# CVE-2026-55011 — Microsoft Malware Protection Engine

> As of 2026-10-08, CVE-2026-55011 is a HIGH-severity vulnerability in Microsoft Malware Protection Engine, CVSS v3.1 7.8, EPSS 0.4% (38.4th percentile). Threadlinqs Intelligence links 3 tracked threat campaigns to CVE-2026-55011, most recently “Microsoft July 2026 Patch Tuesday: 570 Flaws Fixed, 3 Zero-Days Including AD FS and SharePoint Privilege Escalation”.

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

## What is CVE-2026-55011?

Integer underflow (wrap or wraparound) in Microsoft Defender allows an unauthorized attacker to execute code locally.

The record classifies CVE-2026-55011 under weakness class [CWE-191](https://cwe.mitre.org/data/definitions/191.html). Its CVSS v3 base vector states that the flaw requires local access to the host, needs no prior authentication, needs a user to take an action first, and has high impact on confidentiality, integrity, availability. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 106 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 7.8 — HIGH (`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C`)
- **EPSS (FIRST):** 0.4% probability of exploitation in the next 30 days, higher than 38.4% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 5.4/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-07-14, last modified 2026-10-08

## Is CVE-2026-55011 being exploited?

It currently carries a trending score of 33 in the Threadlinqs vulnerability feed.

## Affected products and versions

- [Microsoft](https://intel.threadlinqs.com/vendors/microsoft): Malware Protection Engine

## How to fix CVE-2026-55011

No vendor patch reference has been recorded for CVE-2026-55011 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-2026-55011

3 tracked threats in the Threadlinqs corpus reference CVE-2026-55011, either in the campaign’s CVE list or as an indicator on the campaign record.

- [Microsoft July 2026 Patch Tuesday: 570 Flaws Fixed, 3 Zero-Days Including AD FS and SharePoint Privilege Escalation](https://intel.threadlinqs.com/threat/TL-2026-1324) — CRITICAL · 2026-07-14
- [Microsoft July 2026 Patch Tuesday: 570 Vulnerabilities Fixed, Including 2 Actively Exploited Zero-Days (CVE-2026-56164, CVE-2026-56155)](https://intel.threadlinqs.com/threat/TL-2026-1325) — CRITICAL · 2026-07-14
- [Microsoft July 2026 Patch Tuesday: Record 622 CVEs Include Two Actively Exploited Zero-Days in AD FS (CVE-2026-56155) and SharePoint Server (CVE-2026-56164)](https://intel.threadlinqs.com/threat/TL-2026-1331) — CRITICAL · 2026-07-14

## Sources

Enriched from CVE.org, FIRST EPSS, GitHub Security Advisories. 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-2026-55011)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-55011
Full detection coverage and IOCs for threats exploiting CVE-2026-55011 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp