Security readout for executives and security teams
Plain-English summary
CVE-2026-21247 is a high-severity Windows Hyper-V flaw that could let an already authorized local attacker run code. Successful exploitation requires user interaction and could compromise confidentiality, integrity, and availability. It is not presented as an unauthenticated network attack.
Executive priority
Treat this as a high-priority patching issue, especially on shared virtualization hosts or systems permitting less-trusted local access. It is serious because successful exploitation can fully affect a system, but available evidence does not indicate an unauthenticated remote path or active exploitation.
Technical view
Improper input validation in Windows Hyper-V is classified under CWE-20, CWE-122, and CWE-125. The CVSS 3.1 vector is local, low complexity, low privilege, user-interaction required, and unchanged scope, with high impact across confidentiality, integrity, and availability.
Likely exposure
Potential exposure includes the listed Windows 10, Windows 11, and Windows Server releases and builds. Organizations should verify exact editions, builds, Hyper-V presence, and applicable Microsoft updates. The supplied evidence does not establish whether every listed system is exploitable under all configurations.
Exploitation context
The bundle reports no CISA KEV listing and provides no evidence of active exploitation or a public proof of concept. Exploitation requires local access, low privileges, and user interaction. The precise interaction, vulnerable processing path, and attack prerequisites are not described.
Researcher notes
The title says remote code execution, while the description and CVSS vector specify an authorized local attacker and AV:L. Researchers should preserve this distinction. The supplied sources do not describe the vulnerable function, memory-corruption mechanics, affected configurations, proof-of-concept availability, or exact fixed build numbers.
Mitigation direction
- Review Microsoft's CVE advisory for the applicable security update, prerequisites, and deployment guidance.
- Apply the Microsoft security update to affected systems using established change-control procedures.
- Prioritize shared Hyper-V hosts and systems accessible to less-trusted local users.
- Check Microsoft guidance for supported mitigations if immediate patching is not possible.
Validation and detection
- Inventory the listed Windows editions and compare their exact OS build numbers with Microsoft guidance.
- Determine where Hyper-V is installed or used, while noting the bundle does not define configuration prerequisites.
- Confirm the applicable Microsoft security update is installed successfully on each affected system.
- Reassess update compliance after deployment and investigate installation failures or pending restarts.
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.
CWE-122: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCWE-125: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCWE-20: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupExecution 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-2026-21247 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
- No
- Published
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/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.
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:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C1.35.9microsoftVulnerability scoring details
Base CVSS 3.1 score
7.3HighVector: CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/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
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.
Heap-based Buffer Overflow
Heap-based Buffer Overflow represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Out-of-bounds Read
Out-of-bounds Read represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Improper Input Validation
Improper Input Validation represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.