Security readout for executives and security teams
Plain-English summary
A user who already has authorized, low-privilege access to an affected Windows system could exploit a timing flaw in HTTP.sys to gain elevated privileges. Successful exploitation could compromise sensitive data, system integrity, and availability. This is primarily a post-compromise escalation risk, not a remote, unauthenticated entry point.
Executive priority
Treat this as a high-priority patching issue, especially on multi-user servers, administrative workstations, and systems already exposed to local account compromise. It is less urgent than a remotely exploitable unauthenticated flaw, but successful escalation could enable complete system compromise.
Technical view
CVE-2026-21240 is a CWE-367 time-of-check time-of-use race condition in Windows HTTP.sys. Its CVSS 3.1 score is 7.8: local attack vector, low complexity, low privileges required, no user interaction, unchanged scope, and high confidentiality, integrity, and availability impact.
Likely exposure
Exposure includes the listed Windows 10, Windows 11, and Windows Server versions, including specified Server Core installations. Exploitation requires local authorized access. The bundle does not define precise vulnerable-to-fixed build boundaries, so organizations should compare installed editions, builds, and update status with Microsoft's advisory.
Exploitation context
The source bundle does not identify active exploitation, and the CVE is not listed as KEV. The supplied CVSS vector records exploit maturity as unproven and remediation as an official fix. No public exploit, indicators of compromise, or attack campaign evidence is provided.
Researcher notes
The weakness is a local HTTP.sys TOCTOU race condition requiring low privileges and no user interaction. The bundle provides no vulnerable function, race window, exploit method, patch identifier, or detection signature. Product labels include potentially inconsistent Windows 11 naming, so exact applicability should be verified against Microsoft.
Mitigation direction
- Review Microsoft's CVE advisory for applicable security updates and affected build details.
- Apply the Microsoft security update appropriate to each affected Windows edition and build.
- Prioritize systems where untrusted or broadly assigned users have local access.
- Restrict unnecessary local accounts and privileges while remediation is pending.
Validation and detection
- Inventory Windows editions, versions, build numbers, and Server Core installations.
- Compare installed update status with the remediation details in Microsoft's advisory.
- Confirm updated systems no longer match Microsoft's affected build criteria.
- Review endpoint telemetry for unexplained privilege elevation; no CVE-specific indicators are provided.
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.