CVE-2026-40420: Microsoft Office Click-To-Run Elevation of Privilege Vulnerability
CVE-2026-40420 is an elevation of privilege vulnerability in Microsoft Office Click-To-Run, rated Important by Microsoft, with a CVSS 3.1 base score of 8.8 (High). An attacker can exploit it by an attacker with local access, with low privileges and without user interaction. Microsoft has not reported exploitation in the wild and rates exploitation as less likely. The EPSS model estimates a 0.21% probability of exploitation activity in the next 30 days, higher than 12% of all scored vulnerabilities. Microsoft released the fix on 12 May 2026 in the May 2026 Patch Tuesday. It affects 8 products in 1 product family (Microsoft Office).
Key facts
- CVE ID
- CVE-2026-40420
- Severity
- High CVSS 8.8
- Microsoft severity
- Important
- Impact
- Elevation of Privilege
- Exploited in the wild
- No
- Publicly disclosed
- No
- Exploitation likelihood
- Less Likely
- Weakness
- CWE-284
- EPSS
- 0.21% (percentile 12%)
- CISA KEV
- Not listed
- Released
- Last updated
- Patch Tuesday
- May 2026 Patch Tuesday
- Product families
- Microsoft Office
CVSS score and vector
Microsoft scores CVE-2026-40420 at 8.8 (temporal 7.7) using CVSS 3.1. 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
| Metric | Value |
|---|---|
| Attack vector | Local |
| Attack complexity | Low |
| Privileges required | Low |
| User interaction | None |
| Scope | Changed |
| Confidentiality impact | High |
| Integrity impact | High |
| Availability impact | High |
| Exploit code maturity | Unproven |
| Remediation level | Official fix |
| Report confidence | Confirmed |
Description
Use after free in Microsoft Office allows an authorized attacker to elevate privileges locally.
Frequently asked questions (Microsoft)
Is the Preview Pane an attack vector for this vulnerability?
No, the Preview Pane is not an attack vector.
According to the CVSS metric, a successful exploitation could lead to a scope change (S:C). What does this mean for this vulnerability?
This vulnerability could lead to a browser sandbox escape.
What privileges could be gained by an attacker who successfully exploited this vulnerability?
An attacker who successfully exploited this vulnerability could gain SYSTEM privileges.
Affected products (8)
Microsoft Office
| Product | Security update | Type | Fixed build | Restart |
|---|---|---|---|---|
| Microsoft Office 2019 for 32-bit editions | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
| Microsoft Office 2019 for 64-bit editions | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
| Microsoft 365 Apps for Enterprise for 32-bit Systems | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
| Microsoft 365 Apps for Enterprise for 64-bit Systems | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
| Microsoft Office LTSC 2021 for 64-bit editions | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
| Microsoft Office LTSC 2021 for 32-bit editions | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
| Microsoft Office LTSC 2024 for 32-bit editions | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
| Microsoft Office LTSC 2024 for 64-bit editions | Click to Run | Security Update | https://aka.ms/OfficeSecurityReleases | No |
NIST NVD data
- NVD status
- Modified
- NVD CVSS
- 8.8 (HIGH)
- NVD weakness
- CWE-284
- Published to NVD
- NVD last modified
Affected configurations (CPE)
| Product | Vulnerable versions |
|---|---|
| 365 apps (x64) | all listed versions |
| 365 apps (x86) | all listed versions |
| office 2019 (x64) | all listed versions |
| office 2019 (x86) | all listed versions |
| office long term servicing channel 2021 (x64) | all listed versions |
| office long term servicing channel 2021 (x86) | all listed versions |
| office long term servicing channel 2024 (x64) | all listed versions |
| office long term servicing channel 2024 (x86) | all listed versions |
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40420 (Vendor Advisory)
Acknowledgments
- Ben Lichtman with Microsoft
Revision history
| Version | Date | Description |
|---|---|---|
| 1.0 | Information published. |