Security readout for executives and security teams
Plain-English summary
CVE-2026-21245 is a Windows Kernel flaw that could let someone who already has local, low-privileged access gain full system-level control. It is high severity because successful abuse can compromise confidentiality, integrity, and availability on affected Windows 11 and Windows Server 2025 systems.
Executive priority
Prioritize patch validation in the normal high-severity Windows update cycle, especially for servers, privileged workstations, and shared endpoints. This is not currently evidenced as actively exploited in the supplied sources, but the impact is full local compromise.
Technical view
The source describes a heap-based buffer overflow in the Windows Kernel, classified as CWE-122. The CVSS 3.1 vector is local, low complexity, low privileges required, no user interaction, unchanged scope, and high impact across confidentiality, integrity, and availability. Microsoft lists a patch advisory.
Likely exposure
Exposure is most relevant on Microsoft Windows 11 24H2, 25H2, 26H1, and Windows Server 2025, including Server Core, at the listed base versions. Systems with local users, endpoint compromise risk, or shared administrative workstations deserve faster review.
Exploitation context
The provided bundle does not show CISA KEV listing or cited active exploitation. The CVSS temporal vector marks exploit maturity as unproven. Treat this as a serious local privilege-escalation risk, not confirmed in-the-wild exploitation.
Researcher notes
Key constraints are local attack vector and prior low privileges. The source bundle does not provide vulnerable kernel component details, exploit primitives, or workaround specifics. Avoid assuming exposure beyond the listed Windows versions and Microsoft’s advisory.
Mitigation direction
- Review the Microsoft MSRC advisory and apply the listed security update.
- Prioritize affected Windows 11 and Windows Server 2025 assets.
- Limit unnecessary local user access on high-value Windows systems.
- Monitor vendor guidance for revised affected-version or remediation details.
Validation and detection
- Inventory Windows 11 24H2, 25H2, 26H1, and Windows Server 2025 endpoints.
- Confirm whether Microsoft’s security update for CVE-2026-21245 is installed.
- Check vulnerability scanners for authenticated detection of this CVE.
- Review EDR telemetry for suspicious local privilege-escalation behavior.
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 lookupPrivilege behavior lookup
The CVE wording references privilege impact, so privilege escalation and authorization behavior review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2026-21245 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
- 7.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/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:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C1.85.9microsoftVulnerability scoring details
Base CVSS 3.1 score
7.8HighVector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/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 Kernel Elevation of Privilege 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.