Microsoft Office Excel: Remote Code Execution via heap-based buffer overflow
Summary
A heap-based buffer overflow in Microsoft Office Excel allows an unauthenticated remote attacker to execute code. Exploitation requires that a user opens a crafted Excel file, for example from an email attachment, a download or a link. The CVSS base score is 8.8 (High), with high impact on confidentiality, integrity and availability.
Affected
- Microsoft 365 Apps for Enterprise: 16.0.1 up to the fixed build (exact fixed build unknown)
- Office LTSC 2021: 16.0.1 to below 16.0.14334.20974
- Office LTSC 2024: 16.0.0 to below 16.0.17932.21056
- Office 365 for Mac and Office LTSC for Mac 2021/2024: below 16.114.26101123
Fixed in
- Office LTSC 2021 (16.0.14334.20974) and LTSC 2024 (16.0.17932.21056): listed by third parties as builds released 13 October 2026; not yet confirmed as fixed builds in the vendor table
- Microsoft 365 Apps and Office for Mac: fixed build unknown, verify in the MSRC table
- Office LTSC for Mac 2021 and 2024: no update available at release, check vendor for current status
Workaround
- No vendor workaround known.
- Interim measures (recommendation, not from the vendor): keep Protected View enabled, block or restrict macros and external content from email and internet, apply Office file-block policies for untrusted file types.
Exploitation
- Not listed in CISA KEV.
- CISA SSVC: exploitation none, automatable no.
- EPSS 0.00893.
- No public proof of concept found.
- Qualys reports that one October 2026 fix is exploited in the wild, but does not name it; unknown whether it is this CVE.
Our recommendation
- Patch Microsoft 365 Apps and LTSC clients with the October 2026 Office update first; verify the fixed builds in the MSRC table.
- Check Mac clients separately, as no LTSC for Mac update was available at release.
- Until patched, keep Protected View enabled and restrict macros and external content.
- Monitor for exploitation and patch within the normal monthly cycle or sooner, since Excel files from email are a common attack path.
Information from Microsoft (Patch Tuesday October 2026)
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide: CVE-2026-77902 msrc.microsoft.com
- Microsoft 365 Apps security updates learn.microsoft.com
- Office for Mac release notes learn.microsoft.com
- OpenCVE: CVE-2026-77902 app.opencve.io
- Strix: CVE-2026-77902 www.strix.ai
- Vulners: NVD:CVE-2026-77902 vulners.com
- Qualys Patch Tuesday October 2026 review blog.qualys.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 31 Oct 2026, 18:41.
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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