CVE-2021-33766
Confirmed PUBLISHEDMicrosoft Exchange Server Information Disclosure Vulnerability
Recommended Action
Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.
At a Glance
Microsoft Exchange Server Information Disclosure Vulnerability
- CVE Published
- Jul 14, 2021
- Exploitation Reported
- Jan 18, 2022
- CVSS
- 7.3 High
- EPSS
- —
Affected Versions
| Vendor | Product | Version | Status |
|---|---|---|---|
| Microsoft |
Microsoft Exchange Server 2019 Cumulative Update 9
|
15.02.0 to < 15.02.0858.010 |
Affected |
| Microsoft |
Microsoft Exchange Server 2016 Cumulative Update 20
|
15.01.0 to < 15.01.2242.008 |
Affected |
| Microsoft |
Microsoft Exchange Server 2013 Cumulative Update 23
|
15.00.0 to < 15.00.1497.015 |
Affected |
| Microsoft |
Microsoft Exchange Server 2016 Cumulative Update 19
|
15.01.0 to < 15.01.2176.012 |
Affected |
| Microsoft |
Microsoft Exchange Server 2019 Cumulative Update 8
|
15.02.0 to < 15.02.0792.013 |
Affected |
CVE References
- portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766 portal.msrc.microsoft.com · CVE Record https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CV...
- zerodayinitiative.com/advisories/ZDI-21-798 zerodayinitiative.com · CVE Record https://www.zerodayinitiative.com/advisories/ZDI-21-798/
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 | 2022-01-18 00:00 UTC |
Scanner Artifacts
Nuclei and Metasploit references linked to this CVE.
| Scanner | Reference | Detected |
|---|---|---|
| Nuclei | https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-33766.yaml | Jun 01, 2026 |
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:L/PR:N/UI:N/S:U/C:L/I:L/A:L/E:U/RL:O/RC:C
Exploitation Status
Exploited in the wild
Recorded 2022-01-18 00:00:00 UTC · CISA
Proof of concept available
Recorded 2021-08-31 22:03:13 UTC · GitHub
Scanner Integrations
| Scanner | Reference | Detected |
|---|---|---|
| Nuclei | https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-33766.yaml | Jun 01, 2026 |
Potential Proof of Concepts
These PoCs are unverified and could contain malware. Use at your own risk.
github · Created 2021-09-15 09:09:20 UTC · 10 stars
CVE-2021-33766-poc
github · Created 2021-08-31 22:03:13 UTC · 47 stars
ProxyToken (CVE-2021-33766) : An Authentication Bypass in Microsoft Exchange Server POC exploit
nuclei · Created Unknown
Timeline
Key exploitation, disclosure, scanner coverage, and KEV attestation events for this CVE.
-
15:34 UTC about 2 months ago15:34 UTC · about 2 months ago
Nuclei template available
Scanner coverage available
-
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
-
22:03 UTC almost 5 years ago22:03 UTC · almost 5 years ago
Public PoC available
Public proof-of-concept code published
-
17:53 UTC about 5 years ago17:53 UTC · about 5 years ago
CVE published
Vulnerability disclosed publicly
-
00:00 UTC about 5 years ago00:00 UTC · about 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-2021-33766
{
"cve_id": "CVE-2021-33766",
"title": "Microsoft Exchange Server Information Disclosure Vulnerability",
"affected_vendor": "Microsoft",
"affected_product": "Microsoft Exchange Server 2019 Cumulative Update 9, Microsoft Exchange Server 2016 Cumulative Update 20, Microsoft Exchange Server 2013 Cumulative Update 23, 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": 7.3,
"epss_score": null,
"exploit_status": {
"exploited_in_the_wild": true,
"active_exploitation_observed": false
},
"sensor_telemetry": { "...": "Pro API fields" },
"proof_of_concepts": [ "..." ],
"scanner_integrations": [ "..." ]
}