CVE-2021-33766 — Exchange Server
HIGHCVE-2021-33766 is a high-severity (CVSS 7.3) vulnerability in microsoft exchange server. It is in CISA's Known Exploited Vulnerabilities catalog (added 2022-01-18) — treat it as actively exploited and patch now. No vendor fix is recorded yet; mitigation options are listed below.
Microsoft Exchange Server Information Disclosure Vulnerability
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2022-01-18. Federal agencies were required to remediate by 2022-02-01.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-33766.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-33766 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Counts from FIRST.org, log-scaled.
Description
Microsoft Exchange Server Information Disclosure Vulnerability
Affected Products
exchange servermicrosoftResearch 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
Microsoft Exchange Server Information Disclosure Vulnerability
Microsoft Exchange Server Information Disclosure Vulnerability
Microsoft Exchange Server Information Disclosure Vulnerability
Microsoft Exchange Server Information Disclosure Vulnerability
Microsoft Exchange Server Information Disclosure Vulnerability
Detection & mitigation playbook
Vendor / applianceDetect
Inventory every microsoft exchange server deployment and check each version against the affected-products list above.
Remediation status
No patch has shipped for CVE-2021-33766 yet — track the microsoft exchange server advisory for a fixed release and apply the workarounds below in the meantime.
Mitigate without a patch
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
How to detect CVE-2021-33766
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2021-33766 -u https://target- Template
- Microsoft Exchange - Authentication Bypass
- Severity
- high
- Impact
- Unauthenticated attackers can bypass authentication using a SecurityToken cookie, gaining access to Exchange Server's internal API endpoints and sensitive information.
- Remediation
- Apply security updates provided by Microsoft to fix the authentication bypass vulnerability.
Template by ProjectDiscovery nuclei-templates (daffainfo), MIT licensed. View the full template. Scan only systems you are authorised to test.
Frequently Asked Questions
Is CVE-2021-33766 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.