CVE-2026-12259 Improper Input Validation in nltk/nltk
Attackers can install malicious packages by tampering with downloads before checksum validation, risking code execution in applications using nltk.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-12259?
CVE-2026-12259 is a medium-severity vulnerability in nltk/nltk, published on 3 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
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.
Weakness: CWE-494
Which versions are affected by CVE-2026-12259?
It affects 1 product.
Products include Nltk Nltk.
How do I fix CVE-2026-12259?
Upgrade to a fixed version.
Is CVE-2026-12259 being exploited?
There is no known exploitation of CVE-2026-12259 in the wild and no public exploit code.
Other vulnerabilities in nltk/nltk
- CVE-2026-0848 Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading
- CVE-2025-14009 Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution
- CVE-2026-78683 NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79657 NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79675 NLTK before 3.10.3 JVM Argument Injection via Per-Call Options
Sources
Also known as DEBIAN-CVE-2026-12259, ECHO-34ff-644c-194f, UBUNTU-CVE-2026-12259.