{"id":"CVE-2026-112909","kind":"vulnerability","aliases":["CVE-2026-112909"],"related":[],"title":"filelock: Unix lock path can truncate or re-mode a local file","description":"filelock is a platform-independent file lock for Python. Prior to 4.2.1, UnixFileLock can truncate a regular file renamed onto the lock path when another local user has actual rename permission, or hard-linked there under a permissive hard-link policy; when an explicit mode is requested, affected versions from 3.10.2 can also change that file's mode. This issue is fixed in version 4.2.1.","summary":"filelock's UnixFileLock can truncate a regular file that another local user renamed or hard-linked onto the lock path before 4.2.1. When an explicit mode is requested, affected versions from 3.10.2 can also change that file's permissions.","severity":"medium","cvss":6.1,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["package","python","shared-hosts","unix"],"not_affected_if":["private-lock-directory","windows-only"],"how_to_check":"- Run `pip show filelock` or `python -c \"import filelock; print(filelock.__version__)\"` and compare against `4.2.1`\n- Check whether lock paths sit in shared, world-writable directories such as /tmp where other local users can rename files","fix":"- Upgrade the `filelock` Python package to `4.2.1` or later\n- Keep lock files in directories only the service user can write to","status":"open","published_at":"2026-10-20T15:17:15+00:00","first_seen_at":"2026-10-23T21:57:20+00:00","triaged_at":"2026-10-25T04:04:10+00:00","updated_at":"2026-10-25T04:04:10+00:00","category":"package","confidence":0.7,"triage":{"at":"2026-10-25T04:04:10Z","model":"claude-haiku-5-5","output":{"fix":"- Upgrade the `filelock` Python package to `4.2.1` or later\n- Keep lock files in directories only the service user can write to","reach":"local","title":"filelock: Unix lock path can truncate or re-mode a local file","impact":"medium","summary":"filelock's UnixFileLock can truncate a regular file that another local user renamed or hard-linked onto the lock path before 4.2.1. When an explicit mode is requested, affected versions from 3.10.2 can also change that file's permissions.","category":"package","nicknames":[],"who_cares":["python","unix","shared-hosts"],"confidence":0.7,"needs_auth":"yes","worst_case":"local user truncates or re-modes a victim's regular file","analyst_note":"","how_to_check":"- Run `pip show filelock` or `python -c \"import filelock; print(filelock.__version__)\"` and compare against `4.2.1`\n- Check whether lock paths sit in shared, world-writable directories such as /tmp where other local users can rename files","needs_analyst":false,"not_affected_if":["windows-only","private-lock-directory"],"who_is_affected":"Applications on Unix-like systems that use filelock's UnixFileLock with lock paths in directories other local users can write to. Windows-only deployments and systems where lock files live in private directories are less exposed. Teams that pin filelock below 4.2.1 should check their lock locations.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"unknown"},"cost_usd":0.000592,"input_tokens":3327,"output_tokens":519,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-112909':16A,19A '-2026':15A,18A '3.10.2':58B,119C '4.2.1':48B,78C,133C 'actual':95C 'affect':55B,116C 'also':60B,121C 'anoth':35B,91C 'chang':61B,122C 'cve':14A,17A 'explicit':51B,112C 'file':13A,33B,63B,72C,84C,124C 'filelock':1A,26B,66C 'fix':130C 'hard':41B,100C,107C 'hard-link':40B,99C,106C 'host':24B 'independ':71C 'issu':128C 'link':42B,101C,108C 'local':12A,36B,92C 'lock':3A,45B,73C,88C 'mode':10A,52B,113C,126C 'onto':43B,86C 'packag':20B 'path':4A,46B,89C 'permiss':65B,97C,105C 'platform':70C 'platform-independ':69C 'polici':109C 'prior':76C 'python':21B,75C 're':9A 're-mod':8A 'regular':32B,83C 'renam':38B,85C,96C 'request':54B,115C 'share':23B 'shared-host':22B 'truncat':6A,30B,81C 'unix':2A,25B 'unixfilelock':28B,79C 'user':37B,93C 'version':56B,117C,132C","nicknames":[],"mitigations":[],"who_affected":"Applications on Unix-like systems that use filelock's UnixFileLock with lock paths in directories other local users can write to. Windows-only deployments and systems where lock files live in private directories are less exposed. Teams that pin filelock below 4.2.1 should check their lock locations.","tier":"C","verified":{},"analyst_revisit_at":null,"first_public_at":"2026-10-20T15:17:15+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:N","cvss_by":"security-advisories@github.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-112909","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-112909","severity":null,"published_at":"2026-10-20T15:17:15+00:00","modified_at":"2026-10-22T22:18:02+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"}