Inefficient Regular Expression Complexity in nltk (word_tokenize, sent_tokenize)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2021-43854: 2.79% estimated exploitation activity in the next 30 days; percentile 86.44 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact The vulnerability is present in PunktSentenceTokenizer, senttokenize and wordtokenize. Any users of this class, or these two functions, are vulnerable to a Regular Expression Denial of Service (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. The effect of this vulnerability is noticeable with the following example: Which gave the following output during testing: I canceled the execution of the program after running it for several hours. 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, or applying the workaround described below. Patches The problem has been patched in NLTK 3.6.6. After the fix, running the above program gives the following result: This output shows a linear relationship in execution time versus input length,…

Published by the source

6 Jan 2022, 17:38 UTC

Last modified by the source

8 Jul 2026, 06:28 UTC

Source last checked

26 Oct 2026, 13:18 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-f8m6-h2c7-8h9x

2. Revision 2 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-f8m6-h2c7-8h9x

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# Patch Next advisory brief: CVE-2021-43854

Advisory: Inefficient Regular Expression Complexity in nltk (word_tokenize, sent_tokenize)
IDs: CVE-2021-43854, GHSA-f8m6-h2c7-8h9x
Record: https://patchnext.com/advisories/cve-2021-43854 (revision 3, source checked 26 Oct 2026, 13:18 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.6.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected < 3.6.6; fixed in 3.6.6

## Suggested inspection (read-only)
- `python -m pip show nltk`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nltk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-f8m6-h2c7-8h9x (OSV record GHSA-f8m6-h2c7-8h9x lists affected ranges: PyPI nltk >=0 <3.6.6 (fixed))
- Verified: https://github.com/advisories/GHSA-f8m6-h2c7-8h9x (GitHub-reviewed advisory GHSA-f8m6-h2c7-8h9x lists affected ranges: PyPI nltk >=0 <3.6.6)

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