CVE-2026-55011 – Microsoft Defender Remote Code Execution Vulnerability

An integer underflow vulnerability in the Microsoft Malware Protection Engine allows an unauthorized attacker to execute code locally. Although the CVE is classified as a Remote Code Execution vulnerability, the attack itself occurs locally after a user interacts with malicious content. Microsoft has released an official fix for the affected Microsoft Malware Protection Engine.

CVSS Score: 7.8

SEVERITY: Critical

THREAT:
This vulnerability affects the Microsoft Malware Protection Engine used by Microsoft Defender. By exploiting an integer underflow condition, an attacker can trigger memory handling errors that may result in arbitrary code execution. The attack requires no privileges but does require user interaction, typically by convincing a user to open specially crafted malicious content. Successful exploitation allows code execution with the privileges of the current user.

EXPLOITS:
At the time of publication, the vulnerability has not been publicly disclosed and Microsoft has not detected active exploitation. Microsoft assesses exploitation as less likely. The CVSS v3.1 temporal metrics list exploit code maturity as Unproven, and the provided information does not confirm the existence of publicly available proof-of-concept exploit code.

TECHNICAL SUMMARY:
The vulnerability is caused by an integer underflow (wrap or wraparound) (CWE-191) within the Microsoft Malware Protection Engine. Improper handling of numeric values can lead to unexpected memory operations that allow arbitrary code execution. Microsoft notes that although the vulnerability is categorized as Remote Code Execution, the attack is executed locally after user interaction, such as opening a specially crafted malicious file delivered through social engineering.

EXPLOITABILITY:
The affected product is the Microsoft Malware Protection Engine. Exploitation requires no privileges but does require user interaction, typically involving a user opening specially crafted malicious content. The attack has low complexity and executes locally on the affected system.

BUSINESS IMPACT:
Microsoft Defender is a core security component used across enterprise environments. A successful attack against the malware protection engine could allow attackers to execute malicious code, compromise endpoints, install malware, steal sensitive information, or establish persistence for further attacks. Because organizations rely on Defender as a primary layer of endpoint protection, vulnerabilities affecting its protection engine should be addressed promptly.

WORKAROUND:
No mitigations or workarounds are available. Microsoft recommends applying the official fix for the Microsoft Malware Protection Engine.

URGENCY:
This vulnerability should be deployed urgently because it is rated Critical and affects the Microsoft Malware Protection Engine, a key security component protecting Windows systems. Although Microsoft has not observed active exploitation and user interaction is required, successful exploitation could result in arbitrary code execution on protected endpoints. Prompt deployment of the official fix is recommended.