Exploited vulnerability · CISA KEV
Is CVE-2021-33766 exploited? Yes: in CISA KEV since 18 Jan 2022
Gociux · Microsoft Exchange Server · data: CISA KEV, CVE record, FIRST EPSS
Yes. CVE-2021-33766, Microsoft Exchange Server Information Disclosure, is being exploited in the wild. CISA added it to its Known Exploited Vulnerabilities catalog on 18 Jan 2022, 188 days after it was published. It affects Microsoft Exchange Server. If you run it, CISA's required action is: Apply updates per vendor instructions.
exploitedYesCISA KEV · 18 Jan 2022
ransomwareNot knownno confirmed use
EPSStop 1%among the most likely CVEs to be exploited
severity7.3CVSS 3.1 · high
US federal due1 Feb 2022BOD 22-01 deadline
What it is
Microsoft Exchange Server contains an information disclosure vulnerability which can allow an unauthenticated attacker to steal email traffic from target.
| Vendor | Microsoft (391 exploited CVEs in KEV) |
| Product | Exchange Server |
| Vulnerability | Microsoft Exchange Server Information Disclosure |
| Weakness | CWE-287 |
| Published | 14 Jul 2021 |
| Added to KEV | 18 Jan 2022 |
| Reported by (CNA) | microsoft |
What to do
- Check whether you run it. Look for Exchange Server in your asset inventory, including appliances and cloud images. Stack check lists every exploited Microsoft CVE at once.
- Apply the fix. Apply updates per vendor instructions.
- If you can't patch today, take the affected service off the internet or restrict who can reach it, and check its logs for signs of compromise since 14 Jul 2021.
- Get told next time. Email alerts when CISA adds a CVE for Microsoft.
Other exploited CVEs in Exchange Server
- CVE-2023-21529: Microsoft Exchange Server Deserialization of Untrusted Data Vulnerability (13 Apr 2026)
- CVE-2021-31196: Microsoft Exchange Server Information Disclosure Vulnerability (21 Aug 2024)
- CVE-2024-21410: Microsoft Exchange Server Privilege Escalation Vulnerability (15 Feb 2024)
- CVE-2022-41080: Microsoft Exchange Server Privilege Escalation Vulnerability (10 Jan 2023)
- CVE-2022-41082: Microsoft Exchange Server Remote Code Execution Vulnerability (30 Sep 2022)
- CVE-2022-41040: Microsoft Exchange Server Server-Side Request Forgery Vulnerability (30 Sep 2022)
References
Questions
Is CVE-2021-33766 being exploited?
Yes. CISA added CVE-2021-33766 to the Known Exploited Vulnerabilities catalog on 18 Jan 2022, which means there is reliable evidence of exploitation in the wild.
Is CVE-2021-33766 used in ransomware attacks?
CISA does not list known ransomware use for it. That can change, and exploitation alone is reason to fix it first.
How do I fix CVE-2021-33766?
Apply updates per vendor instructions. Check Microsoft's advisory for the fixed versions, and if you can't patch quickly, restrict access to the affected Exchange Server and watch it for signs of compromise.
Sources: CISA KEV (exploitation, ransomware use, required action, due date), the CVE record (publication date, CVSS, CNA), NVD and FIRST EPSS (live score). More: all KEV additions in January 2022 · Microsoft scorecard · act-now radar.
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