Microsoft Office Excel: Remote code execution via heap-based buffer overflow
Summary
A heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code over a network. The attack requires a user to open or interact with a malicious workbook, typically delivered by email or download. Microsoft rates the issue as Critical-level with CVSS 8.8 and reports no known exploitation at the time of publication.
Affected
- Microsoft 365 Apps for Enterprise: 16.0.1 to before the fixed build (build number not given in the CVE record; see MSRC advisory)
- Office 365 for Mac: before 16.115.26110837
- Office LTSC 2021: before 16.0.14334.21048
- Office LTSC 2024: before 16.0.17932.21140 (from secondary source, to be verified)
- Office LTSC for Mac 2021: before 16.115.26110837
- Office LTSC for Mac 2024: before 16.115.26110837
- Office Online Server: before 16.0.10417.20236 (from secondary source, to be verified)
Fixed in
Fixed in the builds listed above, per the CVE record. Microsoft 365 Apps build number: see https://aka.ms/OfficeSecurityReleases. Confirm all build numbers on the MSRC advisory before rollout.
Workaround
Unknown.
- No vendor workaround found in the sources. Check the MSRC advisory for any published mitigation.
Exploitation
- Exploitation status: no known exploitation as of publication; Microsoft's exploit code maturity is 'unproven'.
- No public exploit code found.
- CISA KEV listing: unknown; check the CISA catalog directly.
Our recommendation
- Patch Microsoft Office Excel on all Windows and Mac systems as soon as the fixed builds are available, prioritising systems that open attachments from external senders.
- Verify the exact fixed build numbers in the MSRC advisory before rollout, including LTSC 2024 and Online Server.
- Until patched, restrict opening Excel files from unknown senders and keep Protected View enabled for internet-sourced files.
- Check the CISA KEV catalog and NVD for updates on exploitation status.
Information from Microsoft (Patch Tuesday November 2026)
- Severity (Microsoft)
- Important
- Impact
- Remote code execution – Microsoft Office Excel
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation less likely
- Product family
- Microsoft Office
- Updates (KB)
- KB5002946
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- MSRC Security Update Guide: CVE-2026-118721 msrc.microsoft.com
- CVE Record CVE-2026-118721 www.cve.org
- VulDB entry 434127 vuldb.com
- Strix.ai CVE-2026-118721 www.strix.ai
- ThreatInt CVE-2026-118721 cve.threatint.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 10 Nov 2026, 21:43.
Original description (NVD)
Heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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