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CVE-2026-79657 critical CVSS 9.8

Unsafe Deserialization in nltk (CVE-2026-79657)

Summary

Vulnerability in nltk (CVE-2026-79657). Successful exploitation can lead to full system takeover. Exploitable via nltk.picklesec.allowlisted_pickle_load. Mitigation: upgrade to 3.10.3 or later.

Original description

The description as published by the source.

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.allowlisted_pickle_load, nltk.tokenize.punkt.punkt_pickle_load, 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.

punkt_pickle_load() 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 allowlisted_pickle_load(..., allowed_modules=("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.

PoC

Preconditions

  • The application loads an attacker-controlled tokenizer or model artifact through these public loaders.

**…

Source: GitHub Security Advisories (Global) NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-79657 has been found in nltk.
Attackers can target a specific entry point like nltk.picklesec.allowlisted_pickle_load over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 9.8/10.
What to do: upgrade nltk to 3.10.3 or later.
If unsure, ask your IT team or search "nltk CVE-2026-79657" on the vendor's site.

CVE-2026-79657 (nltk) — CWE-502 / CVSS v3 9.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: nltk.picklesec.allowlisted_pickle_load / nltk.tokenize.punkt.punkt_pickle_load / nltk.tokenize.repp.ReppTokenizer._execute / numpy.f2py.crackfortran.myeval
Patched: 3.10.3 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

Unsafe deserialization (CWE-502) exists in nltk. Attackers reach the vulnerable code path via nltk.picklesec.allowlisted_pickle_load without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of nltk. Attack surface: nltk.picklesec.allowlisted_pickle_load / nltk.tokenize.punkt.punkt_pickle_load / nltk.tokenize.repp.ReppTokenizer._execute / numpy.f2py.crackfortran.myeval.

🔥 Severity

Severity: Critical (CVSS 9.8/10). Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to 3.10.3.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'nltk' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'nltk' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で nltk を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade nltk to 3.10.3

    ステージング環境で 3.10.3 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

pip nltk
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.3"}]}]
PyPI nltk
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.3"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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