Security readout for executives and security teams
Plain-English summary
CVE-2026-21240 is a Windows privilege-escalation flaw in HTTP.sys. An attacker who already has local authorized access could race the system and gain higher privileges. This is not described as remote initial access, but it can turn a limited foothold into full system compromise on affected Windows endpoints or servers.
Executive priority
Treat this as a high-priority patching item, especially for servers and high-value workstations. It is not presented as remote code execution, but it can materially increase damage from an existing foothold. Prioritize remediation in normal emergency or accelerated patch cycles based on asset criticality.
Technical view
Microsoft describes a time-of-check time-of-use race condition in Windows HTTP.sys, classified as CWE-367. CVSS 3.1 is 7.8: local attack vector, low complexity, low privileges required, no user interaction, and high confidentiality, integrity, and availability impact. Microsoft lists official remediation availability.
Likely exposure
Exposure is likely where affected Windows 10, Windows 11, Windows Server 2019, Server 2022, or Server 2025 builds are present and unpatched. HTTP.sys is a Windows component used by multiple services, so asset owners should validate by OS build and Microsoft update status rather than only by installed applications.
Exploitation context
The provided sources do not show known active exploitation, and the bundled KEV flag is false. Exploitation requires local authorized access, so risk is highest after phishing, credential theft, insider misuse, or compromise of a low-privileged account.
Researcher notes
Evidence is limited to the CVE record and Microsoft advisory metadata. The issue is a local TOCTOU race in HTTP.sys with official remediation marked available. No public exploit details or active exploitation evidence are included in the provided sources.
Mitigation direction
- Review Microsoft’s CVE-2026-21240 advisory for affected builds and updates.
- Apply the relevant Microsoft security update to affected Windows systems.
- Prioritize servers, internet-facing Windows workloads, and shared endpoints.
- Limit local account privileges until patching is complete.
- Monitor Microsoft guidance for revised affected versions or mitigation notes.
Validation and detection
- Inventory Windows assets against Microsoft’s affected product list.
- Check OS build numbers against the versions named in the advisory.
- Confirm the Microsoft security update is installed.
- Validate Server Core systems separately where applicable.
- Review local privilege-escalation alerts from endpoint security tools.
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-367: 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-2026-21240 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 HTTP.sys 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.
Time-of-check Time-of-use (TOCTOU) Race Condition
Time-of-check Time-of-use (TOCTOU) Race Condition represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.