Microsoft Office Excel: Out-of-bounds read information disclosure

Summary

An out-of-bounds read in Microsoft Office Excel allows an unauthorized attacker to disclose information over a network. Exploitation requires that a victim opens a crafted Excel file, for example one delivered by email or download. The impact is limited to confidentiality, and Microsoft rates the issue Important rather than Critical.

Affected

• Microsoft 365 Apps for Enterprise: 16.0.1 before the build listed at https://aka.ms/OfficeSecurityReleases (exact build unknown)
• Microsoft Office 365 for Mac: 1.0.0 before 16.115.26110837
• Microsoft Office LTSC 2021: 16.0.1 before 16.0.14334.21048
• Microsoft Office LTSC for Mac 2021: 16.0.1 before 16.115.26110837
• Microsoft Office LTSC 2024 and LTSC for Mac 2024, Office Online Server: listed only by a third-party tracker, not verified in the CNA record

Fixed in

• Microsoft 365 Apps for Enterprise: Unknown (verify on the MSRC page)
• Microsoft Office 365 for Mac: 16.115.26110837 or later
• Microsoft Office LTSC 2021: 16.0.14334.21048 or later
• Other products: Unknown (verify on the MSRC page)

Workaround

Unknown. No workaround or mitigation is documented in the sources reviewed. Updating is the fix.

Exploitation

• CISA SSVC: Exploitation none, Automatable no, Technical Impact partial
• CVSS temporal vector E:U (Unproven exploit maturity)
• Rapid7 rates it Exploitation Less Likely, with no public disclosure
• No public proof of concept found; no known in-the-wild exploitation of this CVE

Our recommendation

• Install the November 2026 Office security update on Microsoft 365 Apps, Office LTSC and Mac systems through the normal update channel.
• Verify the installed build number against the MSRC advisory, since the Microsoft 365 Apps fixed build could not be confirmed.
• Prioritise the Outlook NTLM vulnerability released the same day, which is known to be exploited in the wild.
• Consider blocking files from the internet and enabling Protected View as general hardening; their effect on this specific bug is not verified.

Public exploits iAttack code is publicly available (PoC on GitHub, Metasploit or Exploit-DB).

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

• Microsoft Security Response Center advisory CVE-2026-124178 msrc.microsoft.com
• CVE Record CVE-2026-124178 www.cve.org
• Microsoft 365 Apps security updates learn.microsoft.com
• Rapid7 Patch Tuesday November 2026 analysis www.rapid7.com
• Zero Day Initiative November 2026 security update review www.zerodayinitiative.com
• Strix CVE-2026-124178 (third-party tracker, unverified) www.strix.ai

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 11 Nov 2026, 12:02.