CVE-2026-108135 in Windows
Summary
by MITRE • 10/14/2026
Improper link resolution before file access ('link following') in Windows Backup Engine 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 prior to file access within the Microsoft Windows Backup Engine represents a critical class of security flaw known as path traversal or symbolic link following. This specific issue arises from a failure by the application to properly validate and resolve file system links before performing operations on them, such as reading, writing, or deleting files. In typical software development practices, applications must ensure that any user-supplied input representing a file path is canonicalized and verified against an allowlist of permitted directories. When this validation step is omitted or implemented incorrectly, the application may inadvertently follow symbolic links or junction points created by an attacker to redirect operations into sensitive system areas outside the intended scope.
From a technical perspective, Windows allows for the creation of symbolic links that point to arbitrary locations on the file system. If the Backup Engine processes these links without verifying their final destination against security boundaries, it can be tricked into overwriting or modifying critical operating system files, configuration settings, or other protected resources. This behavior is particularly dangerous because backup operations often run with elevated privileges to ensure they have access to all necessary data for a complete system image. An attacker who has already gained local access and authenticated credentials can exploit this race condition or logic error to manipulate the file paths processed by the engine, effectively bypassing standard permission checks that would otherwise prevent unauthorized modifications.
The operational impact of this vulnerability is severe, primarily due to its potential for local privilege escalation. By exploiting improper link resolution, an authorized attacker can overwrite system binaries, inject malicious code into trusted processes, or modify critical configuration files such as those governing authentication mechanisms. This capability allows the attacker to gain higher-level access rights than originally granted, potentially achieving SYSTEM level privileges on the affected machine. Once elevated privileges are obtained, the attacker has unrestricted control over the compromised host, enabling them to install backdoors, exfiltrate sensitive data, disable security logging, and pivot further into the network infrastructure.
This vulnerability aligns with Common Weakness Enumeration (CWE) category CWE-59: Improper Link Resolution Before File Access, which describes flaws where a program uses user-supplied input in constructing file paths but fails to ensure that the resolved path is within an expected directory structure. Furthermore, from a tactical standpoint related to the MITRE ATT&CK framework, this flaw facilitates techniques associated with Privilege Escalation via Local Exploitation and potentially Defense Evasion through manipulation of system files. The exploitation vector typically involves creating malicious symbolic links in directories accessible to low-privileged users and then triggering the backup engine to process these paths during a scheduled or manual backup operation.
Mitigation strategies for this vulnerability focus on both immediate patching and long-term architectural improvements. Organizations must apply the latest security updates provided by Microsoft, which address the underlying logic errors within the Windows Backup Engine codebase. In addition to applying patches, system administrators should enforce strict file permission policies that restrict the creation of symbolic links or junction points in directories accessible to standard users where possible. Implementing application whitelisting and monitoring for unusual file system activity can also help detect attempts to exploit this flaw before significant damage occurs. Regular audits of backup configurations and ensuring that backup processes run with the minimum necessary privileges, rather than full administrative rights unless absolutely required, further reduces the attack surface associated with such high-privilege operations.