# CVE-2026-78509 — Microsoft 365 Apps for Enterprise

> As of 2026-10-08, CVE-2026-78509 is a CRITICAL-severity vulnerability in Microsoft 365 Apps for Enterprise, CVSS v3.1 9.8, EPSS 0.9% (60.9th percentile). Threadlinqs Intelligence links 2 tracked threat campaigns to CVE-2026-78509, most recently “September 2026 Microsoft Patch Tuesday — Record 966+ Vulnerabilities with Two Actively Exploited Zero-Days (CVE-2026-81963, CVE-2026-85880) and Multiple Critical Wormable RCEs”.

**Last updated:** 2026-10-08

## What is CVE-2026-78509?

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

The record classifies CVE-2026-78509 under weakness class [CWE-122](https://intel.threadlinqs.com/cwe/CWE-122). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality, integrity, availability. 8 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 44 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 9.8 — CRITICAL (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C`)
- **EPSS (FIRST):** 0.9% probability of exploitation in the next 30 days, higher than 60.9% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 7.3/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-09-08, last modified 2026-10-08

## Is CVE-2026-78509 being exploited?

It currently carries a trending score of 47 in the Threadlinqs vulnerability feed.

## Affected products and versions

- [Microsoft](https://intel.threadlinqs.com/vendors/microsoft): 365 Apps for Enterprise, Office 2019, Office 365 for Mac, Office LTSC 2021, Office LTSC 2024, Office LTSC for Mac 2021, Office LTSC for Mac 2024, Word 2016

## How to fix CVE-2026-78509

No vendor patch reference has been recorded for CVE-2026-78509 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-78509

2 tracked threats in the Threadlinqs corpus reference CVE-2026-78509, either in the campaign’s CVE list or as an indicator on the campaign record.

- [September 2026 Microsoft Patch Tuesday — Record 966+ Vulnerabilities with Two Actively Exploited Zero-Days (CVE-2026-81963, CVE-2026-85880) and Multiple Critical Wormable RCEs](https://intel.threadlinqs.com/threat/TL-2026-2398) — CRITICAL · 2026-09-08
- [Microsoft September 2026 Patch Tuesday — 999 CVEs, 3 actively exploited zero-days (CVE-2026-85880, CVE-2026-81963, CVE-2026-85046)](https://intel.threadlinqs.com/threat/TL-2026-2407) — CRITICAL · 2026-09-08

## Sources

Enriched from CVE.org, FIRST EPSS. Last verified by Threadlinqs on 2026-10-10.

**Vendor advisory and patch**

- [msrc.microsoft.com — vendor advisory](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78509)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-78509
Full detection coverage and IOCs for threats exploiting CVE-2026-78509 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp