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Response snapshot Oct 27, 2026, 01:01 UTC

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VulnerabilityCVE-2026-112907

filelock: On Windows an oplock on a soft lock's marker holds SoftFileLock.acquire() past its timeout and blocks the stale break

On Windows the soft locks (`SoftFileLock`, `MarkerSoftFileLock`, `SoftFileLease`, their async forms, and a `FileLock` that fell back to a soft lock) read their marker by path. The marker is the lock file whose presence says a process holds the lock.

Published Updated

At a glance

Exploited
Not known
Severity
2.5 Low (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: filelock: 4.2.1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-567q-5qvw-pf92 (opens a new tab)

What happened

On Windows the soft locks (`SoftFileLock`, `MarkerSoftFileLock`, `SoftFileLease`, their async forms, and a `FileLock` that fell back to a soft lock) read their marker by path. The marker is the lock file whose presence says a process holds the lock. Each `acquire()` attempt that finds a marker opens it with `os.open` to read the holder's process id, and the lock renames and deletes a marker it judges stale, by path.

On Windows each of these calls waits for an oplock. An oplock (opportunistic lock) is a request by a process that holds a handle on a file to be told before another process opens, renames or deletes that file; the other process waits until the holder answers the break or closes its handle.

Another local account that can open the marker for reading can take a Level 1 oplock on it while no process holds it open, and leave the break unanswered. Every `acquire()` by the user running the lock (below: the lock user) then waits inside the open, past its `timeout`, `blocking=False` and `cancel_check`, and a stale marker under that oplock is not broken.

Mutual exclusion holds and the lock touches no other file. Affected versions are filelock 3.22.0 through 4.2.0 on Windows. The maintainers found this while reviewing the fix for the report Anthropic's OSS Scanner sent on 15 October 2026 (review round 4, finding F4); it is not among the scanner's findings.

Continue reading

## Severity

**CVSS 3.1 `CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L` scores 2.5 (Low).**

`AV:L` (local): the other account needs a logon on the same host. `AC:H` (high attack complexity): Windows grants a Level 1 or batch oplock only to the sole open handle on a file, so the marker must have no holder handle when the other account asks. That account cannot bring this about; it waits for a holder that died, closed its descriptor, or left a marker past its `lifetime`.

`PR:L` (low privileges): an ordinary account with read access to the marker through the directory's inherited ACL (access control list). `UI:N` (no user interaction). `S:U` (scope unchanged). `C:N` and `I:N`: that account can read the marker's content (process id and host name) anyway, and mutual exclusion holds.

`A:L` (partial availability impact): one thread, or the event loop with `run_in_executor=False`, waits until the holder answers; 10 s per attempt in the two-account runs, no crash. Metrics in doubt take the lower reading.

Classification from the review: **security, conditional** on an unheld marker that another account can open.

## Scope and preconditions

- Platform and classes: Windows only. `SoftFileLock`, `MarkerSoftFileLock`, `SoftFileLease`, `AsyncSoftFileLock` and the other async soft locks, and `FileLock` after its `ENOSYS` fallback to a soft lock. `WindowsFileLock` opens its lock file on a separate code path that this advisory does not cover. - Marker access: the soft locks create the marker with `os.

open`, which on Windows applies no more of `mode` than the read-only attribute (GHSA-8mx5-7m5m-r77g). The marker carries the lock directory's inheritable entries. Another account that those entries grant read access to new files can open the marker with a read handle that shares read, write and delete. - Unheld marker: no handle may be open on the marker when the other account requests the oplock.

That is a holder that died and left its marker, a holder that closed its descriptor, or a marker whose `lifetime` has passed. While the holder keeps its handle, the request fails. - Holder behavior: the other account requests the oplock through an overlapped `DeviceIoControl` and never acknowledges the break. Windows cancels a thread's pending request when the thread exits, so the holder thread stays alive.

A same-account process that answers slowly, such as an indexer, has the same effect for its answer time. - What the lock user sees: `SoftFileLock.acquire(timeout=1)` returned `Timeout` after 10.02 s with the second account's handle. In the same-account probe `os.open` with `O_RDONLY` was still running after 15 s and returned once the holder closed.

The retry loop consults `timeout`, `blocking=False` and `cancel_check` between attempts only. With `AsyncSoftFileLock(run_in_executor=False)` the event loop waits. - Stale break: `break_lock_file` renames and unlinks by path. Both calls wait in the same way, so a stale marker under such an oplock stays, and no process acquires the lock until the holder closes.

- Not affected: mutual exclusion, and the lock reads, writes and deletes no file other than the marker.

## Affected code

Scanned commit `b831496b0273`: the [`os.open` in `_read_lock_file`](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_soft.py#L323) ([function](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_soft.py#L312-L335)), called from [`_try_break_stale_lock`](https://github.

com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_soft.py#L107-L123) on every attempt that finds a marker, and from [`_unprobeable_holder`](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_soft.py#L137), [`pid`](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_soft.

py#L164) and [`is_lock_held_by_us`](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_soft.py#L179). [`break_lock_file`](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_util.py#L84-L122) renames at L116 and unlinks at L118 and L122, by path. [`_poll_until_acquired`](https://github.

com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_api.py#L1378-L1408) and [`_check_give_up`](https://github.com/tox-dev/filelock/blob/b831496b0273ecd11bf02b8d080ca8b673a538c1/src/filelock/_api.py#L1590-L1608) test the deadline, `blocking` and `cancel_check` only between attempts.

## Introduced

[e35

Affected package ranges reported by GitHub: filelock: >= 3.22.0, <= 4.2.0

Scores and how they were measured

CVSS base severity

2.5

Version 3.1As of

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-567q-5qvw-pf92 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-567q-5qvw-pf92, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 23). filelock: On Windows an oplock on a soft lock's marker holds SoftFileLock.acquire() past its timeout and blocks the stale break (CVE-2026-112907). https://kevwire.com/record/CVE-2026-112907

MLA

"filelock: On Windows an oplock on a soft lock's marker holds SoftFileLock.acquire() past its timeout and blocks the stale break (CVE-2026-112907)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-112907.

Chicago

KEVWire. "filelock: On Windows an oplock on a soft lock's marker holds SoftFileLock.acquire() past its timeout and blocks the stale break (CVE-2026-112907)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-112907.

Evidence summary (plain text)

filelock: On Windows an oplock on a soft lock's marker holds SoftFileLock.acquire() past its timeout and blocks the stale break (Oct 23, 2026).
On Windows the soft locks (`SoftFileLock`, `MarkerSoftFileLock`, `SoftFileLease`, their async forms, and a `FileLock` that fell back to a soft lock) read their marker by path. The marker is the lock file whose presence says a process holds the lock. Each `acquire()` attempt that finds a marker opens it with `os.open` to read the holder's process id, and the lock renames and deletes a marker it judges stale, by path. On Windows each of these calls waits for an oplock. An oplock (opportunistic lock) is a request by a process that holds a handle on a file to be told before another process opens, renames or deletes that file; the other process waits until the holder answers the break or closes its handle.

Another local account that can open the marker for reading can take a Level 1 oplock on it while no process holds it open, and leave the break unanswered. Every `acquire()` by the user running the lock (below: the lock user) then waits inside the open, past its `timeout`, `blocking=False` and `cancel_check`, and a stale marker under that oplock is not broken. Mutual exclusion holds and the lock touches no other file. Affected versions are filelock 3.22.0 through 4.2.0 on Windows. The maintainers found this while reviewing the fix for the report Anthropic's OSS Scanner sent on 15 October 2026 (review round 4, finding F4); it is not among the scanner's findings.
CVSS 2.5 (version 3.1; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-567q-5qvw-pf92).
Sources:
GitHub-reviewed advisory GHSA-567q-5qvw-pf92: https://github.com/advisories/GHSA-567q-5qvw-pf92
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-112907.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 20:55 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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