CVE-2021-33766

HIGHNVD 7.39.0▲Trending — 5 sources updated this weekExploited in the wild

7.3

Microsoft Exchange Server Information Disclosure Vulnerability

CVSS v3: 7.3
EG Score: 9.0(high)
EPSS: 99.9%
KEV: ⚠ Exploited

Published

July 14, 2021

Last Modified

October 29, 2025

References (5)

secure@microsofthttps://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766
secure@microsofthttps://www.zerodayinitiative.com/advisories/ZDI-21-798/
af854a3a-2127-422b-91ae-364da2661108https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766
af854a3a-2127-422b-91ae-364da2661108https://www.zerodayinitiative.com/advisories/ZDI-21-798/
134c704f-9b21-4f2e-91b3-4a467353bcc0https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

CWE-287Improper Authentication

All Vendor Advisories

(1)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Microsoft MSRCCVE-2021-337662021-07-13
Microsoft Exchange Server Information Disclosure Vulnerability

Data Freshness Timeline

(refreshed 20× in last 7d / 20× in last 30d)

Every time one of our enrichment pipelines (NVD, MITRE cvelistV5, EPSS, CISA KEV, GHSA, OSV, vendor advisories) ran against this CVE. Most recent first.

2026-06-06 11:25 UTCEG score recompute
2026-06-06 11:25 UTCVendor advisory
2026-06-06 11:25 UTCGHSA enrichment
2026-06-06 07:27 UTCEG score recompute
2026-06-06 07:27 UTCVendor advisory
2026-06-06 07:27 UTCGHSA enrichment
2026-06-06 03:28 UTCEG score recompute
2026-06-06 03:28 UTCVendor advisory
2026-06-06 03:28 UTCGHSA enrichment
2026-06-05 23:29 UTCEG score recompute
2026-06-05 23:29 UTCVendor advisory
2026-06-05 23:29 UTCGHSA enrichment
2026-06-05 22:43 UTCkev_newly_listed
2026-06-05 19:31 UTCEG score recompute
2026-06-05 19:31 UTCVendor advisory
2026-06-05 19:31 UTCGHSA enrichment
2026-06-05 15:32 UTCEG score recompute
2026-06-05 15:32 UTCVendor advisory
2026-06-05 15:32 UTCGHSA enrichment
2026-06-05 11:33 UTCEG score recompute

Publicly available exploits

(3 references)

Working exploit code is in the public domain (2 GitHub PoCs). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

GitHub PoCdemossl/CVE-2021-33766-ProxyToken
First seen Sep 15, 2021
CVE-2021-33766-poc
Open source ↗
GitHub PoCbhdresh/CVE-2021-33766
First seen Aug 31, 2021
ProxyToken (CVE-2021-33766) : An Authentication Bypass in Microsoft Exchange Server POC exploit
Open source ↗
Nucleihttp/cves/2021/CVE-2021-33766.yaml
First seen Jan 1, 2021
Microsoft Exchange - Authentication Bypass
Open source ↗

Frequently asked(6)

What is CVE-2021-33766?

CVE-2021-33766 is a high vulnerability published on July 14, 2021. Microsoft Exchange Server Information Disclosure Vulnerability

When was CVE-2021-33766 disclosed?

CVE-2021-33766 was first published in the National Vulnerability Database on July 14, 2021, with the most recent update on October 29, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.

Is CVE-2021-33766 actively exploited?

Yes. CISA added CVE-2021-33766 to the Known Exploited Vulnerabilities catalog on January 18, 2022, affecting Microsoft Exchange Server. KEV listing indicates confirmed exploitation in the wild; this CVE warrants immediate patching attention.

What is the CVSS score of CVE-2021-33766?

CVE-2021-33766 has a CVSS v3 base score of 7.3 (NVD). EchelonGraph synthesises NVD + CISA KEV + FIRST EPSS + GHSA into a combined EG score of 9.0.

Which products are affected by CVE-2021-33766?

CVE-2021-33766 affects Microsoft Exchange Server. The full affected-products list, including version ranges and fixed versions, is shown in the Affected Packages section of this page.

How do I remediate CVE-2021-33766?

Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-33766, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2021-33766

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CVE-2021-33766

Published

Last Modified

References (5)

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Publicly available exploits

Frequently asked(6)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-33766?

Same vendor

Same CWE

CVE-2021-33766

📅 Published

🔄 Last Modified

🔗 References (5)

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

Publicly available exploits

❓ Frequently asked(6)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-33766?

🔗 Related CVEs(same vendor + same CWE)

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References (5)

Frequently asked(6)

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