Security readout for executives and security teams
Plain-English summary
This flaw affects specified Microsoft Exchange Server 2013, 2016, and 2019 cumulative updates. A remote attacker can target a reachable vulnerable server without credentials or user interaction. The rated impact is limited across confidentiality, integrity, and availability, but confirmed exploitation makes remediation urgent.
Executive priority
Prioritize immediate verification and patching of affected Exchange servers, starting with externally reachable systems. The CVSS score is high rather than critical, but confirmed exploitation materially increases business urgency. Escalate any unpatched affected instance to security leadership.
Technical view
CVE-2021-33766 is a network-accessible Exchange Server information-disclosure vulnerability rated CVSS 3.1 7.3. The vector indicates low attack complexity, no privileges, no user interaction, unchanged scope, and low confidentiality, integrity, and availability impacts. The supplied record identifies no CWE or root cause.
Likely exposure
Exposure applies to Exchange 2013 CU23, Exchange 2016 CU19 or CU20, and Exchange 2019 CU8 or CU9. Risk is greatest where affected servers are reachable from untrusted networks. The bundle does not identify other products or versions as affected.
Exploitation context
CISA KEV inclusion confirms exploitation in the wild. The supplied evidence does not describe exploitation frequency, victims, indicators, prerequisites beyond the CVSS vector, or whether exploitation remains ongoing. No exploit procedure is provided here.
Researcher notes
The evidence supports remote, unauthenticated, low-complexity exposure and confirmed exploitation. It does not provide a CWE, root cause, affected component, exploitation mechanics, indicators, or detailed patch prerequisites. Researchers should consult the Microsoft and ZDI advisories for additional public technical context without assuming details absent from those sources.
Mitigation direction
- Inventory Exchange servers and identify installations running the listed cumulative updates.
- Apply the Microsoft security update referenced for CVE-2021-33766.
- Follow current Microsoft guidance for installation requirements and any superseding updates.
- Reduce unnecessary network reachability to affected Exchange services until remediation is verified.
- Treat suspected compromise as an incident because CISA lists the vulnerability as exploited.
Validation and detection
- Confirm each Exchange server version and cumulative-update level against the affected list.
- Verify the Microsoft security update or an applicable superseding update is installed.
- Reassess network reachability of Exchange services from untrusted networks.
- Review available security telemetry for suspicious activity; the supplied sources provide no indicators.
- Confirm vulnerability-management tooling no longer reports CVE-2021-33766 after remediation.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CVE-2021-33766 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- High
- CVSS
- 7.3 (3.1)
- Known Exploited
- Yes
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L/E:U/RL:O/RC:C
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CISA KEV status
SSVC decision data
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L/E:U/RL:O/RC:C3.93.4microsoftVulnerability scoring details
Base CVSS 3.1 score
7.3HighVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L/E:U/RL:O/RC:C
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- Added to KEVCISA-ADP
CISA Known Exploited Vulnerabilities metadata lists this CVE as known exploited.
- ADP timelineCISA-ADP
CVE-2021-33766 added to CISA KEV
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
- 2022-01-18T00:00:00.000Z: CVE-2021-33766 added to CISA KEV
Source materials
- CVE List V5 sourceCVE List V5
- Microsoft Exchange Server Information Disclosure VulnerabilityCVE reference, microsoft · vendor-advisory, patch
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766CVE reference, CVE · x_refsource_MISC, x_transferred
- https://www.zerodayinitiative.com/advisories/ZDI-21-798/CVE reference, CVE · x_refsource_MISC, x_transferred
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766CVE reference, CISA-ADP · government-resource
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.