CVE-2021-33766: Microsoft Exchange Server information disclosure flaw

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 16 September 2026. Confidence: medium.

high priorityIt is in CISA's KEV catalog with an EPSS probability above 0.98, but the record lacks root-cause and exploit detail, so it is rated high rather than critical.

What it is

CVE-2021-33766 is an information disclosure vulnerability in Microsoft Exchange Server. The record gives only a generic description and no root-cause detail, so the exact mechanism is not documented here. It matters because it is listed in CISA's Known Exploited Vulnerabilities catalog and carries a very high EPSS score, indicating active exploitation and high likelihood of attack.

Impact

An attacker could obtain information from the Exchange server, with the CVSS vector also indicating low-level integrity and availability impact. The record does not specify what data is exposed or how it could be used.

Attack surface

The CVSS vector is network-reachable with no privileges and no user interaction required, so exploitation does not need authentication or a victim to click anything. The description does not state which Exchange component or endpoint is involved.

Exploitation

CVE-2021-33766 is in CISA's KEV catalog with a due date of 2022-02-01, and EPSS gives a 30-day probability of 0.98136 (99.9th percentile). No ransomware campaign use is documented, and the references are patch, vendor advisory and third-party advisory entries rather than public exploit write-ups.

What to do

• Apply the Microsoft Exchange Server updates referenced in the MSRC advisory for CVE-2021-33766.
• Confirm the patch level on all internet-facing Exchange servers and prioritize any that are unpatched.
• Restrict external exposure of Exchange services where operationally possible and enforce MFA on administrative access.
• Monitor Microsoft and CISA guidance for any updated remediation steps tied to this CVE.

Detection

• Review Exchange server logs for anomalous requests or information-gathering activity against Exchange endpoints.
• Hunt for unexpected outbound connections or data transfers from Exchange hosts.
• Correlate Exchange host activity with known post-exploitation behavior such as web shell creation or new administrative accounts.
• Verify patch status across Exchange servers and alert on hosts still missing the relevant update.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.