{"id":"CVE-2026-112939","kind":"vulnerability","aliases":["CVE-2026-112939"],"related":[],"title":"filelock: forged process ID in SoftFileLock marker lets a rival take the lock","description":"filelock is a platform-independent file lock for Python. From 3.22.0 until 4.2.1, SoftFileLock can trust a forged process identifier in a marker that an attacker can modify but cannot remove, allowing an authorized contender such as another process of the holder's account or root to act as a confused deputy, remove the live marker, and admit another holder. This issue is fixed in version 4.2.1.","summary":"Versions of the Python filelock package from 3.22.0 up to 4.2.1 let a process trust a forged process identifier in a SoftFileLock marker file. A local user who can modify that marker can cause the live lock to be removed and another holder admitted, breaking mutual exclusion.","severity":"medium","cvss":4.7,"epss":null,"exploited":false,"exploited_at":null,"products":[{"name":"tox-dev filelock","fixed":["4.2.1"],"affected":">= 3.22.0, < 4.2.1","ecosystem":"vendor"}],"ecosystems":["vendor"],"tags":["filelock","multi-user-systems","package","python","shared-hosts"],"not_affected_if":["filelock-os-backend-only","single-user-host","version-4.2.1-or-later","version-below-3.22.0"],"how_to_check":"- Check the installed version with `pip show filelock` or `python -c \"import filelock; print(filelock.__version__)\"`, and flag anything from `3.22.0` up to but not including `4.2.1`.\n- Find code that uses `SoftFileLock` (for example `grep -rn \"SoftFileLock\" .`) on shared hosts or in multi-user environments.","fix":"- Upgrade the Python package `filelock` to `4.2.1` or later, for example `pip install --upgrade \"filelock>=4.2.1\"`.\n- Pin the fixed version in requirements or lock files and rebuild.","status":"open","published_at":"2026-10-20T15:17:16+00:00","first_seen_at":"2026-10-23T21:57:20+00:00","triaged_at":"2026-10-25T04:35:12+00:00","updated_at":"2026-10-25T04:35:12+00:00","category":"package","confidence":0.8,"triage":{"at":"2026-10-25T04:35:12Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade the Python package `filelock` to `4.2.1` or later, for example `pip install --upgrade \"filelock>=4.2.1\"`.\n- Pin the fixed version in requirements or lock files and rebuild.","reach":"local","title":"filelock: forged process ID in SoftFileLock marker lets a rival take the lock","impact":"medium","summary":"Versions of the Python filelock package from 3.22.0 up to 4.2.1 let a process trust a forged process identifier in a SoftFileLock marker file. A local user who can modify that marker can cause the live lock to be removed and another holder admitted, breaking mutual exclusion.","category":"package","nicknames":[],"who_cares":["python","filelock","shared-hosts","multi-user-systems"],"confidence":0.8,"needs_auth":"yes","worst_case":"A local account removes a live lock and another holder enters the critical section.","analyst_note":"","how_to_check":"- Check the installed version with `pip show filelock` or `python -c \"import filelock; print(filelock.__version__)\"`, and flag anything from `3.22.0` up to but not including `4.2.1`.\n- Find code that uses `SoftFileLock` (for example `grep -rn \"SoftFileLock\" .`) on shared hosts or in multi-user environments.","needs_analyst":false,"not_affected_if":["single-user-host","fileLock-os-backend-only","version-below-3.22.0","version-4.2.1-or-later"],"who_is_affected":"Only projects that use SoftFileLock (the file-based lock that is not OS-level) on a host where another local account, including root, can write to the lock directory. Single-user machines and containers where only one trusted process runs are unlikely to be exposed. Projects that use the OS-level FileLock backend or are pinned below 3.22.0 can skip it.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000673,"input_tokens":3374,"output_tokens":672,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-112939':16A,19A '-2026':15A,18A '3.22.0':41B,92C '4.2.1':44B,94C,148C 'account':125C 'act':129C 'admit':77B,139C 'allow':113C 'anoth':75B,119C,140C 'attack':107C 'author':115C 'break':78B 'cannot':111C 'caus':67B 'confus':132C 'contend':116C 'cve':14A,17A 'deputi':133C 'dev':22A 'exclus':80B 'file':57B,87C 'filelock':1A,23A,24B,38B,81C 'fix':145C 'forg':2A,50B,99C 'holder':76B,123C,141C 'host':33B 'id':4A 'identifi':52B,101C 'independ':86C 'issu':143C 'let':8A,45B 'live':69B,136C 'local':59B 'lock':13A,70B,88C 'marker':7A,56B,65B,104C,137C 'modifi':63B,109C 'multi':26B 'multi-user-system':25B 'mutual':79B 'packag':29B,39B 'platform':85C 'platform-independ':84C 'process':3A,47B,51B,100C,120C 'python':30B,37B,90C 'remov':73B,112C,134C 'rival':10A 'root':127C 'share':32B 'shared-host':31B 'softfilelock':6A,55B,95C 'system':28B 'take':11A 'tox':21A 'tox-dev':20A 'trust':48B,97C 'user':27B,60B 'version':34B,147C","nicknames":[],"mitigations":[],"who_affected":"Only projects that use SoftFileLock (the file-based lock that is not OS-level) on a host where another local account, including root, can write to the lock directory. Single-user machines and containers where only one trusted process runs are unlikely to be exposed. Projects that use the OS-level FileLock backend or are pinned below 3.22.0 can skip it.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-20T15:17:16+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N","cvss_by":"security-advisories@github.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-112939","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-112939","severity":null,"published_at":"2026-10-20T15:17:16+00:00","modified_at":"2026-10-21T18:02:59+00:00","first_seen_at":"2026-10-23T21:57:20+00:00","fetched_at":"2026-10-23T22:28:58+00:00","note":null}],"documents":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}