Exploitation evidence
Known exploited · CISA KEV listed
Added 2021-11-17. Ransomware campaign use: Known.
CISA required action: Apply updates per vendor instructions.
CISA due date: 2021-12-01. 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 2016 Cumulative Update 21
Default status: Not specified
- affected: 15.01.0 to < 15.01.2308.020 (custom)
Microsoft / Microsoft Exchange Server 2016 Cumulative Update 22
Default status: Not specified
- affected: 15.0.0 to < 15.01.2375.017 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 10
Default status: Not specified
- affected: 15.02.0 to < 15.02.0792.019 (custom)
Microsoft / Microsoft Exchange Server 2019 Cumulative Update 11
Default status: Not specified
- affected: 15.02.0 to < 15.02.0986.014 (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-2021-42321
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42321
- http://packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.html
- http://packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Execution.html
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42321
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",
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"percentile": 0.99818
}10 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-10",
"epss": 0.91737,
"percentile": 0.99817
}09 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-09",
"epss": 0.91737,
"percentile": 0.99817
}08 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-08",
"epss": 0.91737,
"percentile": 0.99816
}07 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-07",
"epss": 0.91737,
"percentile": 0.99816
}06 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-06",
"epss": 0.91737,
"percentile": 0.99816
}06 Nov 2026, 06:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-05",
"epss": 0.91737,
"percentile": 0.99816
}05 Nov 2026, 06:20 Jerusalem · CVE Program record changed
Stored CNA snapshot at this observation.
{
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}04 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
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"epss": 0.91737,
"percentile": 0.99816
}03 Nov 2026, 18:20 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
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"epss": 0.91737,
"percentile": 0.99815
}03 Nov 2026, 02:22 Jerusalem · EPSS observation updated
Stored EPSS snapshot at this observation.
{
"date": "2026-11-02",
"epss": 0.91737,
"percentile": 0.99815
}03 Nov 2026, 02:22 Jerusalem · CISA KEV status changed
Stored KEV snapshot at this observation.
{
"cveID": "CVE-2021-42321",
"cwes": [
"CWE-184",
"CWE-502"
],
"dateAdded": "2021-11-17",
"dueDate": "2021-12-01",
"forensicTriage": "No",
"knownRansomwareCampaignUse": "Known",
"notes": "https://nvd.nist.gov/vuln/detail/CVE-2021-42321",
"product": "Exchange",
"requiredAction": "Apply updates per vendor instructions.",
"shortDescription": "An authenticated attacker could leverage improper validation in cmdlet arguments within Microsoft Exchange and perform remote code execution.",
"vendorProject": "Microsoft",
"vulnerabilityName": "Microsoft Exchange Server Remote Code Execution Vulnerability"
}