CVE-2021-31207
Confirmed PUBLISHEDMicrosoft Exchange Server Security Feature Bypass Vulnerability
Recommended Action
Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.
At a Glance
Microsoft Exchange Server Security Feature Bypass Vulnerability
- CVE Published
- May 11, 2021
- Exploitation Reported
- Nov 03, 2021
- CVSS
- 6.6 Medium
- EPSS
- —
Affected Versions
| Vendor | Product | Version | Status |
|---|---|---|---|
| Microsoft |
Microsoft Exchange Server 2013 Cumulative Update 23
|
15.00.0 to < 15.00.1497.018 |
Affected |
| Microsoft |
Microsoft Exchange Server 2019 Cumulative Update 9
|
15.02.0 to < 15.02.0858.012 |
Affected |
| Microsoft |
Microsoft Exchange Server 2016 Cumulative Update 20
|
15.01.0 to < 15.01.2242.010 |
Affected |
| Microsoft |
Microsoft Exchange Server 2016 Cumulative Update 19
|
15.01.0 to < 15.01.2176.014 |
Affected |
| Microsoft |
Microsoft Exchange Server 2019 Cumulative Update 8
|
15.02.0 to < 15.02.0792.015 |
Affected |
CVE References
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207 portal.msrc.microsoft.com · CVE Record https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CV...
- zerodayinitiative.com/advisories/ZDI-21-819 zerodayinitiative.com · CVE Record https://www.zerodayinitiative.com/advisories/ZDI-21-819/
- packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-R... packetstormsecurity.com · CVE Record http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyS...
Recommended Actions
- Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.
- Check enrichment artifacts for scanner coverage and available PoCs before rolling remediation validation.
- Use the Pro API to automate enrichment, telemetry, and workflow delivery for VM, SOC, and CTI pipelines.
Known Exploited Vulnerability Sources
Catalogues that list this CVE as a known exploited vulnerability.
Per-source evidence links for KEV attestations are available through the KEVIntel Pro API.
Learn about Pro API access| Source | Added |
|---|---|
| CISA First | 2021-11-03 00:00 UTC |
Scanner Artifacts
Nuclei and Metasploit references linked to this CVE.
| Scanner | Reference | Detected |
|---|---|---|
| Metasploit | https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_proxyshell_rce.rb | Apr 28, 2025 |
Virtual Patch
Compensating WAF rules to help reduce exposure to this CVE. Rule content and deployable vendor exports are available with KEVIntel Enterprise.
KEVIntel does not currently have a virtual patch for this CVE. When available, KEVIntel virtual patches ship as deployable ModSecurity, Cloudflare, and AWS WAF rules.
Enterprise feature. Virtual patch rule content and deployable vendor exports (ModSecurity, Cloudflare, AWS WAF) are available to KEVIntel Enterprise users.
CVSS Scores
CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
Exploitation Status
Exploited in the wild
Recorded 2021-11-03 00:00:00 UTC · CISA
Used in malware
Recorded 2021-11-03 00:00:00 UTC · CISA
Proof of concept available
Recorded 2025-04-28 15:02:52 UTC
Weaknesses (CWE)
-
Unrestricted Upload of File with Dangerous Type
Scanner Integrations
| Scanner | Reference | Detected |
|---|---|---|
| Metasploit | https://github.com/rapid7/metasploit-framework/blob/master/modules/exploits/windows/http/exchange_proxyshell_rce.rb | Apr 28, 2025 |
Potential Proof of Concepts
These PoCs are unverified and could contain malware. Use at your own risk.
Timeline
Key exploitation, disclosure, scanner coverage, and KEV attestation events for this CVE.
-
15:02 UTC about 1 year ago15:02 UTC · about 1 year ago
Metasploit module available
Exploit module available
-
15:02 UTC about 1 year ago15:02 UTC · about 1 year ago
Public PoC available
Public proof-of-concept code published
-
00:00 UTC over 4 years ago00:00 UTC · over 4 years ago
Added to CISA KEV
Listed in the CISA Known Exploited Vulnerabilities catalog
-
00:00 UTC over 4 years ago00:00 UTC · over 4 years ago
First public exploitation report
Exploit observed in malware
-
19:11 UTC about 5 years ago19:11 UTC · about 5 years ago
CVE published
Vulnerability disclosed publicly
-
00:00 UTC over 5 years ago00:00 UTC · over 5 years ago
CVE ID reserved
Identifier reserved by the CNA
Automate This Intelligence with the Pro API
Confidence scoring, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.
Pro API Example
GET /api/v2/pro/kevs/CVE-2021-31207
{
"cve_id": "CVE-2021-31207",
"title": "Microsoft Exchange Server Security Feature Bypass Vulnerability",
"affected_vendor": "Microsoft",
"affected_product": "Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Exchange Server 2019 Cumulative Update 9, Microsoft Exchange Server 2016 Cumulative Update 20, Microsoft Exchange Server 2016 Cumulative Update 19, Microsoft Exchange Server 2019 Cumulative Update 8",
"affected_versions": [
{ "vendor": "...", "product": "...", "status": "affected", "display_label": "..." }
],
"confidence": "Confirmed",
"cvss_score": 6.6,
"epss_score": null,
"exploit_status": {
"exploited_in_the_wild": true,
"active_exploitation_observed": false
},
"sensor_telemetry": { "...": "Pro API fields" },
"proof_of_concepts": [ "..." ],
"scanner_integrations": [ "..." ]
}