{"id":"CVE-2026-12259","summary":"Improper Input Validation in nltk/nltk","details":"In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package bytes to disk and may extract them before enforcing SHA-256 or MD5 checksum validation. This allows an attacker to tamper with the package response body for `info.url` through a compromised mirror, malicious proxy, or other source-substitution condition, leading to the installation of attacker-controlled package bytes. The vulnerability can result in malicious corpus or model content being trusted by downstream users or applications.","aliases":["CVE-2026-63310","GHSA-5wp5-5229-5g6q","GHSA-gf32-cmjh-8m9v","PYSEC-2026-3729","PYSEC-2026-4318"],"modified":"2026-10-17T02:48:28.197218276Z","published":"2026-08-03T07:09:35.262Z","database_specific":{"cna_assigner":"@huntr_ai","cwe_ids":["CWE-494"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/12xxx/CVE-2026-12259.json"},"references":[{"type":"WEB","url":"https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/12xxx/CVE-2026-12259.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-12259"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/nltk/nltk","events":[{"introduced":"ad9c96ba00a16923ffe917eacf63f1707bfa2d08"},{"last_affected":"ad9c96ba00a16923ffe917eacf63f1707bfa2d08"}],"database_specific":{"cpe":"cpe:2.3:a:nltk:nltk:3.9.4:*:*:*:*:*:*:*","extracted_events":[{"introduced":"3.9.4"},{"last_affected":"3.9.4"}],"source":"CPE_STRING"}}],"versions":["3.9.4"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-12259.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N"}]}