CVE-2026-78683: nltk (PyPI)

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

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

From the GitHub advisory

Summary

The NLTK library's TransitionParser.parse() method deserializes model files using pickleload() with the default restricted=False parameter, allowing arbitrary Python code execution when loading a malicious model file. The library provides a RestrictedUnpickler class for safe deserialization, but it is never used by production code paths, leaving the vulnerability unpatched.

Root Cause

File: nltk/parse/transitionparser.py (lines 542-557)

The parse() method calls pickleload(f) without restricted=True, routing through WarningUnpickler which inherits from pickle.Unpickler and does NOT override findclass(). This allows arbitrary class/function resolution during unpickling, enabling RCE via standard pickle gadgets (e.g., os.system, subprocess.Popen).

Weakness type: CWE-502.

Product: nltk (PyPI) · Assigned by: VulnCheck · Published: 2026-08-25

Affected and fixed versions

Ecosystem	Package	Vulnerable	First fixed
PyPI	`nltk`	<= 3.9.4	3.10.0

Vendor	Product	Affected versions (per CVE record)
nltk	nltk	< 3.10.0

Severity scores

Scored by	CVSS	Score	Vector
VulnCheck	4.0	9.4 Critical	`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	3.1	9.6 Critical	`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H`
GitHub	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`

Attack vector: network (remotely reachable)
Attack complexity: low
Attack requirements: none
Privileges required: none (no login)
User interaction: passive

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

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.

Timeline

2026-08-25 — CVE record last updated
2026-08-25 — CVE record published by VulnCheck
2026-09-08 — GitHub-reviewed advisory GHSA-rhp5-r9x4-f5g2 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.