CVE-2026-22701: filelock Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-01-10 · updated 2026-06-17
Weakness: CWE-59

Description

filelock is a platform-independent file lock for Python. Prior to version 3.20.3, 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. This issue has been patched in version 3.20.3.

Affected products

tox-dev filelock: before 3.20.3 — fixed in 3.20.3
Pypi Filelock: before 3.20.3; 0.2.0, 0.2.1, 0.2.2, 1.0.0, 1.0.1, 1.0.2 (+74) — fixed in 3.20.3

How to fix

Upgrade to a fixed version: tox-dev filelock 3.20.3; Pypi Filelock 3.20.3. Check the vendor advisory for mitigations.

References

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