# CVE-2026-78505 — Microsoft 365 Apps for Enterprise

> As of 2026-09-09, CVE-2026-78505 is a HIGH-severity vulnerability in Microsoft 365 Apps for Enterprise, CVSS v3.1 8.8. Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2026-78505, most recently “Microsoft September 2026 Patch Tuesday — 999 CVEs, 3 actively exploited zero-days (CVE-2026-85880, CVE-2026-81963, CVE-2026-85046)”.

**Last updated:** 2026-09-09

## What is CVE-2026-78505?

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

The record classifies CVE-2026-78505 under weakness class [CWE-122](https://cwe.mitre.org/data/definitions/122.html). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs a user to take an action first, and has high impact on confidentiality, integrity, availability. 8 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 5 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 8.8 — HIGH (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C`)
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 4.4/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-09-08, last modified 2026-09-09

## Is CVE-2026-78505 being exploited?

No public exploitation evidence for CVE-2026-78505 is recorded in the sources Threadlinqs tracks — CISA KEV, Exploit-DB, public proof-of-concept repositories and ProjectDiscovery Nuclei — as of 2026-09-09. That is an absence of evidence in those feeds, not a guarantee that the vulnerability is unexploited.

## Affected products and versions

- **Microsoft**: 365 Apps for Enterprise, Office 2016, Office 2019, Office 365 for Mac, Office LTSC 2021, Office LTSC 2024, Office LTSC for Mac 2021, Office LTSC for Mac 2024

## How to fix CVE-2026-78505

No vendor patch reference has been recorded for CVE-2026-78505 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-2026-78505

1 tracked threat in the Threadlinqs corpus references CVE-2026-78505, either in the campaign’s CVE list or as an indicator on the campaign record.

- [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, GitHub Security Advisories. Last verified by Threadlinqs on 2026-09-09.

**Vendor advisory and patch**

- [msrc.microsoft.com — vendor advisory](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78505)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-78505
Full detection coverage and IOCs for threats exploiting CVE-2026-78505 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp