CVE-2026-55899
HIGH 7.8 ALL-YEARSStack-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code locally.
Severe if exploited (CVSS 7.8), but no confirmed exploitation. Patch on a normal cadence.
Exploitation likelihood
0.5%chance of exploitation in 30 days · 40th percentile
Impact if exploited
7.8CVSS 3.1 · HIGH · CNA
- ConfidentialityHigh
- IntegrityHigh
- AvailabilityHigh
What an attacker needs
- ⚠Access: Requires local access to the host
- ✓Privileges: No account or privileges required
- ⚠User interaction: A user must take an action (click / open a file)
- ✓Complexity: No special conditions — reliably repeatable
✓ lowers the bar for an attacker · ⚠ raises it
Proof of concept
No public PoC is currently catalogued.
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Tracked sources are checked weekly for the full archive, including PoCs published after the original disclosure. A later release appears after a tracked source lists it and a refresh succeeds.
Affected
Vendors Microsoft
Products Microsoft 365 Apps For Enterprise Microsoft Excel 2016 Microsoft Office 2019 Microsoft Office 365 For Mac Microsoft Office Ltsc 2021 Microsoft Office Ltsc 2024 Microsoft Office Ltsc For Mac 2021 Microsoft Office Ltsc For Mac 2024 Office Online Server 365 Apps Excel Microsoft 365 Office 2019 Office 2021 Office 2024
Weakness (CWE)
- CWE-121: : Stack-based Buffer Overflow
- CWE-20: : Improper Input Validation
CVSS vector
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C
References and research data (JSON)
Known Affected Software Configurations
| Vendor | Product | Version range |
|---|---|---|
| Microsoft | 365 Apps | * |
| Microsoft | 365 Apps | * |
| Microsoft | Excel | 2016 |
| Microsoft | Excel | 2016 |
| Microsoft | Microsoft 365 | * |
| Microsoft | Office 2019 | * |
| Microsoft | Office 2019 | * |
| Microsoft | Office 2021 | * |
| Microsoft | Office 2021 | * |
| Microsoft | Office 2021 | * |
| Microsoft | Office 2024 | * |
| Microsoft | Office 2024 | * |
| Microsoft | Office 2024 | * |
| Microsoft | Office Online Server | < 16.0.10417.20175 |
All CVSS metrics
- HIGH 7.8 v3.1 · CNA Primary
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C - HIGH 7.8 v3.1 · NVD Secondary
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
References
Patches & mitigations
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55899 CVE record · microsoft · Updated 2026-10-08T18:13:22.438Z
NVD · CVE-2026-55899 · Updated 2026-07-16T11:51:04.013
Advisories
- https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-44040 EUVD · EUVD-2026-44040
Sources: NVD · CVE.org · All sources and attribution · EPSS