# CVE-2026-40420: Microsoft Office Click-To-Run Elevation of Privilege Vulnerability

> An authorized local attacker can exploit a use-after-free flaw in Microsoft Office to elevate privileges.

- **Severity:** High 8.8 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 12 May 2026
- **Updated:** 10 Aug 2026
- **Also known as:** GHSA-q3w4-cjqg-q4x3

## What is CVE-2026-40420?

CVE-2026-40420 is a high-severity use after free vulnerability in Microsoft 365 Apps, published on 12 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Use after free in Microsoft Office allows an authorized attacker to elevate privileges locally.

Weakness: CWE-284 (Improper Access Control)

## Which versions are affected by CVE-2026-40420?

It affects 3 products.


Products: Microsoft 365 Apps, Microsoft Office, Microsoft Office Long Term Servicing Channel.

## How do I fix CVE-2026-40420?

No fix has been published for CVE-2026-40420 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-40420 being exploited?

There is no known exploitation of CVE-2026-40420 in the wild and no public exploit code.

## Other vulnerabilities in Microsoft 365 Apps

- [CVE-2024-21413](https://insights.sbomify.com/CVE-2024-21413/): Microsoft Outlook Improper Input Validation (critical 9.8, exploited in the wild)
- [CVE-2023-23397](https://insights.sbomify.com/CVE-2023-23397/): Microsoft Office Outlook Privilege Escalation (critical 9.8, exploited in the wild)
- [CVE-2024-38189](https://insights.sbomify.com/CVE-2024-38189/): Microsoft Project Remote Code Execution Vulnerability (high 8.8, exploited in the wild)
- [CVE-2023-35311](https://insights.sbomify.com/CVE-2023-35311/): Microsoft Outlook Security Feature Bypass (high 8.8, exploited in the wild)
- [CVE-2026-21514](https://insights.sbomify.com/CVE-2026-21514/): Microsoft Office Word Reliance on Untrusted Inputs in a Security Decision (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40420 and fix it if it is affected.

What it is: CVE-2026-40420 is a high-severity use after free vulnerability in Microsoft 365 Apps, published on 12 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-40420 in the wild and no public exploit code.

Affected product: Microsoft 365 Apps. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-40420/ (Markdown: https://insights.sbomify.com/CVE-2026-40420/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40420
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40420
- OSV: https://osv.dev/vulnerability/CVE-2026-40420

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40420/
Summaries are generated and may contain mistakes; the official description above is the record.