{"cve":"CVE-2026-81727","nvd":{"published":"2026-08-27T17:21:03.533","modified":"2026-08-31T19:08:45.560","status":"Analyzed","description":"NLTK versions before 3.10.3 contain a filesystem containment bypass vulnerability in the Downloader.download and Downloader.incr_download methods that allows attackers to overwrite files outside the install root through pre-existing hardlinks. Attackers with write access to a shared downloader directory can create hardlinks pointing to outside-root files that are then overwritten during normal package extraction, mutating files outside the intended install tree.","cvss_v4":{"vector":"CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N","score":6.9},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H","score":7.1},"cwes":["CWE-59"],"references":["https://github.com/nltk/nltk/security/advisories/GHSA-f794-5jv7-7672","https://www.vulncheck.com/advisories/nltk-before-3.10.3-hardlink-file-overwrite-via-downloader","https://github.com/nltk/nltk/security/advisories/GHSA-f794-5jv7-7672"],"products":["nltk:nltk"],"source":"nvd","synced_at":"2026-10-19T22:50:17.453294893+00:00"},"epss":{"cve":"CVE-2026-81727","score":0.00187,"percentile":0.08358,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GHSA-f794-5jv7-7672","url":"https://osv.dev/vulnerability/GHSA-f794-5jv7-7672","summary":"NLTK: Downloader.download follows hardlinks and overwrites outside-root files","modified":"2026-09-10T03:51:15.298800760Z","packages":[{"ecosystem":"PyPI","name":"nltk","purl":"pkg:pypi/nltk","ranges":["<3.10.3"],"fixed":["3.10.3"]}]},{"id":"PYSEC-2026-3741","url":"https://osv.dev/vulnerability/PYSEC-2026-3741","summary":"","modified":"2026-09-01T09:15:03.611585308Z","packages":[{"ecosystem":"PyPI","name":"nltk","purl":"pkg:pypi/nltk","ranges":["<3.10.3"],"fixed":["3.10.3"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}