CVE-2026-61371 in AVML
Summary
by MITRE • 07/15/2026
Microsoft AVML before 0.17.0 could follow a symlink when opening a destination output path on Unix, allowing truncation/overwrite of the symlink target. The destructive effect is performed at open-time via O_TRUNC, and can happen before full input validation completes (“truncation-before-validation”).
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Analysis
by VulDB Data Team • 07/15/2026
This vulnerability exists in Microsoft AVML version 0.17.0 and earlier on Unix systems where the application fails to properly validate file paths when handling symbolic links. The flaw occurs during the file opening process when the system follows a symlink to determine the destination output path, creating an opportunity for malicious actors to manipulate the target file through a race condition or path traversal attack. The vulnerability stems from a classic truncation-before-validation pattern where the system opens a file with O_TRUNC flag before performing complete input validation, allowing an attacker to control what gets truncated if they can manipulate the symlink target. This represents a fundamental security issue in Unix file system handling that aligns with CWE-22 Path Traversal and CWE-170 Improper Null Termination vulnerabilities.
The operational impact of this vulnerability is significant as it allows for arbitrary file truncation or overwrite operations on the symlink target, potentially enabling privilege escalation attacks or data corruption. Attackers could exploit this by creating a symbolic link to a critical system file and then triggering AVML to open that path, resulting in the truncation of important files such as configuration files, binaries, or log files. The timing aspect of this vulnerability is particularly dangerous because it occurs at open-time before full input validation completes, meaning there's no opportunity for the application to detect malicious intent during the validation phase. This type of vulnerability can be classified under ATT&CK technique T1059 Command and Scripting Interpreter where attackers manipulate file system operations to achieve their objectives.
The root cause lies in the improper handling of symbolic links during file creation operations, specifically when the application uses O_TRUNC flag without adequate path resolution or validation checks. This creates a window of opportunity for attackers to substitute the intended target file with a malicious symlink pointing to a different location. The vulnerability is particularly concerning because it bypasses normal access controls and validation mechanisms, as the system follows the symlink at open-time rather than performing path canonicalization before the truncation operation. Security practitioners should note that this issue demonstrates the importance of proper file system abstraction in Unix environments where symbolic links can be manipulated to redirect operations.
Mitigation strategies include implementing proper path validation routines that check for symbolic links before opening files, using more secure file creation patterns such as O_EXCL flag to prevent overwriting existing files, and ensuring all input paths are fully resolved and validated before any file system operations occur. Organizations should also implement monitoring for suspicious symlink creation activities and consider using restricted file access controls to limit the ability of unprivileged users to create symlinks that could be exploited. The fix requires updating AVML to version 0.17.0 or later where proper symlink handling has been implemented, and system administrators should verify that all instances of the application are updated to prevent exploitation. Additionally, implementing principle of least privilege and file system permissions can help limit the potential damage from this type of vulnerability while maintaining operational security standards.