Impact

A Time-of-Check-Time-of-Use (TOCTOU) race condition allows local attackers to corrupt or truncate arbitrary user files through symlink attacks. The vulnerability exists in both Unix and Windows lock file creation where filelock checks if a file exists before opening it with OTRUNC. An attacker can create a symlink pointing to a victim file in the time gap between the check and open, causing os.open() to follow the symlink and truncate the target file.

Who is impacted:

All users of filelock on Unix, Linux, macOS, and Windows systems. The vulnerability cascades to dependent libraries:

- virtualenv users: Configuration files can be overwritten with virtualenv metadata, leaking sensitive paths - PyTorch users: CPU ISA cache or model checkpoints can be corrupted, causing crashes or ML pipeline failures - poetry/tox users: through using virtualenv or filelock on their own.

Attack requires local filesystem access and ability to create symlinks (standard user permissions on Unix; Developer Mode on Windows 10+). Exploitation succeeds within 1-3 attempts when lock file paths are predictable.

Patches

Fixed in version 3.20.1.

Unix/Linux/macOS fix: Added ONOFOLLOW flag to os.open() in UnixFileLock.\acquire() to prevent symlink following.

Windows fix: Added GetFileAttributesW API check to detect reparse points (symlinks/junctions) before opening files in WindowsFileLock.\acquire().

Users should upgrade to filelock 3.20.1 or later immediately.

Workarounds

If immediate upgrade is not possible:

1. Use SoftFileLock instead of UnixFileLock/WindowsFileLock (note: different locking semantics, may not be suitable for all use cases) 2. Ensure lock file directories have restrictive permissions (chmod 0700) to prevent untrusted users from creating symlinks 3. Monitor lock file directories for suspicious symlinks before running trusted applications

Warning: These workarounds provide only partial mitigation. The race condition remains exploitable. Upgrading to version 3.20.1 is strongly recommended.

Technical Details: How the Exploit Works

The Vulnerable Code Pattern

Unix/Linux/macOS (src/filelock/unix.py:39-44):

python def acquire(self) -> None: ensuredirectoryexists(self.lockfile) openflags = os.ORDWR | os.OTRUNC # (1) Prepare to truncate if not Path(self.lockfile).exists(): # (2) CHECK: Does file exist? openflags |= os.OCREAT fd = os.open(self.lockfile, openflags, ...) # (3) USE: Open and truncate

Windows (src/filelock/windows.py:19-28):

python def acquire(self) -> None: raiseonnotwritablefile(self.lockfile) # (1) Check writability ensuredirectoryexists(self.lockfile) flags = os.ORDWR | os.OCREAT | os.OTRUNC # (2) Prepare to truncate fd = os.open(self.lockfile, flags, ...) # (3) Open and truncate

The Race Window

The vulnerability exists in the gap between operations:

Unix variant:

Time Victim Thread Attacker Thread ---- ------------- --------------- T0 Check: lockfile exists? → False T1 ↓ RACE WINDOW T2 Create symlink: lock → victimfile T3 Open lockfile with OTRUNC → Follows symlink → Opens victimfile → Truncates victimfile to 0 bytes! ☠️

Windows variant:

Time Victim Thread Attacker Thread ---- ------------- --------------- T0 Check: lockfile writable? T1 ↓ RACE WINDOW T2 Create symlink: lock → victimfile T3 Open lockfile with OTRUNC → Follows symlink/junction → Opens victimfile → Truncates victimfile to 0 bytes! ☠️

Step-by-Step Attack Flow

1. Attacker Setup:

python Attacker identifies target application using filelock lockpath = "/tmp/myapp.lock" # Predictable lock path victimfile = "/home/victim/.ssh/config" # High-value target

2. Attacker Creates Race Condition:

python import os import threading

def attackerthread(): # Remove any existing lock file try: os.unlink(lockpath) except FileNotFoundError: pass

# Create symlink pointing to victim file os.symlink(victimfile, lockpath) print(f"[Attacker] Created: {lockpath} → {victimfile}")

Launch attack threading.Thread(target=attackerthread).start()

3. Victim Application Runs:

python from filelock import UnixFileLock

Normal application code lock = UnixFileLock("/tmp/myapp.lock") lock.acquire() # ← VULNERABILITY TRIGGERED HERE At this point, /home/victim/.ssh/config is now 0 bytes!

4. What Happens Inside os.open():

On Unix systems, when os.open() is called:

c // Linux kernel behavior (simplified) int open(const char pathname, int flags) { struct file f = pathlookup(pathname); // Resolves symlinks by default!

if (flags & OTRUNC) { truncatefile(f); // ← Truncates the TARGET of the symlink }

return filedescriptor; }

Without ONOFOLLOW flag, the kernel follows the symlink and truncates the target file.

Why the Attack Succeeds Reliably

Timing Characteristics:

- Check operation (Path.exists()): ~100-500 nanoseconds - Symlink creation (os.symlink()): ~1-10 microseconds - Race window: ~1-5 microseconds (very small but exploitable) - Thread scheduling quantum: ~1-10 milliseconds

Success factors:

1. Tight loop: Running attack in a loop hits the race window within 1-3 attempts 2. CPU scheduling: Modern OS thread schedulers frequently context-switch during I/O operations 3. No synchronization: No atomic file creation prevents the race 4. Symlink speed: Creating symlinks is extremely fast (metadata-only operation)

Real-World Attack Scenarios

Scenario 1: virtualenv Exploitation

python Victim runs: python -m venv /tmp/myenv Attacker racing to create: os.symlink("/home/victim/.bashrc", "/tmp/myenv/pyvenv.cfg")

Result: /home/victim/.bashrc overwritten with: home = /usr/bin/python3 include-system-site-packages = false version = 3.11.2 ← Original .bashrc contents LOST + virtualenv metadata LEAKED to attacker

Scenario 2: PyTorch Cache Poisoning

python Victim runs: import torch PyTorch checks CPU capabilities, uses filelock on cache Attacker racing to create: os.symlink("/home/victim/.torch/compiledmodel.pt", "/home/victim/.cache/torch/cpuisacheck.lock")

Result: Trained ML model checkpoint truncated to 0 bytes Impact: Weeks of training lost, ML pipeline DoS

Why Standard Defenses Don't Help

File permissions don't prevent this:

- Attacker doesn't need write access to victimfile - os.open() with OTRUNC follows symlinks using the victim's permissions - The victim process truncates its own file

Directory permissions help but aren't always feasible:

- Lock files often created in shared /tmp directory (mode 1777) - Applications may not control lock file location - Many apps use predictable paths in user-writable directories

File locking doesn't prevent this:

- The truncation happens during the open() call, before any lock is acquired - fcntl.flock() only prevents concurrent lock acquisition, not symlink attacks

Exploitation Proof-of-Concept Results

From empirical testing with the provided PoCs:

Simple Direct Attack (filelocksimplepoc.py):

- Success rate: 33% per attempt (1 in 3 tries) - Average attempts to success: 2.1 - Target file reduced to 0 bytes in \<100ms

virtualenv Attack (weaponizedvirtualenv.py):

- Success rate: ~90% on first attempt (deterministic timing) - Information leaked: File paths, Python version, system configuration - Data corruption: Complete loss of original file contents

PyTorch Attack (weaponizedpytorch.py):

- Success rate: 25-40% per attempt - Impact: Application crashes, model loading failures - Recovery: Requires cache rebuild or model retraining

Discovered and reported by: George Tsigourakos (@tsigouris007)

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 raiseonnotwritablefile() (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 ONOFOLLOW flag to prevent symlink following during lock file creation.

Patched Version: Next release (commit: 255ed068bc85d1ef406e50a135e1459170dd1bf0)

Mitigation Details: - The ONOFOLLOW flag is added conditionally and gracefully degrades on platforms without support - On platforms with ONOFOLLOW support (most modern systems): symlink attacks are completely prevented - On platforms without ONOFOLLOW (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 - ONOFOLLOW documentation: https://man7.org/linux/man-pages/man2/open.2.html - GitHub Repository: https://github.com/tox-dev/filelock

---

Reported by: George Tsigourakos (@tsigouris007)