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9.9
CVE-2026-12259: NLTK may install tampered packages without checking integrity
CVE-2026-12259 · published 2 months ago
Summary
Version 3.9.4 of the NLTK library can write a downloaded language package to disk before confirming its checksum, so a malicious mirror or proxy could supply altered files that are then installed. This could let attackers place harmful code or data into applications that rely on NLTK. Update NLTK to a newer release that verifies checksums first, or only download packages from trusted sources and verify their integrity manually.
What to do
- Update nltk to version 3.9.3.
Affected software
| Ecosystem | Vendor | Product | Affected versions |
|---|---|---|---|
| PyPI | – | nltk |
< 3.9.3 Fix: upgrade to 3.9.3
|
| Debian:14 | debian | nltk | All versions |
| pip | – | nltk |
<= 3.9.2 Fix: upgrade to 3.9.3
|
| – | nltk | nltk/nltk | <= latest |
| Debian:11 | debian | nltk | All versions |
| Debian:12 | debian | nltk | All versions |
| Debian:13 | debian | nltk | All versions |
| Ubuntu:Pro:14.04:LTS | canonical | nltk | All versions |
| Ubuntu:Pro:16.04:LTS | canonical | nltk | All versions |
| Ubuntu:Pro:18.04:LTS | canonical | nltk | All versions |
| Ubuntu:Pro:20.04:LTS | canonical | nltk | All versions |
| Ubuntu:Pro:22.04:LTS | canonical | nltk | All versions |
| Ubuntu:Pro:24.04:LTS | canonical | nltk | All versions |
| Ubuntu:Pro:26.04:LTS | canonical | nltk | All versions |
Original advisory text
Duplicate Advisory: NLTK: Missing Post-Download Integrity Verification Allows Malicious Package Injection
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.
References
- https://security-tracker.debian.org/tracker/CVE-2026-12259 Vendor Advisory
- https://www.cve.org/CVERecord?id=CVE-2026-12259 Third Party Advisory
- https://github.com/nltk/nltk/security/advisories/GHSA-5wp5-5229-5g6q
- https://nvd.nist.gov/vuln/detail/CVE-2026-12259
- https://github.com/nltk/nltk/pull/3449
- https://github.com/nltk/nltk/releases/tag/3.9.3
- https://ubuntu.com/security/CVE-2026-12259 Third Party Advisory
- https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63310... Vendor Advisory
- https://nvd.nist.gov/vuln/detail/CVE-2026-63310 Vendor Advisory
- https://github.com/nltk/nltk Product
- https://huntr.com/bounties/659ccf6d-12d4-4d4a-84c0-078633c35a5d
- https://www.vulncheck.com/advisories/nltk-before-missing-post-download-integrity...
- https://github.com/nltk/nltk/commit/0e26734a61094b628d93e26dc18dd7302567ac46
- https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3729.y...
- https://github.com/advisories/GHSA-5wp5-5229-5g6q
Internet-facing
60 days
Internal
At next upgrade
- Not known to be exploited
- Needs hands-on effort to exploit
- Gives an attacker partial control
Severity
9.9
Critical
Type
CWE-494Download of Code Without Integrity Check
Timeline
Published3 Aug 2026
Updated7 Oct 2026
First seen3 Aug 2026
Sources
CVE-2026-12259 · NVD
CVE-2026-12259 · MITRE
DEBIAN-CVE-2026-12259 · OSV
UBUNTU-CVE-2026-12259 · OSV
PYSEC-2026-3729 · OSV
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