CVE-2026-79657: 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 1.3%, percentile 70: FIRST’s model estimates a 1.3% chance of exploitation activity in the next 30 days, higher than 70% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
Only applies if your dependency tree includes nltk (PyPI) <= 3.10.2, fixed in 3.10.3. Check your lockfile, including transitive dependencies.

What it is

NLTK versions before 3.10.3 contain a remote code execution vulnerability in allowlisted pickle loaders that trust entire module namespaces instead of specific safe callables. Attackers can craft malicious pickle payloads invoking dangerous in-namespace functions like ReppTokenizer.execute and numpy.f2py.crackfortran.myeval through pickle REDUCE to execute arbitrary commands during model or tokenizer artifact loading.

From the GitHub advisory

Summary

The current source tree still allows arbitrary code execution during supposedly safer allowlisted pickle loading. The allowlist trusts whole module namespaces instead of exact safe globals, so crafted pickles can invoke dangerous in-namespace callables through pickle REDUCE.

Details

- Vulnerability type: Remote code execution via unsafe deserialization - Affected component: nltk.picklesec.allowlistedpickleload, nltk.tokenize.punkt.punktpickleload, nltk.parse.transitionparser.TransitionParser.parse - Affected versions: Current source v3.10.0-rc2; published 3.9.4 was not the claim target for this bypass. - Patched versions: Not yet patched - Root cause: Module-prefix allowlists include dangerous callables such as nltk.tokenize.repp.ReppTokenizer.execute and numpy.f2py.crackfortran.myeval.

punktpickleload() allowlists both nltk.tokenize.punkt and the whole nltk.tokenize namespace, which exposes ReppTokenizer.execute() and its subprocess.Popen(...) sink during unpickling. TransitionParser.parse() uses allowlistedpickleload(..., allowedmodules=("numpy", "scipy", "sklearn")), which permits numpy.f2py.crackfortran.myeval() and its attacker-controlled eval(...) path. I confirmed both gadgets create marker files before the caller returns or later aborts on type misuse.

Weakness type: CWE-502, CWE-693.

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

Affected and fixed versions

Ecosystem	Package	Vulnerable	First fixed
PyPI	`nltk`	<= 3.10.2	3.10.3

Vendor	Product	Affected versions (per CVE record)
nltk	nltk	< 3.10.3

Severity scores

Scored by	CVSS	Score	Vector
VulnCheck	4.0	9.3 Critical	`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`
VulnCheck	3.1	9.8 Critical	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`
GitHub	4.0	9.3	`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`

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

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 yes, technical impact total.
FIRST EPSS (model of 2026-10-21): 1.27% probability of exploitation activity in the next 30 days; percentile 70.

Timeline

2026-08-25 — CVE record published by VulnCheck
2026-09-08 — GitHub-reviewed advisory GHSA-x99w-6fgc-pmfw published (critical)
2026-10-08 — CVE record last updated

Sources and references

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