---
id: "CVE-2026-123380"
page: "https://infrastack.ai/live/CVE-2026-123380"
kind: "vulnerability"
aliases: ["CVE-2026-123380"]
title: "Microsoft Outlook: heap overflow lets an attacker run code over network"
severity: "high"
cvss: 8.8
epss: null
exploited: false
status: "open"
category: "other"
products: []
ecosystems: []
tags: ["microsoft-office", "other", "outlook", "windows"]
not_affected_if: []
published_at: "2026-11-10T19:20:41+00:00"
first_seen_at: "2026-11-10T19:25:12+00:00"
updated_at: "2026-11-10T21:33:17+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-123380", "first_seen_at": "2026-11-10T19:25:12+00:00"}, {"source": "analyst", "url": null, "first_seen_at": "2026-11-10T21:33:17+00:00"}]
exploit_sources: []
---

# Microsoft Outlook: heap overflow lets an attacker run code over network

Link for people: https://infrastack.ai/live/CVE-2026-123380 (this .md is for agents)

A heap-based buffer overflow in Microsoft Office Outlook lets an unauthorized attacker execute code over a network. The sources rate it high (CVSS 8.8) and do not describe the trigger or any user action needed.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

Check File > Account > About Outlook (Office Account) for your build and compare it to the fixed build listed on the MSRC page for CVE-2026-123380; apply the November 2026 Office security update (see Microsoft 365 Apps security release notes). Fixed build numbers: unknown, not read from a primary source.

## Timeline

- 2026-11-10T19:25:12+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-123380
- 2026-11-10T21:33:17+00:00 analyst — MSRC page not retrievable (empty via web-cli); NVD page empty. ZDI Nov 2026 review (fetched, truncated) did not show this CVE in the readable part; NextGuard lists it as Outlook RCE CVSS 8.8, Nov 9. No primary source read, so nothing verified; trigger, builds and fix unknown. Not the exploited Outlook zero-day (that is CVE-2026-124194). Needs revisit once MSRC page is reachable.

## From the sources

Heap-based buffer overflow in Microsoft Office Outlook allows an unauthorized attacker to execute code over a network.

---

_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_