How severe is CVE-2021-33766?

CVE-2021-33766 has a CVSS score of 7.3/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-33766?

Affected package information for CVE-2021-33766 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-33766?

No fixed version has been published for CVE-2021-33766 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-33766 actively exploited in the wild?

Yes. There are 8 known exploit references for CVE-2021-33766, including 6 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-33766?

CVE-2021-33766 has an EPSS (Exploit Prediction Scoring System) score of 93.8%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

Is CVE-2021-33766 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-33766 (Microsoft Exchange Server Information Disclosure) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2022-01-18. CISA required all US federal civilian agencies to remediate this vulnerability by 2022-02-01. The required action was: Apply updates per vendor instructions. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

Which products and versions are affected by CVE-2021-33766?

CVE-2021-33766 affects microsoft exchange server (3 versions, e.g. 2013, 2016, 2019). Check the affected-products list on this page against your deployed version to confirm exposure.

Is there exploit tooling available for CVE-2021-33766?

A Nuclei detection template exists for CVE-2021-33766, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-33766 published, and has it been updated?

CVE-2021-33766 was published on July 14, 2021 and was last updated on October 29, 2025. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2022-01-18 — agencies required to remediate by 2022-02-01

CVE-2021-33766

HIGH

Microsoft Exchange Server Information Disclosure Vulnerability

Published

Jul 14, 2021

Updated

Oct 29, 2025

Affected

0 pkgs

Patched

None yet

Exploits

8 known

EPSS Exploitation Probability

via FIRST.org ↗

93.8%probability of exploitation in next 30 days

Very High Risk100th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

Microsoft Exchange Server Information Disclosure Vulnerability

Affected Products

1 product · 5 configurations

Application

exchange servermicrosoft

3 versions

201320162019

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Microsoft Exchange Server Information Disclosure Vulnerability

github.comGithubAug 2021

Microsoft Exchange Server Information Disclosure Vulnerability

github.comGithubSep 2021

Microsoft Exchange Server Information Disclosure Vulnerability

github.comGithubAug 2021

Microsoft Exchange Server Information Disclosure Vulnerability

github.comAPIOct 2021

Microsoft Exchange Server Information Disclosure Vulnerability

unit42.paloaltonetworks.comAPIDec 2021

Microsoft Exchange Server Information Disclosure Vulnerability

cisa.govCISAJan 2022

Frequently Asked Questions

Microsoft Exchange Server Information Disclosure Vulnerability

O3 Security · Impact-Aware SCA

Is CVE-2021-33766 in your stack?

O3 detects CVE-2021-33766 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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