CVE-2026-112911VulnerabilityPublished 28 Oct 2026

filelock: StrictSoftFileLock and SoftReadWriteLock adopt a protocol directory another user created first, losing

Affects filelockSource GCVE Database

Authoritative record:CVE-2026-112911· Medium 6.3

Not known to be exploited · CVSS 6.3 Medium : upgrade to 4.2.1 when convenient.

Data as of 28 Oct 2026, 22:12 UTC

CVSS 3.1 base score

6.3Medium

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

Patch: Available – fixed in 4.2.1

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 20 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: 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 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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 filelock to 4.2.1.

Affected software

Product	Affected	Fixed in
PyPIfilelock	Affected >=3.30.0 <4.2.1	Fixed in 4.2.1

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Description

`StrictSoftFileLock`, `AsyncStrictSoftFileLock`, `SoftReadWriteLock` and `AsyncSoftReadWriteLock` keep their whole ownership state as files in a directory derived from the lock path (`<path>.filelock/claims`, `<path>.rw/{gen,holders}`). When that directory already exists they adopt it after checking only that it is a real directory, not who owns it. Another local user who can create names in the lock's directory can create the tree before the first acquire on that path by the user running the lock (below: the lock user). In `/tmp`, `/var/tmp` or another sticky shared directory any user can do that; the sticky bit is a directory flag that lets only a file's owner remove or rename it. The other user then owns the tree. They can unlink the lock user's claims while the lock user holds the lock, so a second process of the lock user, root included, enters the same critical section and neither process sees an error. Against `SoftReadWriteLock` the same position also admits two writers with `on_compromise` silent. It redirects the lock user's own creates and renames into a directory the other user picks (the scanner replaced a repository's `.git/HEAD` with a filelock record written with the lock user's privileges). It also keeps the lock user's acquire or release running past any `timeout`. Affected versions are filelock 3.30.0 through 4.2.0 (finding `ANT-2026-Q5VK5JEE`). ## Severity **CVSS 3.1 `CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H` scores 6.3 (Medium).** `AV:L` (local): the other user needs an account on the same host. `PR:L` (low privileges): an ordinary account is enough. `UI:N` (no user interaction). `AC:H` (high attack complexity): the other user must create the protocol directory before the lock user's first acquire on that path. That depends on the path being new, or on a reboot, a tmp cleaner or the documented `<path>.rw` recovery having removed its tree; once planted, the rest needs no race. `I:H` (high integrity impact): two holders write whatever the lock protects, and for `SoftReadWriteLock` the lock user's process creates, replaces or deletes files named `HEAD`, `epoch` and `0000000000000000000N` in any directory the lock user can write. `A:H` (high availability impact): the owner of `<path>.rw` can make the lock user's `read_lock(timeout=1)` spin at full CPU without end, cost 22 s per acquire, or pin 1 MiB of memory per planted file until a heartbeat thread dies of `MemoryError`. `C:N` (no confidentiality impact). `S:U` (scope unchanged). Classification from the review: **security, conditional** on the other user winning initial ownership of the protocol directory. The scanner rated it high (CWE-283). With `AC:L` the same impacts would score 7.1. ## Scope and preconditions - The other user can create names in the directory that holds the lock. In a sticky `1777` directory such as `/tmp` any local user can. The sticky bit stops that user from renaming or removing a directory the lock user created, but does nothing for one the other user created first. - The other user gets there first: `<path>.filelock` (strict) or `<path>.rw` (soft read-write) must not exist when the lock user's process first acquires. `_strict.py` does not remove the directory at any point, so a path the lock user has already used stays safe for as long as the tree exists. The exposure is a new lock path, or one whose tree a reboot, a tmp cleaner, a cache purge or the recovery described in `docs/how-to.rst` removed. - For the strict lock the planted directory must be writable by the lock user (`0777`); a planted `0755` directory makes the lock user's acquire fail with `PermissionError` instead. Planting only the outer `<path>.filelock` is enough. The lock user creates `claims` inside it, and the other user, as owner of the parent, renames that `claims` aside and creates an empty one. - The Linux `fs.protected_regular`, `fs.protected_symlinks`, `fs.protected_hardlinks` and `fs.protected_fifos` sysctls (kernel settings, all enabled in the scanner's image) do not interfere. They cover regular files, symlinks, hard links and FIFOs, not directories. The `SoftReadWriteLock` symlink redirect below also passes `fs.protected_symlinks=1`, because the symlink (a file that points at another path) sits in the other user's own non-sticky directory. - The affected classes are `StrictSoftFileLock` and `AsyncStrictSoftFileLock` (which inherits `_acquire`), and `SoftReadWriteLock` and `AsyncSoftReadWriteLock` with default options. `FileLock` gives way to the same position (when the other user creates the lock file first with mode `0666` and unlinks it during a hold, a second lock user process acquires), so the strict lock is not weaker than the native one here. This advisory reports it because `StrictSoftFileLock` is the lock documented as fail-closed and already raises `SoftFileLockProtocolError` for a symlink or a regular file at the same names, while it uses a directory that belongs to someone else without any signal. The `SoftReadWri

CWE-283Unverified Ownership (opens in a new tab)
Base weaknessOWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The product does not properly verify that a critical resource is owned by the proper entity.
Architecture and Design · Operation
Very carefully manage the setting, management, and handling of privileges. Explicitly manage trust zones in the software.
Architecture and Design · Separation of Privilege
Consider following the principle of separation of privilege. Require multiple conditions to be met before permitting access to a system resource.
OWASP Cheat Sheets
Docker Security (opens in a new tab)
Third Party Javascript Management (opens in a new tab)
Virtual Patching (opens in a new tab)
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V1.4.4 (opens in a new tab)
V1.4.5 (opens in a new tab)
V1.10.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-283

Technical details

References

Community reviews

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