CVE-2026-55012 – Microsoft Defender Remote Code Execution Vulnerability

An integer overflow and heap-based buffer overflow 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 is carried out 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 integer overflow and heap-based buffer overflow conditions, an attacker can execute arbitrary code on a target system. The attack requires no privileges but does require user interaction, such as convincing a user to download and 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 integer overflow or wraparound (CWE-190) and heap-based buffer overflow (CWE-122) within the Microsoft Malware Protection Engine. An attacker can trigger these memory handling flaws using specially crafted malicious content, resulting in arbitrary code execution. Although the vulnerability is categorized as Remote Code Execution, Microsoft clarifies that the attack is executed locally after user interaction, such as opening a malicious file obtained 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 widely deployed security component across enterprise environments. A successful attack against the malware protection engine could allow attackers to execute malicious code, compromise endpoints, install additional malware, steal sensitive information, or use the affected device as a launching point for further attacks. Because security software is a trusted component, vulnerabilities within it require prompt attention.

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 core security component used to protect Windows systems. Although Microsoft has not observed active exploitation and user interaction is required, successful exploitation could allow arbitrary code execution on protected endpoints. Prompt deployment of the official fix reduces the risk of compromise.