CVE-2026-12259: In nltk version 3.9.4, the `nltk.downloader.Downloader._download_package()` function writes downloaded package

Routine

MEDIUM 6.5DOWNGRADE

CVE-2026-12259 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.

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

Why this priority

No elevated risk signals yet

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	6.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.1% (top 99%)
CISA exploited list	No
Weakness	CWE-494
Published	2026-08-03
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

2026-08-03 08:17 UTCPublished CVE record published
2026-10-23 22:10 UTCUpdated Record last modified

nltk nltk DOWNGRADE

Affected: 3.9.4

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.9.4

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 3.9.4 (Debian / Ubuntu)

apt-cache madison nltk sudo apt install nltk=<VERSION_BEFORE_3.9.4> sudo apt-mark hold nltk

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list nltk sudo dnf downgrade nltk-<VERSION_BEFORE_3.9.4> sudo dnf versionlock add nltk

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

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-2026-12259?

nltk: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-12259 being exploited?

CVE-2026-12259 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-2026-12259 affect?

CVE-2026-12259 affects nltk. Severity is medium (CVSS 6.5). 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 th

How urgent is CVE-2026-12259?

Priority 23/100. No elevated risk signals yet.

Related CVEs

CVE-2026-12261 MEDIUM 6.5 UPGRADE A vulnerability in `nltk.downloader` in nltk/nltk versions <= 3.9.4 allows for cross-package resource and mode
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

← Back to latest CVEs