The vulnerability CVE-2025-68146 affects the tox-dev project's filelock Python library, which provides platform-independent file locking mechanisms. In versions before 3.20.1, a TOCTOU race condition exists during lock file creation on both Unix-like and Windows systems. The flaw occurs because filelock first checks if a lock file exists and then opens it with the O_TRUNC flag to truncate it if present. Between the existence check and the open call, an attacker with local filesystem access can create a symbolic link pointing to an arbitrary target file. When filelock opens the lock file path, it follows the symlink and truncates the target file, leading to corruption or data loss. This attack requires the ability to create symlinks, which is standard for non-privileged users on Unix systems and possible on Windows 10+ with Developer Mode enabled. The attack is facilitated when lock file paths are predictable, and exploitation typically succeeds within 1-3 attempts. Because filelock is widely used and included as a dependency in many Python projects, this vulnerability can cascade to numerous applications across multiple platforms. The vulnerability impacts integrity and availability of files but does not directly affect confidentiality. The issue is fixed in version 3.20.1 of filelock. Until upgrading, mitigations include switching to SoftFileLock (which uses different locking semantics and may not be suitable for all use cases), enforcing strict permissions (chmod 0700) on lock file directories to prevent unauthorized symlink creation, and monitoring these directories for suspicious symlinks. However, these mitigations only partially reduce risk, as the race condition remains exploitable. No known exploits in the wild have been reported as of publication.

For European organizations, this vulnerability poses a significant risk to the integrity and availability of critical files managed by Python applications that rely on the filelock library. Since filelock is used for coordinating access to shared resources, successful exploitation could lead to data corruption, loss of important files, or disruption of services that depend on reliable file locking. This could affect software development environments, automated deployment pipelines, configuration management, and other operational processes that use filelock for concurrency control. The local nature of the attack limits remote exploitation but insider threats, compromised endpoints, or malicious local users could leverage this vulnerability to cause damage. The cascading effect through dependent libraries increases the attack surface, potentially impacting a broad range of applications. For organizations handling sensitive or regulated data, such as financial institutions, healthcare providers, and critical infrastructure operators, file corruption or service disruption could lead to compliance violations, operational downtime, and reputational damage. The requirement for local access and symlink creation privileges somewhat limits the threat but does not eliminate it, especially in environments with multiple users or less restrictive permissions.

The primary mitigation is to upgrade all instances of the filelock library to version 3.20.1 or later, which contains the fix for the TOCTOU race condition. For environments where immediate upgrade is not feasible, organizations should: 1) Replace UnixFileLock and WindowsFileLock with SoftFileLock cautiously, understanding the different locking semantics and evaluating suitability for their use cases. 2) Restrict permissions on directories used for lock files to mode 0700 or equivalent, ensuring only trusted users can create files or symlinks there. 3) Implement monitoring and alerting for the creation of unexpected symbolic links within lock file directories to detect potential exploitation attempts. 4) Review and harden local user permissions to minimize the ability of untrusted users to create symlinks, especially on Windows systems by disabling Developer Mode if not required. 5) Audit dependent libraries and applications for usage of vulnerable filelock versions and coordinate patching across the software supply chain. 6) Incorporate file integrity monitoring on critical files to detect unexpected truncation or corruption. These steps collectively reduce the risk but do not fully eliminate it until the upgrade is applied.