Security readout for executives and security teams
Plain-English summary
A person who already has limited access to an affected Windows device could exploit a memory-handling flaw in Microsoft Office Click-To-Run to gain higher privileges. Successful exploitation could expose sensitive data, alter the system, or disrupt availability without additional user interaction.
Executive priority
Treat this as a high-priority endpoint patching issue. It is not presented as an unauthenticated remote compromise, but it could let an existing low-privilege user or attacker take greater control of a device. Confirm exposure and deploy Microsoft’s applicable update promptly.
Technical view
CVE-2026-40420 is a local, low-complexity use-after-free vulnerability in Microsoft Office Click-To-Run. It requires existing low privileges but no user interaction. The CVSS 3.1 score is 8.8, with changed scope and high potential impact to confidentiality, integrity, and availability.
Likely exposure
Exposure is limited to devices running affected Microsoft 365 Apps for Enterprise, Office 2019, Office LTSC 2021, or Office LTSC 2024 versions identified in the source bundle. Internet accessibility alone is not sufficient because exploitation is local and requires prior authorized access.
Exploitation context
The source bundle does not identify active exploitation, and the CVE is not listed as KEV. The CVSS vector marks exploit maturity as unproven. However, existing local access and low attack complexity make prompt remediation important, especially on shared endpoints and systems where limited accounts access sensitive resources.
Researcher notes
The bundle describes a use-after-free condition but maps CWE-284, an access-control category; the precise root-cause classification should be confirmed from Microsoft’s full record. Fixed build numbers, affected operating systems, attack mechanics, and public proof-of-concept status are not provided, so conclusions beyond the supplied CVSS vector would be speculative.
Mitigation direction
- Review the Microsoft advisory for the applicable security update and deployment guidance.
- Update affected Microsoft Office installations through the organisation’s approved patch-management process.
- Prioritise shared workstations, privileged-user endpoints, and devices processing sensitive information.
- Restrict unnecessary local access while remediation is pending.
Validation and detection
- Inventory Office products, editions, versions, and Click-To-Run status across managed endpoints.
- Compare installed Office builds and update state with Microsoft’s CVE advisory.
- Confirm the applicable Microsoft update completed successfully on each affected device.
- Monitor endpoint and identity telemetry for unexpected privilege changes or suspicious local activity.
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-284: Authorization and privilege behavior lookup
Authorization weaknesses can support privilege escalation and valid-account review, depending on exploit path. 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-2026-40420 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
- 8.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:L/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.
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:C/C:H/I:H/A:H/E:U/RL:O/RC:C26microsoftVulnerability scoring details
Base CVSS 3.1 score
8.8HighVector: CVSS:3.1/AV:L/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
- Microsoft Office Click-To-Run 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.
Improper Access Control
Improper Access Control represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.