Microsoft Office Outlook: Heap-based buffer overflow allows remote code execution
Summary
A heap-based buffer overflow in Microsoft Office Outlook allows an unauthenticated remote attacker to execute code on the affected system. Exploitation requires that a user interacts with a malicious message or file, which is most likely delivered by email. The vendor rates the impact as high for confidentiality, integrity and availability (CVSS 8.8, Microsoft's CNA score; NVD analysis is still pending).
Affected
- Microsoft 365 Apps for Enterprise: 16.0.1 up to the fixed build (exact build not stated in the CNA record)
- Microsoft Office LTSC 2021: 16.0.1 before 16.0.14334.21048
- Microsoft Office LTSC 2024: 16.0.0 before 16.0.17932.21140
- Outlook 2016, Office 2019 and Outlook for Mac: not listed in the record reviewed; check MSRC
Fixed in
- Microsoft Office LTSC 2021: 16.0.14334.21048
- Microsoft Office LTSC 2024: 16.0.17932.21140
- Microsoft 365 Apps for Enterprise: Unknown. See the Microsoft release page https://aka.ms/OfficeSecurityReleases
Workaround
- Unknown. No workaround was found; apply the security update.
Exploitation
- Microsoft's CVSS vector marks exploit code maturity as Unproven (E:U), meaning no exploit code was known at publication.
- No report of in-the-wild exploitation or public proof-of-concept code was found.
- Not to be confused with CVE-2026-124194, a separate Outlook flaw reported as exploited.
Our recommendation
- Check the MSRC advisory for the exact fixed Microsoft 365 Apps build and for any other affected Outlook or Office versions in your environment.
- Install the security updates for Microsoft 365 Apps, Office LTSC 2021 and Office LTSC 2024 as a priority.
- Remind users not to open unexpected attachments or links from external senders.
- Prioritise CVE-2026-124194 in the same update cycle.
Information from Microsoft (Patch Tuesday November 2026)
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide: CVE-2026-123376 msrc.microsoft.com
- CVE Record CVE-2026-123376 www.cve.org
- BleepingComputer: November 2026 Patch Tuesday www.bleepingcomputer.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, 21:16.
Original description (NVD)
Heap-based buffer overflow 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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