CVE-2026-70190 in .NET
Summary
by MITRE • 10/14/2026
Improper link resolution before file access ('link following') in .NET allows an authorized attacker to elevate privileges locally.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability described constitutes a classic race condition known as Time of Check to Time of Use, specifically manifesting through improper link resolution prior to file access within the Microsoft .NET framework ecosystem. This flaw arises when the application performs security checks or validates permissions on a symbolic link target before actually opening the file descriptor for subsequent operations. In many operating systems, particularly Windows and Linux environments where .NET applications are deployed, symbolic links can be manipulated by users with write access to specific directories. If an attacker creates a malicious symlink pointing to a sensitive system file or configuration directory after the initial check but before the actual file operation occurs, the application may inadvertently operate on the unintended target. This discrepancy between the state verified during authorization and the state accessed during execution allows for unauthorized privilege escalation, as the process runs with elevated privileges associated with its original context rather than restricting access to only what was initially validated.
From a technical perspective, this issue aligns directly with CWE-59 Improper Link Resolution Before File Access, often referred to as link following or TOCTOU race conditions involving symbolic links. The core mechanism involves the application relying on static path information for security decisions without ensuring that the underlying file system object remains immutable during the interval between verification and usage. In .NET applications, this can occur when methods such as File.Open, FileStream constructors, or other I/O operations are used in a way that does not atomically verify permissions against the final inode or handle target. An authorized attacker with local access to the application's working directory or any path component leading to sensitive resources can exploit this window of opportunity by swapping the symlink target dynamically. This exploitation technique is categorized under MITRE ATT&CK tactic T1068 Exploitation for Privilege Escalation, specifically leveraging techniques that abuse system configuration or file permissions to gain higher-level access than originally granted.
The operational impact of this vulnerability is severe, as it enables a low-privileged user to achieve high-privilege execution on the host machine. Once an attacker successfully exploits the race condition, they can read, modify, or delete critical system files, inject malicious code into legitimate processes, or alter configuration settings that control application behavior and security boundaries. This effectively breaks the principle of least privilege, allowing lateral movement within a network if the compromised account is used to access other resources. The severity is further compounded by the fact that .NET applications often run with significant permissions depending on their deployment context, such as running under service accounts or integrated Windows authentication contexts, which can amplify the blast radius of a successful local exploit.
Mitigation strategies must focus on eliminating the race condition window and enforcing strict file access controls. Developers should avoid using symbolic links in security-sensitive paths whenever possible. If symlinks are unavoidable, applications must use atomic operations that verify permissions at the exact moment of file opening rather than beforehand. On Windows systems, utilizing CreateFile with appropriate flags to detect reparse points can help prevent unintended link following. Additionally, implementing strict input validation and ensuring that all path components are resolved securely using APIs designed for safe path handling is critical. Security teams should also enforce least-privilege principles by running applications in restricted environments or containers where the impact of local privilege escalation is contained. Regular code reviews focusing on file I/O operations and static analysis tools configured to detect CWE-59 patterns can help identify these vulnerabilities before deployment, ensuring that security checks remain synchronized with actual resource access states.