CVE-2026-102204 in Defender for Endpoint
Summary
by MITRE • 10/13/2026
Improper link resolution before file access ('link following') in Microsoft Defender for Endpoint allows an authorized attacker to elevate privileges locally.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified as improper link resolution before file access, commonly referred to as a symlink race condition or TOCTOU (Time-of-Check-to-Time-of-Use) flaw within the context of Microsoft Defender for Endpoint, represents a critical security deficiency in how the application handles symbolic links during its operational workflow. This specific class of vulnerabilities arises when an application checks the properties of a file path and subsequently performs an operation on that same path without ensuring that the underlying resource has remained unchanged between the check and the use. In this instance, Microsoft Defender for Endpoint fails to properly validate or lock the target of symbolic links before proceeding with file access operations. This oversight creates a window of opportunity where an attacker can manipulate the filesystem state by replacing a legitimate file reference with a symlink pointing to a sensitive system resource, such as a configuration file containing credentials, kernel modules, or other critical binaries required for privilege escalation.
From a technical perspective, this flaw is rooted in the lack of atomic operations when resolving paths that may involve symbolic links. When an authorized user creates a maliciously crafted directory structure containing symlinks pointing to high-privilege targets, they can exploit the timing gap inherent in non-atomic file handling routines. The attacker leverages their existing access rights to create these links and then triggers actions within Microsoft Defender for Endpoint that inadvertently follow them. Because the application does not verify that the inode or security descriptor of the target remains consistent with its initial check, it proceeds to read from or write to the symlinked destination as if it were a trusted file under normal control. This mechanism allows the attacker to redirect input/output operations toward protected areas of the operating system, effectively bypassing standard access controls and integrity checks designed to prevent unauthorized modifications.
The operational impact of this vulnerability is severe, primarily due to its potential for local privilege escalation. An authenticated user on the affected system can exploit this flaw to gain higher-level permissions than originally granted, potentially achieving SYSTEM or root-equivalent privileges depending on the context in which Microsoft Defender for Endpoint operates. Once elevated access is obtained, an attacker can install persistent backdoors, disable security monitoring tools, exfiltrate sensitive data, and move laterally across the network with minimal resistance. This undermines the core purpose of endpoint protection solutions by allowing adversaries to neutralize defensive mechanisms from within the trusted environment, thereby facilitating full system compromise without requiring external exploitation vectors or zero-day exploits against other components.
This vulnerability aligns closely with Common Weakness Enumeration (CWE) categories such as CWE-59: Improper Link Resolution Before File Access and CWE-367: Time-of-check Time-of-use Race Condition. These classifications highlight the fundamental failure in input validation and race condition mitigation that characterizes this defect. Furthermore, from a tactical standpoint related to the MITRE ATT&CK framework, this exploit technique falls under Privilege Escalation via Local Exploitation (T1068) and potentially involves techniques associated with Defense Evasion (T1562), as disabling or manipulating security software is often a precursor to broader malicious activity. The ability to escalate privileges locally using trusted applications like endpoint protection agents is particularly dangerous because it bypasses many perimeter-based defenses that assume internal trust boundaries are secure.
To mitigate this risk, immediate patching of Microsoft Defender for Endpoint components affected by this flaw is essential. Organizations should verify their current build versions against the latest security advisories released by Microsoft and apply updates promptly to close the race condition window in file handling routines. In addition to software remediation, administrators should enforce strict least-privilege principles on endpoints, ensuring that standard user accounts do not have write access to directories where symlinks could be created or modified without oversight. Implementing robust filesystem monitoring solutions can also help detect anomalous symlink creation patterns indicative of exploitation attempts. Furthermore, configuring application control policies such as Windows Defender Application Control (WDAC) can restrict the execution and file modification capabilities of untrusted processes, thereby reducing the attack surface available to local adversaries attempting to leverage this vulnerability for privilege escalation.