 to < 15.01.1979.008 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 17
Default status: Not specified
- affected: 15.01.0 to < 15.01.2044.013 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 18
Default status: Not specified
- affected: 15.01.0 to < 15.01.2106.013 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 19
Default status: Not specified
- affected: 15.01.0 to < 15.01.2176.009 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 8
Default status: Not specified
- affected: 15.01.0 to < 15.01.1415.010 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 9
Default status: Not specified
- affected: 15.01.0 to < 15.01.1466.016 (custom)
Microsoft / Microsoft Exchange Server 2019
Default status: Not specified
- affected: 15.02.0 to < 15.02.0221.018 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 1
Default status: Not specified
- affected: 15.02.0 to < 15.01.1979.006 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 2
Default status: Not specified
- affected: 15.02.0 to < 15.02.0397.011 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 3
Default status: Not specified
- affected: 15.02.0 to < 15.02.0464.015 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 4
Default status: Not specified
- affected: 15.02.0 to < 15.02.0659.012 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 5
Default status: Not specified
- affected: 15.02.0 to < 15.02.0595.008 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 6
Default status: Not specified
- affected: 15.02.0 to < 15.02.0529.013 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 7
Default status: Not specified
- affected: 15.02.0 to < 15.02.0721.013 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 8
Default status: Not specified
- affected: 15.02.0 to < 15.02.0792.010 (custom)
Fixes and workarounds
No explicit solution text in the downloaded CVE record. Consult the vendor advisory; unaffected version ranges are shown above as reported, not inferred as a patch recommendation.
Sources and related reporting
CVE Program record ↗ · Download tracked evidence as JSON
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
- http://packetstormsecurity.com/files/161846/Microsoft-Exchange-2019-SSRF-Arbitrary-File-Write.html
- http://packetstormsecurity.com/files/161938/Microsoft-Exchange-ProxyLogon-Remote-Code-Execution.html
- http://packetstormsecurity.com/files/162610/Microsoft-Exchange-2019-Unauthenticated-Email-Download.html
- http://packetstormsecurity.com/files/162736/Microsoft-Exchange-ProxyLogon-Collector.html
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26855
Observed change timeline
History begins when Egret first downloads each source. It is not the complete upstream CVE history.
12 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-12",
"epss": 0.99996,
"percentile": 0.99988
}10 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-10",
"epss": 0.99996,
"percentile": 0.99989
}09 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-09",
"epss": 0.99996,
"percentile": 0.99989
}08 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-08",
"epss": 0.99996,
"percentile": 0.99989
}07 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-07",
"epss": 0.99996,
"percentile": 0.99989
}06 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-06",
"epss": 0.99996,
"percentile": 0.99989
}06 Nov 2026, 06:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-05",
"epss": 0.99996,
"percentile": 0.99989
}05 Nov 2026, 18:20 Jerusalem · CVE Program record changed
Stored CNA snapshot at this observation.
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"status": "affected",
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},
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"product": "Microsoft Exchange Server 2016 Cumulative Update 14",
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"vendor": "Microsoft",
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"product": "Microsoft Exchange Server 2016 Cumulative Update 18",
"vendor": "Microsoft",
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"assignerOrgId": "f38d906d-7342-40ea-92c1-6c4a2c6478c8",
"assignerShortName": "microsoft",
"cveId": "CVE-2021-26855",
"datePublished": "2021-03-02T23:55:26.000Z",
"dateReserved": "2021-02-08T00:00:00.000Z",
"dateUpdated": "2026-08-19T16:30:15.282Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}04 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-04",
"epss": 0.99996,
"percentile": 0.99989
}03 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-03",
"epss": 0.99996,
"percentile": 0.99988
}03 Nov 2026, 02:22 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-02",
"epss": 0.99996,
"percentile": 0.99988
}03 Nov 2026, 02:22 Jerusalem · CISA KEV status changed
Stored KEV snapshot at this observation.
{
"cveID": "CVE-2021-26855",
"cwes": [
"CWE-918"
],
"dateAdded": "2021-11-03",
"dueDate": "2022-05-03",
"forensicTriage": "No",
"knownRansomwareCampaignUse": "Known",
"notes": "Reference CISA's ED 21-02 (https://www.cisa.gov/news-events/directives/ed-21-02-mitigate-microsoft-exchange-premises-product-vulnerabilities) for further guidance and requirements. Note: The due date for addressing this vulnerability aligns with the requirements outlined in ED 21-02. https://nvd.nist.gov/vuln/detail/CVE-2021-26855",
"product": "Exchange Server",
"requiredAction": "Apply updates per vendor instructions.",
"shortDescription": "Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution. This vulnerability is part of the ProxyLogon exploit chain.",
"vendorProject": "Microsoft",
"vulnerabilityName": "Microsoft Exchange Server Remote Code Execution Vulnerability"
}