CVE-2021-33766: Technical Deep-Dive (Auto Refreshed)

Generated on 2026-04-05T17:26:12.293Z. This file is automatically regenerated every 30 minutes by the CVE AI enrichment job using web sources (NVD, MITRE, CISA KEV, GitHub).

Executive Technical Summary

Microsoft Exchange Server Information Disclosure Vulnerability

• Context preserved from previous revision: Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target. 1. Use only isolated environments and systems you own or are explicitly authorized to test.

Technical Details

• CVE: CVE-2021-33766
• KEV date added: 2022-01-18
• KEV due date: 2022-02-01
• NVD published: 2021-07-14
• NVD modified: 2025-10-29
• MITRE modified: 2025-10-21
• CVSS base score: 7.3
• CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• CVSS exploitability score: 3.9
• CVSS impact score: 3.4
• Attack vector: Network
• Attack complexity: Low
• Privileges required: None
• User interaction: None
• Scope: Unchanged
• Confidentiality impact: Low
• Integrity impact: Low
• Availability impact: Low

Versions and Products Impacted

• microsoft / exchange server (versions: 2013)
• microsoft / exchange server (versions: 2016)
• microsoft / exchange server (versions: 2019)
• Microsoft / Microsoft Exchange Server 2019 Cumulative Update 9 (versions: 15.02.0)
• Microsoft / Microsoft Exchange Server 2016 Cumulative Update 20 (versions: 15.01.0)
• Microsoft / Microsoft Exchange Server 2013 Cumulative Update 23 (versions: 15.00.0)
• Microsoft / Microsoft Exchange Server 2016 Cumulative Update 19 (versions: 15.01.0)
• Microsoft / Microsoft Exchange Server 2019 Cumulative Update 8 (versions: 15.02.0)

Weakness Classification

• NVD-CWE-noinfo
• CWE-noinfo Not enough information

Repositories for Lab Validation (Public Examples)

• Ostorlab/KEV | stars: 608 | updated: 2026-03-23 | https://github.com/Ostorlab/KEV
  Notes: Ostorlab KEV: One-command to detect most remotely known exploitable vulnerabilities. Sourced from CISA KEV, Google's Tsunami, Ostorlab's Asteroid and Bug Bounty programs.
• FDlucifer/Proxy-Attackchain | stars: 562 | updated: 2026-04-02 | https://github.com/FDlucifer/Proxy-Attackchain
  Notes: Proxylogon & Proxyshell & Proxyoracle & Proxytoken & All exchange server history vulns summarization :)

People and Organizations Mentioned

• microsoft
• Exchange Server
• Ostorlab
• FDlucifer

Practical Defensive Validation (Authorized Only)

1. Use only isolated environments and systems you own or are explicitly authorized to test.
2. Snapshot infrastructure before validation and preserve baseline logs (EDR, SIEM, OS, app).
3. Classify sensitive fields in microsoft / exchange server (versions: 2013) and verify logs/responses do not leak secrets.
4. Test redaction and response-minimization controls in debug and production-like modes.
5. Monitor bulk export behavior and abnormal low-privilege read spikes.

References

• NVD record: https://nvd.nist.gov/vuln/detail/CVE-2021-33766
• MITRE CVE record: https://www.cve.org/CVERecord?id=CVE-2021-33766
• CISA KEV Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• CISA KEV JSON feed: https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json
• KEV notes: https://nvd.nist.gov/vuln/detail/CVE-2021-33766
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766
• https://www.zerodayinitiative.com/advisories/ZDI-21-798/
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766
• Repository example: https://github.com/Ostorlab/KEV
• Repository example: https://github.com/FDlucifer/Proxy-Attackchain

This content is for defensive security training and authorized validation only.