Security readout for executives and security teams
Plain-English summary
This critical Hyper-V flaw could let a low-privileged attacker remotely execute code across a security boundary, potentially compromising data, systems, and service availability. The supplied CVSS score is 9.9. Prioritize affected Hyper-V hosts, but the provided evidence does not establish active exploitation.
Executive priority
Treat this as an urgent patch-validation issue because successful exploitation could cross a security boundary and cause full confidentiality, integrity, and availability impact. First identify affected Hyper-V hosts, then accelerate Microsoft update deployment according to business criticality and exposure. Do not describe it as actively exploited based on this bundle.
Technical view
CVE-2021-26867 is a Windows Hyper-V remote code execution vulnerability. Its CVSS 3.1 vector indicates network reachability, low attack complexity, low privileges, no user interaction, changed scope, and high confidentiality, integrity, and availability impact. The bundle identifies specific Windows 10 and Windows Server releases; it provides no CWE or technical root-cause details.
Likely exposure
Potential exposure is limited to systems using Hyper-V on the listed Windows 10 1909/2004 and Windows Server 1909/2004/20H2 releases. Confirm Hyper-V enablement, exact edition and build, security-update status, and relevant network reachability. The bundle does not identify other products or versions.
Exploitation context
The CVSS vector describes a network-accessible, low-complexity attack requiring low privileges and no user interaction. The bundle marks this CVE as absent from KEV and supplies no evidence of exploitation in the wild, public proof-of-concept code, or exploitation frequency.
Researcher notes
The available evidence is sparse: it provides the CVSS vector, affected release names, and Microsoft references, but no root cause, vulnerable component details, attack prerequisites beyond CVSS, or named update identifiers. Researchers should consult the linked Microsoft record for authoritative applicability and remediation data without inferring unsupported affected versions.
Mitigation direction
- Review the Microsoft advisory for the security update applicable to each affected operating-system release.
- Apply the applicable Microsoft security update using the organization’s tested emergency-patching process.
- Prioritize internet-facing, shared, sensitive, or otherwise untrusted Hyper-V environments.
- Restrict unnecessary network access to affected hosts while remediation is pending.
Validation and detection
- Inventory systems running the listed Windows releases and determine whether Hyper-V is enabled.
- Compare each system’s edition, build, and installed updates against Microsoft’s CVE guidance.
- Use authenticated vulnerability scanning or endpoint inventory to identify missing applicable security updates.
- Re-scan remediated hosts and document exceptions, unsupported systems, and compensating controls.
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.
Execution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-26867 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
- Critical
- CVSS
- 9.9 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H/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.
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:L/UI:N/S:C/C:H/I:H/A:H/E:U/RL:O/RC:C3.16microsoftVulnerability scoring details
Base CVSS 3.1 score
9.9CriticalVector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H/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.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- Windows Hyper-V Remote Code Execution VulnerabilityCVE reference, microsoft · vendor-advisory, patch
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26867CVE reference, CVE · x_refsource_MISC, x_transferred
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.