5.8

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3.1

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CVE-2026-112938

CVE-2026-112938: filelock: StrictSoftFileLock follows a symbolic link swapped in at <path>.filelock and creates claim files in a directory another local user chooses

filelock is a platform-independent file lock for Python. From 3.30.0 until 4.2.1, On POSIX platforms, StrictSoftFileLock and AsyncStrictSoftFileLock validate the coordination directory in _ensure_protocol_directory but later reach the claims directory through _open_directory by pathname, allowing a local account that can rename the coordination name to swap in a symbolic link between check and use during acquisition. That race can create and leave claim files in another writable directory or strand claims in a different strict lock so the second lock times out. Separately, the claims property and force_break follow a resting symbolic link without a race when the lock user inspects or recovers the lock. Claims inspection discloses a file name to the other account only if the resulting exception text or claim_name reaches that account through a shared log, CI output, or error report. A blind force_break recovery that removes live claims from another strict lock can allow two processes into that lock's critical section; this path reaches other strict locks, not arbitrary files. This issue is fixed in version 4.2.1.

(GitHub Advisory)

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CVE-2026-112938

CVE-2026-112938:

5.8

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
filelock	pip	>= 3.30.0, <= 4.2.0	4.2.1

Technical Details

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Root Cause Analysis

CVE-2026-112938 is a TOCTOU (time-of-check-to-time-of-use) symlink-following vulnerability in filelock's StrictSoftFileLock (and AsyncStrictSoftFileLock). The lock checks <path>.filelock with a single lstat call, then reaches <path>.filelock/claims by pathname for every subsequent operation (mkdir, scan, publish, link, unlink). _open_directory passes O_NOFOLLOW, but that flag only protects the final claims component; the kernel still follows a symlink in the earlier <path>.filelock component. An attacker who can rename <path>.filelock (e.g., in a non-sticky shared directory) uses renameat2(RENAME_EXCHANGE) to swap in a symlink between the lstat and the next mkdir, causing the lock user's process to create the claims directory and claim files inside an attacker-chosen directory. The fix (PR #839, commit bfa066d) replaces by-name resolution with descriptor-relative access: it opens <path>.filelock with O_NOFOLLOW and reaches claims relative to that descriptor via dir_fd/openat, and pins the claim directory during publication, release, and force_break. The vulnerable functions are those that performed by-name resolution of the claim directory: the acquire entry point (_acquire), the doorway (_attempt_doorway), the claims property, force_break, and the helper functions _ensure_protocol_directory, _open_directory, _read_existing_claims, _list_claim_names, _publish_record, _link_no_replace, _unlink_in_directory, _filesystem_now, and _require_exact_name. All of these exist in the vulnerable version (>= 3.30.0, <= 4.2.0) and were modified by the patch to use descriptor-relative access.

Vulnerable functions

StrictSoftFileLock._acquire

src/filelock/_strict.py

This is the entry point that performs the lstat check on the coordination directory and then reaches claims by pathname. The race between the lstat and the subsequent mkdir/scan/publish is the core of CVE-2026-112938.

StrictSoftFileLock._attempt_doorway

src/filelock/_strict.py

This function scans existing claims and publishes the caller's intent/held claims. In the vulnerable version it resolved the claim directory by pathname, so a swapped-in symlink at <path>.filelock redirected claim creation into an attacker-chosen directory.

StrictSoftFileLock.force_break

src/filelock/_strict.py

force_break() removes a named claim. In the vulnerable version it resolved the path by name with no check, so a planted symlink at <path>.filelock let an attacker cause the lock user to remove claims from a different strict lock, breaking mutual exclusion.

StrictSoftFileLock.claims

src/filelock/_strict.py

The claims property reads existing claims by pathname with no symlink check, so a resting symlink at <path>.filelock/claims redirects the scan into an attacker-chosen directory, leaking one file name per planted link.

StrictSoftFileLock._claim_directory

src/filelock/_strict.py

This property builds the claim directory path by string concatenation (Path(f'{_canonical(self.lock_file)}{_COORDINATION_SUFFIX}') / _CLAIM_DIRECTORY_NAME) with no symlink validation, which is the root of the by-name resolution used by claims and force_break.

_ensure_protocol_directory

src/filelock/_strict.py

This helper performs the lstat check and then mkdir by pathname. The TOCTOU gap between the lstat and the mkdir is the exact window the attacker exploits with renameat2(RENAME_EXCHANGE).

_open_directory

src/filelock/_strict.py

This helper opens the claim directory with O_NOFOLLOW, but because it receives the full path, the flag only protects the final component. A symlink at the intermediate <path>.filelock component is still followed, enabling the redirect.

_read_existing_claims

src/filelock/_strict.py

This function scans existing claims using directory.exists() and os.scandir(directory) by pathname, which follow a symlink in the intermediate component, redirecting the scan into an attacker-chosen directory.

_list_claim_names

src/filelock/_strict.py

This function lists claim names via os.scandir(directory) by pathname, following a symlink in the intermediate component.

_publish_record

src/filelock/_strict.py

This function creates the claim files (intent and held) by opening the claim directory by full path, so a symlink at <path>.filelock redirects file creation into an attacker-chosen directory.

_link_no_replace

src/filelock/_strict.py

This function creates the held claim via a hard link by pathname, following a symlink in the intermediate component.

_unlink_in_directory

src/filelock/_strict.py

This function unlinks claim files by pathname. In the vulnerable version, a symlink at <path>.filelock redirects the unlink, and when the name is back at the real directory the unlink gets ENOENT (counted as removed), stranding the claim file in the attacker-chosen directory.

_filesystem_now

src/filelock/_strict.py

This function creates a clock-probe file by pathname. A symlink at the claims component redirects the probe open, leaving one-byte dot-files in the attacker-chosen directory.

_require_exact_name

src/filelock/_strict.py

This function confirms a claim name with a by-name os.scandir, which follows a symlink in the intermediate component, enabling the force_break() redirect.

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CVE-2026-112938: filelock: StrictSoftFileLock follows a symbolic link swapped in at <path>.filelock and creates claim files in a directory another local user chooses

CVE-2026-112938:

5.8

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CVE-2026-112938: filelock: StrictSoftFileLock follows a symbolic link swapped in at <path>.filelock and creates claim files in a directory another local user chooses

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5.8

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CVE-2026-112938: filelock: StrictSoftFileLock follows a symbolic link swapped in at <path>.filelock and creates claim files in a directory another local user chooses

CVE-2026-112938:

5.8

-

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Technical Details

CVE-2026-112938: filelock: StrictSoftFileLock follows a symbolic link swapped in at <path>.filelock and creates claim files in a directory another local user chooses

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Vulnerable functions

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5.8

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