filelock has TOCTOU race condition that allows symlink attacks during lock file creation
filelock is a platform-independent file lock for Python. In versions prior to 3.20.1, 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. All users of filelock on Unix, Linux, macOS, and Windows systems are impacted. The vulnerability cascades to dependent libraries. The 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. The issue is fixed in version 3.20.1. 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; and/or monitor lock file directories for suspicious symlinks before running trusted applications. These workarounds provide only partial mitigation. The race condition remains exploitable. Upgrading to version 3.20.1 is strongly recommended.
- Published
- Dec 16, 2025
- Updated
- Jun 17, 2026
- EPSS
- 0.2% · 9th percentile
- CWE
- CWE-59
Sign in to follow · You’ll be notified if a followed record enters KEV, gets an exploit or is updated.
Report tools
Action score
26
Monitor
Low priority for now.
- CVSS
- 26 / 40 · 6.5 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 0 / 30 · 0.2%
CISA SSVC decision
- Exploitation
- none
- Automatable
- no
- Technical impact
- partial
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- 6.5
- CNA · GitHub_M
- 6.3
- 0.2 point gap
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Noroxi analysis
No Noroxi analysis for this record yet
We don't hand-write analysis for the hundreds of thousands of vulnerabilities in the database; that wouldn't be honest. For notable, high-impact vulnerabilities our team writes the mechanism, detection and remediation steps.
We use this product, ask for helpAffected systems
| Vendor | Product | CPE |
|---|---|---|
| tox-dev | filelock | cpe:2.3:a:tox-dev:filelock |
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- tox-dev filelockbefore 3.20.1
Versions reported by the vendor
Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.
tox-dev filelock
- < 3.20.1affected
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| Debian:12 | python-filelock | before 3.9.0-1+deb12u1 | 3.9.0-1+deb12u1 |
| Debian:13 | python-filelock | before 3.18.0-1+deb13u1 | 3.18.0-1+deb13u1 |
| Debian:14 | python-filelock | before 3.20.2-1 | 3.20.2-1 |
| openSUSE:Leap 16.0 | python-filelock | before 3.18.0-160000.3.1 | 3.18.0-160000.3.1 |
| openSUSE:Tumbleweed | python-filelock | before 3.20.2-1.1 | 3.20.2-1.1 |
| openSUSE:Tumbleweed | python-virtualenv | before 20.36.1-1.1 | 20.36.1-1.1 |
| PyPI | filelock | before 3.20.1 | 3.20.1 |
| SUSE:Linux Enterprise Module for Development Tools 15 SP7 | python-filelock | before 3.0.12-150100.3.6.1 | 3.0.12-150100.3.6.1 |
| SUSE:Linux Enterprise Module for Python 3 15 SP7 | python-filelock | before 3.12.2-150400.10.8.1 | 3.12.2-150400.10.8.1 |
| SUSE:Linux Enterprise Module for Python 3 15 SP7 | python-filelock | before 3.12.2-150400.10.8.1 | 3.12.2-150400.10.8.1 |
| SUSE:Linux Enterprise Server 16.0 | python-filelock | before 3.18.0-160000.3.1 | 3.18.0-160000.3.1 |
Same product
tox-dev: all recordsOther highest-scoring records for the same primary product.
- CVE-2026-22701filelock Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock21Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| tox-dev filelock | 3.20.1 · description | Vendor (CNA) |
| debian:python-filelock | 3.9.0-1+deb12u1 · Debian:12 | Package registry (OSV) |
| opensuse:python-filelock | 3.12.2-150400.10.8.1 · openSUSE:Leap 15.6 | Package registry (OSV) |
| opensuse:python-virtualenv | 20.36.1-1.1 · openSUSE:Tumbleweed | Package registry (OSV) |
| PyPI:filelock | 3.20.1 | Package registry (OSV) |
| suse:python-filelock | 3.0.12-150100.3.6.1 · SUSE:Linux Enterprise Module for Development Tools 15 SP7 | Package registry (OSV) |
| suse:python-filelock | 3.12.2-150400.10.8.1 · SUSE:Linux Enterprise Module for Python 3 15 SP7 | Package registry (OSV) |
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
No credits in the CNA record.
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
- CVE-2026-2270125 days apartfilelock Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock21Monitor
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
Technical details
Attack conditions
- Someone with local access to the system can trigger it.
- A low-privileged account is enough.
- No user action is required.
- Special conditions such as timing or configuration are required.
If successful
- Confidentiality
- low · data can be read
- Integrity
- high · data or configuration can be modified
- Availability
- high · the service can be disrupted
- Attack vector
- Local
- Attack complexity
- High
- Privileges required
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- Low
- Integrity impact
- High
- Availability impact
- High
Weakness class (CWE)
CWE-59 · Improper Link Resolution Before File Access ('Link Following')CVSS vector
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:H
nvd-primary
Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
Attack patterns (CAPEC)
ATT&CK techniques
- Obfuscated Files or Information: HTML SmugglingT1027.006
- Obfuscated Files or Information: Embedded PayloadsT1027.009
- Boot or Logon Autostart Execution:Shortcut ModificationT1547.009
- Hide Artifacts: Resource ForkingT1564.009
- Hijack Execution Flow: Executable Installer File Permissions WeaknessT1574.005
- Hijack Execution Flow: Services File Permissions WeaknessT1574.010
Change log
- Fix✗ → ✓
For records you follow, these changes also arrive as notifications. →
References
- github.com/tox-dev/filelock/commit/4724d7f8c3393ec1f048c93933e6e3e6ec321f0e
- github.com/tox-dev/filelock/pull/461
- github.com/tox-dev/filelock/releases/tag/3.20.1
Vendor advisories and official records. Exploit/PoC links are deliberately left out.