CVE-2026-12261: A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for cross-package resource and mode

Q: How do I fix CVE-2026-12261?

nltk: upgrade to newer than 3.9.4. Exact commands for each OS are on this page.

Q: Is CVE-2026-12261 being exploited?

CVE-2026-12261 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-12261 affect?

CVE-2026-12261 affects nltk. Severity is medium (CVSS 6.5). 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-

Q: How urgent is CVE-2026-12261?

Priority 25/100. No elevated risk signals yet.

Routine

MEDIUM 6.5UPGRADE

CVE-2026-12261 is a medium-severity flaw (CVSS 6.5) affecting nltk. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 names

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.2% (top 89%)
CISA exploited list	No
Weakness	CWE-284
Published	2026-08-07
Last modified	2026-10-23

Publisher (CNA): huntr.dev · Vendor: nltk

Last updated 2026-10-23 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-08-07 07:16 UTCPublished CVE record published
2026-10-23 21:10 UTCUpdated Record last modified

nltk nltk UPGRADE

Affected: <= 3.9.4

Fixed in: newer than 3.9.4

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy nltk || rpm -q nltk || nltk --version

2. UPGRADE to newer than 3.9.4 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade nltk

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade nltk

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-12261?

nltk: upgrade to newer than 3.9.4. Exact commands for each OS are on this page.

Is CVE-2026-12261 being exploited?

CVE-2026-12261 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-12261 affect?

CVE-2026-12261 affects nltk. Severity is medium (CVSS 6.5). 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-

How urgent is CVE-2026-12261?

Priority 25/100. No elevated risk signals yet.

Related CVEs

CVE-2026-12259 MEDIUM 6.5 DOWNGRADE In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package
CVE-2026-12372 LOW 3.7 DOWNGRADE A Server-Side Request Forgery (SSRF) vulnerability exists in nltk/nltk versions 3.9.4 and the current develop

References

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