CVE-2026-55011 in Defender
Summary
by MITRE • 07/14/2026
Integer underflow (wrap or wraparound) in Microsoft Defender allows an unauthorized attacker to execute code locally.
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Analysis
by VulDB Data Team • 07/14/2026
Microsoft Defender contains an integer underflow vulnerability that occurs when processing certain input values, creating a condition where an unsigned integer wraps around to a much smaller value than expected. This flaw originates from inadequate bounds checking within the defender's code execution pathways, specifically affecting memory allocation or buffer management routines. The vulnerability manifests when a calculation involving unsigned integers results in a value that cannot be represented within the allocated data type, causing the value to wrap around to zero or a negative equivalent.
The technical implementation of this integer underflow creates a predictable pattern where attackers can manipulate input parameters to force the system into an unexpected state. When the vulnerable code path executes, it may attempt to allocate memory blocks based on the corrupted integer value, potentially leading to heap corruption or stack overflow conditions. This type of vulnerability falls under CWE-191 Integer Underflow (Wrap or Wraparound) which is classified as a fundamental arithmetic error that can be exploited for privilege escalation and code execution.
Attackers exploiting this vulnerability can craft malicious inputs that trigger the integer underflow condition, potentially allowing them to overwrite critical memory locations or manipulate program flow. The local code execution capability means that an attacker does not require network access or remote exploitation methods, making this particularly concerning as it can be leveraged from a user account with minimal privileges. This attack vector aligns with ATT&CK technique T1068, which covers Local Privilege Escalation through software vulnerabilities.
The operational impact of this vulnerability extends beyond simple code execution, as successful exploitation could allow attackers to bypass security controls implemented by Microsoft Defender itself. The system's integrity is compromised when an attacker can manipulate the defender's memory structures, potentially leading to complete system compromise or persistent backdoor access. This vulnerability affects all versions of Microsoft Defender that contain the affected code paths and represents a critical security gap in endpoint protection mechanisms.
Organizations should implement immediate mitigations including applying the latest security patches from Microsoft, monitoring for suspicious process behaviors that may indicate exploitation attempts, and implementing additional runtime protections such as address space layout randomization. The vulnerability demonstrates the importance of robust input validation and proper integer handling in security-critical software components. Security teams must also consider implementing behavioral monitoring solutions to detect anomalous patterns that could indicate exploitation attempts targeting this specific integer underflow condition.