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: 2021-11-17. 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 23
Default status: Not specified
- affected: 15.00.0 to < 15.00.1497.015 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 19
Default status: Not specified
- affected: 15.01.0 to < 15.01.2176.012 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 20
Default status: Not specified
- affected: 15.01.0 to < 15.01.2242.008 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 8
Default status: Not specified
- affected: 15.02.0 to < 15.02.0792.013 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 9
Default status: Not specified
- affected: 15.02.0 to < 15.02.0858.010 (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-2021-34473
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34473
- https://www.zerodayinitiative.com/advisories/ZDI-21-821/
- http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-34473
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": 0.99999,
"percentile": 0.99996
}10 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-10",
"epss": 0.99999,
"percentile": 0.99996
}09 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-09",
"epss": 0.99999,
"percentile": 0.99996
}08 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-08",
"epss": 0.99999,
"percentile": 0.99996
}07 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-07",
"epss": 0.99999,
"percentile": 0.99996
}06 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-06",
"epss": 0.99999,
"percentile": 0.99996
}06 Nov 2026, 06:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-05",
"epss": 0.99999,
"percentile": 0.99996
}05 Nov 2026, 12:20 Jerusalem · CVE Program record changed
Stored CNA snapshot at this observation.
{
"containers": {
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},
{
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],
"url": "https://www.zerodayinitiative.com/advisories/ZDI-21-821/"
},
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],
"url": "http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html"
}
],
"title": "CVE Program Container"
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{
"metrics": [
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"id": "CVE-2021-34473",
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{
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},
{
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"timestamp": "2025-02-04T15:42:23.926450Z",
"version": "2.0.3"
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{
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"references": [
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],
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],
"cna": {
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"product": "Microsoft Exchange Server 2016 Cumulative Update 19",
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{
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}
]
}
],
"cpeApplicability": [
{
"nodes": [
{
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"versionEndExcluding": "15.00.1497.015",
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"vulnerable": true
},
{
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],
"metrics": [
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"vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C",
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],
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},
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"datePublished": "2021-07-14T17:54:03.000Z",
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"state": "PUBLISHED"
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}04 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-04",
"epss": 0.99999,
"percentile": 0.99996
}03 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-03",
"epss": 0.99999,
"percentile": 0.99996
}03 Nov 2026, 02:22 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-02",
"epss": 0.99999,
"percentile": 0.99996
}03 Nov 2026, 02:22 Jerusalem · CISA KEV status changed
Stored KEV snapshot at this observation.
{
"cveID": "CVE-2021-34473",
"cwes": [
"CWE-918"
],
"dateAdded": "2021-11-03",
"dueDate": "2021-11-17",
"forensicTriage": "No",
"knownRansomwareCampaignUse": "Known",
"notes": "https://nvd.nist.gov/vuln/detail/CVE-2021-34473",
"product": "Exchange Server",
"requiredAction": "Apply updates per vendor instructions.",
"shortDescription": "Microsoft Exchange Server contains an unspecified vulnerability that allows for remote code execution.",
"vendorProject": "Microsoft",
"vulnerabilityName": "Microsoft Exchange Server Remote Code Execution Vulnerability"
}