CVE-2026-112940

filelock is a platform-independent file lock for Python. From 3.20.4 until 4.2.1, ReadWriteLock let SQLite reopen the database and side files by name; on POSIX systems an attacker with rename access in a non-sticky parent can swap those names to cause overwrite or unlink operations, while a writable adjacent hot journal is additionally required for attacker-chosen content overwrites. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 3.20.4 until 4.2.1, ReadWriteLock let SQLite reopen the database and side files by name; on POSIX systems an attacker with rename access in a non-sticky parent can swap those names to cause overwrite or unlink operations, while a writable adjacent hot journal is additionally required for attacker-chosen content overwrites. This issue is fixed in version 4.2.1.

ReadWriteLock and AsyncReadWriteLock (the SQLite-backed reader/writer lock) hand SQLite a name they do not control. On Linux the lock validates its database through a descriptor (the number the kernel gives a process for an open file). It then connects SQLite to /proc/self/fd/<n>, a Linux path that names the process's own open file n. SQLite turns that path back into a plain path and reopens it by name, following symlinks (a symlink is a file that points at another path). On every POSIX route SQLite also acts on -journal, -wal and -shm files it finds beside that name. Those are SQLite's side files: the rollback journal (pages saved to undo an interrupted write), the write-ahead log or WAL (changes kept outside the main file), and the shared-memory index for the WAL.

Another local user who can create or rename entries in the directory that holds the lock database can turn this against the user running the lock (below: the lock user). Such a directory is a 0777 or group-writable cache, build or project directory without the sticky bit (a directory flag that lets only a file's owner remove or rename it). That other user can make the lock user's ordinary read_lock() or write_lock() write bytes of the other user's choosing into the lock user's own files. The same position lets the other user make that call create or unlink files of the lock user, admit a reader beside a writer, or kill the holder. All of this runs with the lock user's privileges. In a sticky directory such as /tmp a planted -wal file makes every acquire fail. Affected versions are filelock 3.20.4 through 4.2.0 (the /proc/self/fd redirect since 4.0.9, the side-file variants since 3.20.4).

Anthropic's OSS Scanner reported this in private as ANT-2026-KMJ2575J [high, CWE-363]: ReadWriteLock: SQLite still follows planted symlinks, overwriting files.

Severity

CVSS 3.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H scores 7.1 (High). 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:L (low attack complexity): the planted-journal variant needs no race, and the redirect won 77 of 77 attempts. I:H (high integrity impact): the other user writes bytes of their choosing into files of the lock user and unlinks files of their choosing. A:H (high availability impact): the other user makes every acquire fail or kills the lock holder. C:N (no confidentiality impact): the other user learns nothing. S:U (scope unchanged): the effect stays within the lock user's own privileges.

Classification from the external review: conditional security, a cross-user effect that exists only when another user can change names in the lock database's directory. The review notes that the 77/77 inotify result (inotify is a Linux interface that reports file events) is stronger evidence than a theoretical TOCTOU (a time-of-check to time-of-use race). The scanner rated it high. The review accepts the mechanism and narrows it to that precondition.

Scope and preconditions

• The other party is a different local account that can create or rename entries in the directory that holds the lock database. That is a 0777 directory or a group-writable one (including a setgid 2775 project directory shared with that account), in each case without the sticky bit. The lock user's lock must be able to open the database read-write, so a database the other user creates is 0666. filelock itself creates the database 0600.
• Sticky directories (/tmp, /var/tmp, any 1777 directory) narrow the finding rather than exclude it. The redirect and the journal replay do not reach a database the lock user owns there (tested with fs.protected_regular=2). What remains is a planted non-empty <db>-wal (any mode; a directory or FIFO works too) that the lock user cannot unlink. It makes every acquire raise sqlite3.OperationalError: disk I/O error. A hard link (a…

filelock is a platform-independent file lock for Python. From 3.20.4 until 4.2.1, ReadWriteLock let SQLite reopen the database and side files by name; on POSIX systems an attacker with rename access in a non-sticky parent can swap those names to cause overwrite or unlink operations, while a writable adjacent hot journal is additionally required for attacker-chosen content overwrites. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 3.20.4 until 4.2.1, ReadWriteLock let SQLite reopen the database and side files by name; on POSIX systems an attacker with rename access in a non-sticky parent can swap those names to cause overwrite or unlink operations, while a writable adjacent hot journal is additionally required for attacker-chosen content overwrites. This issue is fixed in version 4.2.1.

filelock is a platform-independent file lock for Python. From 3.20.4 until 4.2.1, ReadWriteLock let SQLite reopen the database and side files by name; on POSIX systems an attacker with rename access in a non-sticky parent can swap those names to cause overwrite or unlink operations, while a writable adjacent hot journal is additionally required for attacker-chosen content overwrites. This issue is fixed in version 4.2.1.

ReadWriteLock and AsyncReadWriteLock (the SQLite-backed reader/writer lock) hand SQLite a name they do not control. On Linux the lock validates its database through a descriptor (the number the kernel gives a process for an open file). It then connects SQLite to /proc/self/fd/<n>, a Linux path that names the process's own open file n. SQLite turns that path back into a plain path and reopens it by name, following symlinks (a symlink is a file that points at another path). On every POSIX route SQLite also acts on -journal, -wal and -shm files it finds beside that name. Those are SQLite's side files: the rollback journal (pages saved to undo an interrupted write), the write-ahead log or WAL (changes kept outside the main file), and the shared-memory index for the WAL.

Another local user who can create or rename entries in the directory that holds the lock database can turn this against the user running the lock (below: the lock user). Such a directory is a 0777 or group-writable cache, build or project directory without the sticky bit (a directory flag that lets only a file's owner remove or rename it). That other user can make the lock user's ordinary read_lock() or write_lock() write bytes of the other user's choosing into the lock user's own files. The same position lets the other user make that call create or unlink files of the lock user, admit a reader beside a writer, or kill the holder. All of this runs with the lock user's privileges. In a sticky directory such as /tmp a planted -wal file makes every acquire fail. Affected versions are filelock 3.20.4 through 4.2.0 (the /proc/self/fd redirect since 4.0.9, the side-file variants since 3.20.4).

Anthropic's OSS Scanner reported this in private as ANT-2026-KMJ2575J [high, CWE-363]: ReadWriteLock: SQLite still follows planted symlinks, overwriting files.

Severity

CVSS 3.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H scores 7.1 (High). 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:L (low attack complexity): the planted-journal variant needs no race, and the redirect won 77 of 77 attempts. I:H (high integrity impact): the other user writes bytes of their choosing into files of the lock user and unlinks files of their choosing. A:H (high availability impact): the other user makes every acquire fail or kills the lock holder. C:N (no confidentiality impact): the other user learns nothing. S:U (scope unchanged): the effect stays within the lock user's own privileges.

Classification from the external review: conditional security, a cross-user effect that exists only when another user can change names in the lock database's directory. The review notes that the 77/77 inotify result (inotify is a Linux interface that reports file events) is stronger evidence than a theoretical TOCTOU (a time-of-check to time-of-use race). The scanner rated it high. The review accepts the mechanism and narrows it to that precondition.

Scope and preconditions

• The other party is a different local account that can create or rename entries in the directory that holds the lock database. That is a 0777 directory or a group-writable one (including a setgid 2775 project directory shared with that account), in each case without the sticky bit. The lock user's lock must be able to open the database read-write, so a database the other user creates is 0666. filelock itself creates the database 0600.
• Sticky directories (/tmp, /var/tmp, any 1777 directory) narrow the finding rather than exclude it. The redirect and the journal replay do not reach a database the lock user owns there (tested with fs.protected_regular=2). What remains is a planted non-empty <db>-wal (any mode; a directory or FIFO works too) that the lock user cannot unlink. It makes every acquire raise sqlite3.OperationalError: disk I/O error. A hard link (a…

filelock is a platform-independent file lock for Python. From 3.20.4 until 4.2.1, ReadWriteLock let SQLite reopen the database and side files by name; on POSIX systems an attacker with rename access in a non-sticky parent can swap those names to cause overwrite or unlink operations, while a writable adjacent hot journal is additionally required for attacker-chosen content overwrites. This issue is fixed in version 4.2.1.

Root has patched CVE-2026-112940 in the filelock package for Root:PyPI. Multiple fixed versions available.