# 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.

- **Severity:** Medium 5.3 (`CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N`)
- **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:** Yes
- **Published:** 3 Aug 2026
- **Updated:** 3 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-12259, ECHO-34ff-644c-194f, UBUNTU-CVE-2026-12259

## 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 (Download of Code Without Integrity Check)

## Which versions are affected by CVE-2026-12259?

It affects 1 product.


Products: 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](https://insights.sbomify.com/CVE-2026-0848/): Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading (critical 10.0)
- [CVE-2025-14009](https://insights.sbomify.com/CVE-2025-14009/): Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution (critical 10.0)
- [CVE-2026-78683](https://insights.sbomify.com/CVE-2026-78683/): NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization (critical 9.4)
- [CVE-2026-79657](https://insights.sbomify.com/CVE-2026-79657/): NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization (critical 9.3)
- [CVE-2026-79675](https://insights.sbomify.com/CVE-2026-79675/): NLTK before 3.10.3 JVM Argument Injection via Per-Call Options (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-12259 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-12259 in the wild and no public exploit code.

Affected product: nltk/nltk. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-12259/ (Markdown: https://insights.sbomify.com/CVE-2026-12259/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-12259
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-12259
- OSV: https://osv.dev/vulnerability/CVE-2026-12259

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-12259/
Summaries are generated and may contain mistakes; the official description above is the record.