CVE-2021-33766
Microsoft Exchange Server Information Disclosure
Description
CVE-2021-33766 is a high-severity information disclosure vulnerability in Microsoft Exchange Server that allows an unauthenticated attacker to steal email traffic from a target. The flaw affects Microsoft Exchange Server 2013, 2016, and 2019, enabling remote exploitation over the network without requiring any privileges or user interaction. CISA has added CVE-2021-33766 to its Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild. With an EPSS score of 93.48% (99.8th percentile), this Microsoft Exchange Server vulnerability has an extremely high probability of exploitation, making immediate patching essential for all organizations running affected versions.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:LOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| microsoft | exchange server | 2013; 2016; 2019 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-33766(Patch, Vendor Advisory)
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766(Patch, Vendor Advisory)
- https://www.zerodayinitiative.com/advisories/ZDI-21-798/(Third Party Advisory, VDB Entry)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766(US Government Resource)
Weakness Type
This CVE does not have a specifically assigned CWE classification in the NVD database. However, the vulnerability is categorized as an information disclosure flaw in Microsoft Exchange Server that allows unauthorized access to sensitive email data through improperly secured endpoints.
The underlying weakness involves insufficient access controls on Exchange Server components, enabling unauthenticated attackers to intercept and exfiltrate email communications without proper authorization checks.
Impact Analysis
CVE-2021-33766 carries a CVSS 3.1 score of 7.3 (HIGH), reflecting significant risk to organizations running Microsoft Exchange Server. The vulnerability is remotely exploitable over the network with low attack complexity, requires no authentication, and needs no user interaction, making it trivially exploitable by any attacker with network access. While the individual confidentiality, integrity, and availability impacts are rated LOW, the combination of all three being affected increases the overall risk. The EPSS score of 93.48% (99.8th percentile) indicates near-certain exploitation activity, placing this vulnerability among the most actively targeted Exchange Server flaws. Organizations using on-premises Microsoft Exchange Server 2013, 2016, or 2019 face substantial risk of email data theft if this vulnerability remains unpatched.
Exploit Maturity
CISA has confirmed active exploitation of CVE-2021-33766 in the wild by adding it to the Known Exploited Vulnerabilities catalog, with a remediation deadline of 2022-02-01. The EPSS score of 93.48% (99.8th percentile) indicates near-certain exploitation activity, reflecting widespread targeting of this Microsoft Exchange Server information disclosure vulnerability. The Zero Day Initiative advisory at ZDI-21-798 provides additional technical details about the vulnerability. Given the history of Microsoft Exchange Server vulnerabilities being chained together for maximum impact (such as ProxyLogon and ProxyShell attack chains), organizations should treat this vulnerability with the highest urgency.
Remediation
- Apply Microsoft security updates immediately as required by CISA KEV directive. Install the July 2021 cumulative updates for all affected Exchange Server versions (2013, 2016, and 2019) from the Microsoft Security Response Center advisory.
- Verify patch deployment across all Exchange Server instances in your environment. Confirm that Exchange Server 2013 Cumulative Update 23, Exchange Server 2016 CU20/CU21, and Exchange Server 2019 CU9/CU10 (or later) with the July 2021 security update are installed.
- Implement network-level protections by restricting external access to Exchange Server components. Use firewall rules and reverse proxy configurations to limit which endpoints are accessible from the internet, and consider deploying a Web Application Firewall (WAF) in front of Exchange Server.
- Monitor for indicators of compromise by reviewing Exchange Server logs, IIS logs, and network traffic for signs of unauthorized email access or data exfiltration. Conduct a thorough review of email forwarding rules and mailbox delegations for any unauthorized changes.
- Evaluate migration to Exchange Online as a long-term strategy to reduce the attack surface of on-premises Exchange Server deployments, which have been repeatedly targeted by sophisticated threat actors.
Technical Details
CVE-2021-33766 is an information disclosure vulnerability in Microsoft Exchange Server that enables an unauthenticated attacker to intercept email traffic. The CVSS vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L) confirms that the vulnerability is network-accessible with low attack complexity and requires neither privileges nor user interaction, with the scope remaining unchanged. The vulnerability affects the Exchange Server autodiscovery and mail-fetching mechanisms, allowing attackers to access email data without proper authentication. Microsoft Exchange Server versions 2013, 2016, and 2019 are all affected, and the flaw was addressed in the July 2021 Patch Tuesday security updates released by Microsoft.
Frequently Asked Questions
Is CVE-2021-33766 being actively exploited?
Yes. CVE-2021-33766 is listed in the CISA Known Exploited Vulnerabilities catalog, confirming active exploitation in the wild. The EPSS score of 93.48% (99.8th percentile) indicates near-certain exploitation activity, making immediate patching critical.
What products are affected by CVE-2021-33766?
CVE-2021-33766 affects Microsoft Exchange Server versions 2013, 2016, and 2019. All on-premises deployments of these Exchange Server versions are vulnerable until the July 2021 security updates are applied.
How do I fix CVE-2021-33766?
Apply the Microsoft security updates released in July 2021 for Exchange Server 2013, 2016, and 2019. Verify that all Exchange Server instances have been patched and implement network-level restrictions to limit external access to Exchange Server endpoints.
How severe is CVE-2021-33766?
CVE-2021-33766 has a CVSS 3.1 score of 7.3 (HIGH). Despite the individual impact ratings being low, the combination of network-based remote exploitation without authentication and the extremely high EPSS score of 93.48% make this a critical vulnerability requiring immediate attention.
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