# CVE-2026-71513: NLTK AllowlistUnpickler dotted-name validation bypass

Source: https://junglewise.ai/threats/cve-2026-71513-nltk-allowlistunpickler-dotted-name-validation-bypass  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-22. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-71513 · Severity: high · CVSS 8.8 · Published 2026-08-22

NLTK is a popular Python natural language processing library used for text analysis and machine learning tasks. Versions 3.10.0 through 3.10.2 contain a critical flaw in the AllowlistUnpickler security component that fails to properly validate untrusted serialized Python objects (pickles). An attacker can craft malicious transition-parser models that execute arbitrary commands on systems loading these models, potentially compromising the entire system and any data processed by it.

Technologies: [nltk (PyPI)](https://junglewise.ai/threats/technologies/nltk). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-71513 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-71513 being exploited in the wild.

**How severe is CVE-2026-71513?** High severity, with a CVSS score of 8.8, published 22 August 2026.

**Which products and versions are affected?** nltk (PyPI) >= 3.10.0, < 3.10.3, ECOSYSTEM:introduced=3.10.0,fixed=3.10.3.

**Is there a fix for CVE-2026-71513?** Yes. nltk (PyPI): upgrade to 3.10.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-71513 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-71513?** Check whether you run nltk (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- nltk (PyPI): upgrade to 3.10.3 or later

  ```sh
  pip install "nltk>=3.10.3"
  ```


## Technical details

The vulnerability is a deserialization bypass (CWE-502) in NLTK's AllowlistUnpickler class, which is designed to safely load pickled objects by restricting which modules and classes can be unpickled. The flaw lies in the find_class() method's validation logic: it checks only the module name against an allowlist but fails to validate the global name itself. This allows attackers to use dotted-name attribute traversal (valid in pickle protocol ≥ 4) to reach callables outside the intended namespace—for example, crafting a pickle requesting "sklearn.os.system" to reach the dangerous os.system function. The vulnerability is triggered when TransitionParser.parse() loads a model via allowlisted_pickle_load(). Attack vector is network-based with low complexity; no authentication or privileges are required, though the attack requires the user to load a maliciously crafted model file. The fix in version 3.10.3 implements four protective guards: rejecting dotted names, rejecting dunder attributes, denying an explicit set of known-dangerous functions (numpy.load, pandas.read_pickle), and denying modules like os, subprocess, sys, and builtins outright.

## Affected products

- NLTK Project NLTK 3.10.0 through 3.10.2

## Identifiers

- CVE-2026-71513
- GHSA-5gh2-94qg-qppq
- PYSEC-2026-3867

## Timeline

- 2026-08-22: disclosed: Published to GitHub Advisory Database and NVD
- 2026-08-10: patched: Security fix commit c3e3711 authored; fix shipped in version 3.10.3

## References

- [https://github.com/nltk/nltk/commit/c3e37113742a1ebeeb4f2ca58941f320f98805ea](https://github.com/nltk/nltk/commit/c3e37113742a1ebeeb4f2ca58941f320f98805ea)
- [https://github.com/nltk/nltk/blob/v3.10.2/nltk/picklesec.py](https://github.com/nltk/nltk/blob/v3.10.2/nltk/picklesec.py)
- [https://www.vulncheck.com/advisories/nltk-through-remote-code-execution-via-allowlistunpickler-dotted-name-bypass](https://www.vulncheck.com/advisories/nltk-through-remote-code-execution-via-allowlistunpickler-dotted-name-bypass)

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