nltk — security advisory
A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for cross-package resource and model poisoning. The downloader extracts package archives into shared namespaces such as `corpora/` and `taggers/` instead of package-isolated roots, and validates package integrity only after the archive has been written and extracted. This design flaw enables one package to overwrite another package's trusted resources within the same namespace, making the changes immediately active through ordinary NLTK APIs. This issue persists across fresh interpreter restarts and can affect downstream workflows, including machine learning pipelines and reproducibility-sensitive environments.
- Products
- nltk
- Affected versions
- 3.10.0-1, 3.10.1-1, 3.10.3-1, 3.8-1, 3.8.1-1, 3.9.1-1, 3.9.1-2, 3.9.2-1, 3.9.3-1
- Fixed versions
- Not specified
- CVEs
- CVE-2026-12261
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 1 Nov 2026, 04:09 UTC · Revision 1
Source evidence
A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for cross-package resource and model poisoning. The downloader extracts package archives into shared namespaces such as `corpora/` and `taggers/` instead of package-isolated roots, and validates package integrity only after the archive has been written and extracted. This design flaw enables one package to overwrite another package's trusted resources within the same namespace, making the changes immediately active through ordinary NLTK APIs. This issue persists across fresh interpreter restarts and can affect downstream workflows, including machine learning pipelines and reproducibility-sensitive environments.
osv:DEBIAN-CVE-2026-12261
nltk
Fixed versions: Not specified
nltk
Fixed versions: Not specified
nltk
Fixed versions: Not specified
Complete source record
{
"id": "9590113648994da2366a037a034869e19d0ed1ceea80ac46da728135e847f4b6",
"url": "https://osv.dev/vulnerability/DEBIAN-CVE-2026-12261",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "DEBIAN-CVE-2026-12261",
"summary": "A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for cross-package resource and model poisoning. The downloader extracts package archives into shared namespaces such as `corpora/` and `taggers/` instead of package-isolated roots, and validates package integrity only after the archive has been written and extracted. This design flaw enables one package to overwrite another package's trusted resources within the same namespace, making the changes immediately active through ordinary NLTK APIs. This issue persists across fresh interpreter restarts and can affect downstream workflows, including machine learning pipelines and reproducibility-sensitive environments.",
"evidence": "osv:DEBIAN-CVE-2026-12261",
"products": [
{
"name": {
"value": "nltk",
"evidence": "/affected/0/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "3.10.0-1",
"evidence": "/affected/0/versions"
},
{
"value": "3.10.1-1",
"evidence": "/affected/0/versions"
},
{
"value": "3.10.3-1",
"evidence": "/affected/0/versions"
},
{
"value": "3.8-1",
"evidence": "/affected/0/versions"
},
{
"value": "3.8.1-1",
"evidence": "/affected/0/versions"
},
{
"value": "3.9.1-1",
"evidence": "/affected/0/versions"
},
{
"value": "3.9.1-2",
"evidence": "/affected/0/versions"
},
{
"value": "3.9.2-1",
"evidence": "/affected/0/versions"
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{
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}
]
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{
"name": {
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"affected_versions": [
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},
{
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"evidence": "/affected/1/versions"
},
{
"value": "3.10.3-1",
"evidence": "/affected/1/versions"
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{
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"evidence": "/affected/1/versions"
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{
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{
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{
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"prerequisites": [],
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"affected_versions": [
{
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},
{
"value": "3.10.1-1",
"evidence": "/affected/2/versions"
},
{
"value": "3.10.3-1",
"evidence": "/affected/2/versions"
},
{
"value": "3.9.1-2",
"evidence": "/affected/2/versions"
},
{
"value": "3.9.2-1",
"evidence": "/affected/2/versions"
},
{
"value": "3.9.3-1",
"evidence": "/affected/2/versions"
}
]
}
],
"severity": "medium",
"structured": {
"id": "DEBIAN-CVE-2026-12261",
"aliases": [
"DEBIAN-CVE-2026-12261"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N"
}
],
"affected": [
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
}
]
}
],
"package": {
"name": "nltk",
"purl": "pkg:deb/debian/nltk?arch=source&distro=bookworm",
"ecosystem": "Debian:12"
},
"versions": [
"3.10.0-1",
"3.10.1-1",
"3.10.3-1",
"3.8-1",
"3.8.1-1",
"3.9.1-1",
"3.9.1-2",
"3.9.2-1",
"3.9.3-1"
],
"database_specific": {
"source": "https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2026-12261.json"
},
"ecosystem_specific": {
"urgency": "not yet assigned"
}
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
}
]
}
],
"package": {
"name": "nltk",
"purl": "pkg:deb/debian/nltk?arch=source&distro=trixie",
"ecosystem": "Debian:13"
},
"versions": [
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"3.10.1-1",
"3.10.3-1",
"3.9.1-2",
"3.9.2-1",
"3.9.3-1"
],
"database_specific": {
"source": "https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2026-12261.json"
},
"ecosystem_specific": {
"urgency": "not yet assigned"
}
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
}
]
}
],
"package": {
"name": "nltk",
"purl": "pkg:deb/debian/nltk?arch=source&distro=forky",
"ecosystem": "Debian:14"
},
"versions": [
"3.10.0-1",
"3.10.1-1",
"3.10.3-1",
"3.9.1-2",
"3.9.2-1",
"3.9.3-1"
],
"database_specific": {
"source": "https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2026-12261.json"
},
"ecosystem_specific": {
"urgency": "not yet assigned"
}
}
],
"modified": "2026-10-24T09:47:33.892934957Z",
"provider": "osv",
"upstream": [
"CVE-2026-12261"
],
"references": [
"https://security-tracker.debian.org/tracker/CVE-2026-12261"
]
},
"related_cves": [
{
"value": "CVE-2026-12261",
"evidence": "/upstream"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 429028,
"published_at": "2026-08-07T07:16:26.377+00:00"
}