Microsoft Office Outlook: Remote code execution via heap-based buffer overflow
Summary
A heap-based buffer overflow in Microsoft Office Outlook allows an unauthorized remote attacker to execute code on the affected system. The CVSS base score is 8.8 (high), with high impact on confidentiality, integrity and availability. Exploitation requires user interaction, and the exact trigger is not stated in the sources reviewed. Microsoft 365 Apps for Enterprise, Office LTSC 2021 and Office LTSC 2024 are listed as affected.
Affected
- Microsoft 365 Apps for Enterprise: from 16.0.1, before the fixed build listed on the Microsoft Office security releases page
- 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
Fixed in
- Microsoft 365 Apps for Enterprise: Unknown. Check the Microsoft Office security releases page and the MSRC advisory.
- Microsoft Office LTSC 2021: 16.0.14334.21048
- Microsoft Office LTSC 2024: 16.0.17932.21140
- The fix is expected in the November 2026 Patch Tuesday cycle; this was not confirmed.
Workaround
Unknown. No workaround or mitigation is documented in the sources reviewed.
Exploitation
- Exploited in the wild: no evidence found.
- Public proof of concept: none found.
- CISA KEV: not listed according to one aggregator; not confirmed against CISA.
- The CVE record carries exploit code maturity E:U (unproven) and report confidence RC:C (confirmed).
- Not to be confused with CVE-2026-124194, a separate actively exploited Outlook spoofing flaw in the same release.
Our recommendation
- Treat as high priority for patching because of the CVSS score of 8.8.
- Verify the fixed build for each product on the MSRC advisory and install the update on all Microsoft 365 Apps and Office LTSC clients.
- Until patched, restrict attachments and macros from untrusted senders and use Protected View for files from untrusted sources. These are general controls, not CVE-specific mitigations.
- Monitor for Outlook crash reports and suspicious inbound email.
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-2026-123380 msrc.microsoft.com
- CVE.org record CVE-2026-123380 www.cve.org
- BleepingComputer: November 2026 Patch Tuesday www.bleepingcomputer.com
- Strix: CVE-2026-123380 (aggregator, not authoritative) 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 10 Nov 2026, 21:13.
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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