Microsoft Office Outlook: Remote code execution via integer overflow
Summary
Microsoft Office Outlook contains an integer overflow or wraparound flaw that allows remote code execution. According to the CVE record, Microsoft 365 Apps for Enterprise, Office LTSC 2021 and Office LTSC 2024 are affected before the fixed builds. An unauthenticated attacker can exploit it over the network, but attack complexity is high and user interaction is required, so a crafted message opened by a user is the likely attack path, which is not confirmed in the sources.
Affected
- Microsoft 365 Apps for Enterprise: from 16.0.1, before the build listed on https://aka.ms/OfficeSecurityReleases (exact build unknown)
- Microsoft Office LTSC 2021: from 16.0.1, before 16.0.14334.21048
- Microsoft Office LTSC 2024: from 16.0.0, before 16.0.17932.21140
- Office 2016, Office 2019, Outlook for Mac and Outlook mobile: Unknown
Fixed in
- Microsoft 365 Apps for Enterprise: November 2026 Office security update (exact build: check MSRC)
- Microsoft Office LTSC 2021: 16.0.14334.21048 or later
- Microsoft Office LTSC 2024: 16.0.17932.21140 or later
Workaround
Unknown.
Exploitation
- No known exploitation in the sources reviewed. Microsoft does not describe the issue as exploited.
- No public proof of concept found. Not listed in the CISA KEV catalog.
- CISA SSVC: exploitation none, automatable no, technical impact total.
Our recommendation
- Confirm the Microsoft 365 Apps and Office build and update channel on every client.
- Install the November 2026 Office security update and verify the exact fixed build on the MSRC page.
- Install the Outlook zero-day fix CVE-2026-124194 in the same cycle, as it is actively exploited.
- Monitor for public proof of concept or KEV addition and re-check NVD, which is still awaiting analysis.
Information from Microsoft (Patch Tuesday November 2026)
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- CVE Record CVE-2026-123378 www.cve.org
- Microsoft Security Update Guide CVE-2026-123378 msrc.microsoft.com
- Strix CVE-2026-123378 www.strix.ai
- Patch Tuesday November 2026 list (newsbeep) www.newsbeep.com
- Tenable: November 2026 Patch Tuesday (CVE-2026-124194) www.tenable.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 11 Nov 2026, 05:17.
Original description (NVD)
Integer overflow or wraparound in Microsoft Office Outlook 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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