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CVE-2026-78683 critical CVSS 9.6

Unsafe Deserialization in nltk (CVE-2026-78683)

Summary

Vulnerability in nltk (CVE-2026-78683). Successful exploitation can lead to full system takeover. Exploitable via RestrictedUnpickler. Mitigation: upgrade to 3.10.0 or later.

Original description

The description as published by the source.

Summary

The NLTK library's TransitionParser.parse() method deserializes model files using pickle_load() 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 pickle_load(f) without restricted=True, routing through WarningUnpickler which inherits from pickle.Unpickler and does NOT override find_class(). This allows arbitrary class/function resolution during unpickling, enabling RCE via standard pickle gadgets (e.g., os.system, subprocess.Popen).

Vulnerability chain in nltk/picklesec.py:

def pickle_load(file, *, context=None, restricted=False):
    if restricted:
        return RestrictedUnpickler(file).load()  # Safe: blocks all globals
    return WarningUnpickler(file, context=context).load()  # VULNERABLE PATH

WarningUnpickler only emits a warning but does NOT block unsafe class loading — it calls super().load() which is standard pickle.Unpickler.load().

Why this is not by design:

  • NLTK intentionally created RestrictedUnpickler to block unsafe deserialization
  • The restricted=True parameter exists in the API but is never used by any production code path
  • All call sites use the defa…

Source: NIST National Vulnerability Database 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-2026-78683 has been found in nltk.
Attackers can target a specific entry point like RestrictedUnpickler over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 9.6/10.
What to do: upgrade nltk to 3.10.0 or later.
If unsure, ask your IT team or search "nltk CVE-2026-78683" on the vendor's site.

CVE-2026-78683 (nltk) — CWE-502 / CVSS v3 9.6
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: RestrictedUnpickler / WarningUnpickler / pickle.Unpickler / os.system
Patched: 3.10.0 — 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 RestrictedUnpickler without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of nltk. Attack surface: RestrictedUnpickler / WarningUnpickler / pickle.Unpickler / os.system.

🔥 Severity

Severity: Critical (CVSS 9.6/10). Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to 3.10.0.

🛡️ 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 3.10.0

    ステージング環境で 3.10.0 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":"3.10.0"}]}]
PyPI nltk
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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