CVE-2026-12259
MEDIUMDescription
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.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 1% of all known CVEs.
References
Related Vulnerabilities
Other CWE-494 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2020-5398 | High | 7.5 | 89% | - | Fix |
| CVE-2022-40799 | High | 8.8 | 34% | KEV | - |
| CVE-2018-19234 | High | 8.8 | 3.3% | - | Fix |
| CVE-2016-6567 | Critical | 9.8 | 2.9% | - | - |
| CVE-2016-6564 | High | 8.1 | 2.6% | - | - |
| CVE-2001-1125 | Critical | 9.8 | 2.5% | - | Fix |
Common questions
How do I fix CVE-2026-12259?
Upgrade nltk to 3.9.3 or later.
Is CVE-2026-12259 being actively exploited?
Not that we know of. CVE-2026-12259 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.10% is the estimated probability that it will be exploited in the next 30 days. That is higher than 1% of all scored CVEs.
How severe is CVE-2026-12259?
CVE-2026-12259 has a CVSS v3 base score of 6.5, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-12259 affect?
Published advisories record a fix for nltk (PyPI (Python)). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-10-23.