CVE-2021-33766: Microsoft Exchange Server Information Disclosure
- Is it exploited?
Yes. CISA added it to the Known Exploited Vulnerabilities catalog on , 188 days after the CVE record was published.
- How likely is exploitation in the next 30 days?
- 98.2%
EPSS score, percentile 99.9 of all scored records
- How severe?
- 7.3 High
CVSS v3.1 base score, given by the CNA
- What should I do?
CISA deadline . Apply updates per vendor instructions.
- Which versions are affected?
Affected
- from 15.00.0 to before 15.00.1497.015
- from 15.01.0 to before 15.01.2176.012
- from 15.01.0 to before 15.01.2242.008
- from 15.02.0 to before 15.02.0792.013
- from 15.02.0 to before 15.02.0858.010
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of ; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of ; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
What to do
Required action, set by CISA
Apply updates per vendor instructions.
CISA due date for this action: .
Advisories and patches
Linked from CISA's catalog entry
Affected products and versions
As listed in the CVE record.
| Status | Versions |
|---|---|
| Affected | from 15.00.0 to before 15.00.1497.015 |
| Status | Versions |
|---|---|
| Affected | from 15.01.0 to before 15.01.2176.012 |
| Status | Versions |
|---|---|
| Affected | from 15.01.0 to before 15.01.2242.008 |
| Status | Versions |
|---|---|
| Affected | from 15.02.0 to before 15.02.0792.013 |
| Status | Versions |
|---|---|
| Affected | from 15.02.0 to before 15.02.0858.010 |
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
About this vulnerability
Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.
Summary: CISA's KEV catalog, as of .
- CISA added CVE-2021-33766 to its Known Exploited Vulnerabilities (KEV) catalog on 18 Jan 2022.
- CISA's ransomware flag for this entry is "Unknown": no evidence of ransomware use in the catalog, which is not proof of none.
- The due date CISA sets for the required action is 1 Feb 2022.
- The CVE record was published on 14 Jul 2021, 188 days before it was added to KEV.
- The CVE record carries a CVSS v3.1 base score of 7.3 (High), given by its CNA.
- FIRST's EPSS model puts the probability of exploitation in the next 30 days at 98.2% (percentile 99.9), as of 28 Oct 2026.
- CISA's SSVC assessment in the CVE record: exploitation active, automatable yes, technical impact partial.
Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of ; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of ; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of . About the data.
Description
Microsoft Exchange Server Information Disclosure Vulnerability
Microsoft Exchange Server Information Disclosure Vulnerability
Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of . About the data.
Weakness
CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.
Timeline
| Date | Event |
|---|---|
| CVE record published | |
| Added to CISA KEV | |
| CISA due date for the required action | |
| CVE record last updated |
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