---
title: "CVE-2026-78683: NLTK: Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution"
url: https://erroneo.us/security/cve/CVE-2026-78683/
description: "CVE-2026-78683 (nltk (PyPI)): NLTK before 3.10.0 (affected versions Fixes, deadlines and original sources."
type: vulnerability-record
section: "Security Digest"
date_published: 2026-08-25
date_modified: 2026-09-08
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-78683"
kev: false
cvss: 9.4
epss: 0.0051
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-78683: NLTK: Unsafe Pickle Deserialization in TransitionParser Allows Remote Code Execution

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.5% , percentile 43: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 43% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Only applies if your dependency tree includes nltk (PyPI) <= 3.9.4, fixed in 3.10.0 . Check your lockfile, including transitive dependencies.

## What it is

NLTK before 3.10.0 (affected versions

## Affected packages

- PyPI `nltk` <= 3.9.4 → fixed: 3.10.0

## Affected products (CVE record)

- nltk nltk: < 3.10.0

## Severity

- VulnCheck CVSS 4.0: 9.4 `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`
- VulnCheck CVSS 3.1: 9.6 `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H`
- GitHub CVSS 4.0: 9.4 `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.51% probability of exploitation activity in the next 30 days; percentile 43.

## Sources

- [GitHub Security Advisory (GHSA-rhp5-r9x4-f5g2)](https://github.com/nltk/nltk/security/advisories/GHSA-rhp5-r9x4-f5g2)
- [GitHub Advisory GHSA-rhp5-r9x4-f5g2](https://github.com/advisories/GHSA-rhp5-r9x4-f5g2)
- [CVE.org record CVE-2026-78683](https://www.cve.org/CVERecord?id=CVE-2026-78683)
- [NVD entry CVE-2026-78683](https://nvd.nist.gov/vuln/detail/CVE-2026-78683)
- [VulnCheck Advisory: NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization](https://www.vulncheck.com/advisories/nltk-before-remote-code-execution-via-unsafe-pickle-deserialization)
- [github.com](https://github.com/nltk/nltk/pull/3631)
- [github.com](https://github.com/nltk/nltk/commit/f26b3753038d937b68145daf15e9636f8451053c)
- [github.com](https://github.com/nltk/nltk/releases/tag/v3.10.0)
- [github.com](https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3734.yaml)

---
Source page: https://erroneo.us/security/cve/CVE-2026-78683/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt