filelock Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock

Vulnerability Summary

Title: Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock

Affected Component: filelock package - SoftFileLock class File: src/filelock/_soft.py lines 17-27 CWE: CWE-362, CWE-367, CWE-59

Description

A TOCTOU race condition vulnerability exists in the SoftFileLock implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly.

The vulnerability occurs in the _acquire() method between raise_on_not_writable_file() (permission check) and os.open() (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service.

Attack Scenario

1. Lock attempts to acquire on /tmp/app.lock
2. Permission validation passes
3. [RACE WINDOW] - Attacker creates: ln -s /tmp/important.txt /tmp/app.lock
4. os.open() tries to create lock file
5. Lock operates on attacker-controlled target file or fails

Impact

What kind of vulnerability is it? Who is impacted?

This is a Time-of-Check-Time-of-Use (TOCTOU) race condition vulnerability affecting any application using SoftFileLock for inter-process synchronization.

Affected Users:

• Applications using filelock.SoftFileLock directly
• Applications using the fallback FileLock on systems without fcntl support (e.g., GraalPy)

Consequences:

• Silent lock acquisition failure - applications may not detect that exclusive resource access is not guaranteed
• Denial of Service - attacker can prevent lock file creation by maintaining symlink
• Resource serialization failures - multiple processes may acquire "locks" simultaneously
• Unintended file operations - lock could operate on attacker-controlled files

CVSS v4.0 Score: 5.6 (Medium) Vector: CVSS:4.0/AV:L/AT:L/PR:L/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N

Attack Requirements:

• Local filesystem access to the directory containing lock files
• Permission to create symlinks (standard for regular unprivileged users on Unix/Linux)
• Ability to time the symlink creation during the narrow race window

Patches

Has the problem been patched? What versions should users upgrade to?

Yes, the vulnerability has been patched by adding the O_NOFOLLOW flag to prevent symlink following during lock file creation.

Patched Version: Next release (commit: 255ed068bc85d1ef406e50a135e1459170dd1bf0)

Mitigation Details:

• The O_NOFOLLOW flag is added conditionally and gracefully degrades on platforms without support
• On platforms with O_NOFOLLOW support (most modern systems): symlink attacks are completely prevented
• On platforms without O_NOFOLLOW (e.g., GraalPy): TOCTOU window remains but is documented

Users should:

• Upgrade to the patched version when available
• For critical deployments, consider using UnixFileLock or WindowsFileLock instead of the fallback SoftFileLock

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

For users unable to update immediately:

1. Avoid SoftFileLock in security-sensitive contexts - use UnixFileLock or WindowsFileLock when available (these were already patched for CVE-2025-68146)

2. Restrict filesystem permissions - prevent untrusted users from creating symlinks in lock file directories:

   bash

   chmod 700 /path/to/lock/directory

3. Use process isolation - isolate untrusted code from lock file paths to prevent symlink creation

4. Monitor lock operations - implement application-level checks to verify lock acquisitions are successful before proceeding with critical operations

References

Are there any links users can visit to find out more?

• Similar Vulnerability: CVE-2025-68146 (TOCTOU vulnerability in UnixFileLock/WindowsFileLock)
• CWE-362 (Concurrent Execution using Shared Resource): https://cwe.mitre.org/data/definitions/362.html
• CWE-367 (Time-of-check Time-of-use Race Condition): https://cwe.mitre.org/data/definitions/367.html
• CWE-59 (Improper Link Resolution Before File Access): https://cwe.mitre.org/data/definitions/59.html
• O_NOFOLLOW documentation: https://man7.org/linux/man-pages/man2/open.2.html
• GitHub Repository: https://github.com/tox-dev/filelock

Reported by: George Tsigourakos (@tsigouris007)