{"id":"CVE-2026-12259","title":"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","summary":"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…","severity":"medium","cvss":6.5,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N","cwe":["CWE-494","CWE-354"],"vendor":"nltk","product":"nltk","affected":["nltk = 3.9.4"],"patched":["nltk 3.9.3"],"published":"2026-08-03","updated":"2026-10-15","sourceUpdated":"2026-10-15T19:21:42.683","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-12259","references":[{"url":"https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d","label":"security@huntr.dev"},{"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-12259.json"},{"url":"https://access.redhat.com/security/cve/CVE-2026-12259"},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2510340"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-12259"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-12259"},{"url":"https://github.com/nltk/nltk/security/advisories/GHSA-5wp5-5229-5g6q"},{"url":"https://github.com/nltk/nltk/pull/3449"},{"url":"https://github.com/nltk/nltk/commit/0e26734a61094b628d93e26dc18dd7302567ac46"},{"url":"https://github.com/nltk/nltk"},{"url":"https://github.com/nltk/nltk/releases/tag/3.9.3"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3729.yaml"},{"url":"https://www.vulncheck.com/advisories/nltk-before-missing-post-download-integrity-verification"},{"url":"https://github.com/advisories/GHSA-5wp5-5229-5g6q"}],"tags":["nvd","csaf","vex","red-hat","osv","pip","ghsa"],"epss":0.00101,"epssPercentile":0.00891,"aliases":["GHSA-5wp5-5229-5g6q","CVE-2026-63310","GHSA-gf32-cmjh-8m9v","PYSEC-2026-3729","PYSEC-2026-4318"],"ecosystem":"pip","scores":{"nvd":6.5,"vendor":5.3,"osv":5.3,"ghsa":5.3},"ingestedAt":"2026-09-08T15:33:26.986Z","slug":"CVE-2026-12259","body":"## Overview\n\nIn 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.\n\n## Affected\n\n- `nltk = 3.9.4`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.\n\n## Package advisory (CVE-2026-12259)\n\nAffected packages:\n\n- `nltk < 3.9.3`\n\nPatched in:\n\n- `nltk 3.9.3`\n\nSource: https://osv.dev/vulnerability/GHSA-5wp5-5229-5g6q\n\n## Vendor advisories\n\n- **Red Hat VEX** · Moderate · affected: Exploit Intelligence, Lightspeed Core, OpenShift Lightspeed, Red Hat Ansible Automation Platform 2, Red Hat OpenShift AI (RHOAI) · no fix planned: Exploit Intelligence, Red Hat Ansible Automation Platform 2, Lightspeed Core, OpenShift Lightspeed, … · updated 2026-10-15 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-12259.json)","depth":"sunlit","depthScore":36,"depthScoreParts":{"impact":35.8,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[{"seq":222655,"id":"CVE-2026-12259","ts":1792092418289,"field":"cvss","old":"5.3","new":"6.5"}]}