NLTK: Quadratic CPU exhaustion in LegalitySyllableTokenizer.tokenize via unguarded onset reversal loop

Exploitation

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

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-126301: 0.51% estimated exploitation activity in the next 30 days; percentile 44.04 (FIRST EPSS, 2026-11-14). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary `LegalitySyllableTokenizer.tokenize` in the NLTK tokenizer package contains an unguarded O(n^2) processing loop. When the method is called on a single token that consists of a long run of vowels (for example `"a" * n`), it consumes CPU time proportional to the square of the token length. An attacker who can supply an untrusted token to this public tokenizer API can force a single request to saturate one CPU core for a prolonged period, degrading availability of any service that syllabifies attacker-controlled text. No authentication, special configuration, or user interaction is required.

Root Cause The method walks the token in reverse and, on every iteration after the first vowel is seen, evaluates the membership test `char_lower + current_onset[::-1] in self.legal_onsets`. The set `legal_onsets` is built by `onset()`, which returns only the characters up to the first vowel, so every member is vowel-free. For an all-vowel token the first branch can therefore never…

Published by the source

13 Nov 2026, 21:16 UTC

Last modified by the source

14 Nov 2026, 02:56 UTC

Source last checked

14 Nov 2026, 07:50 UTC

1. Revision 2 · amended · 14 Nov 2026

   Amended from osv record GHSA-gpwc-27cw-rh9r

2. Revision 1 · created · 13 Nov 2026

   Published from ghsa record GHSA-gpwc-27cw-rh9r

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# Patch Next advisory brief: CVE-2026-126301

Advisory: NLTK: Quadratic CPU exhaustion in LegalitySyllableTokenizer.tokenize via unguarded onset reversal loop
IDs: CVE-2026-126301, GHSA-gpwc-27cw-rh9r
Record: https://patchnext.com/advisories/ghsa-gpwc-27cw-rh9r (revision 2, source checked 14 Nov 2026, 07:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nltk 3.10.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nltk: affected >= 3.6, < 3.10.4; fixed in 3.10.4

## 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://github.com/advisories/GHSA-gpwc-27cw-rh9r (GitHub-reviewed advisory GHSA-gpwc-27cw-rh9r lists affected ranges: PyPI nltk >=3.6 <=3.10.3)
- Verified: https://osv.dev/vulnerability/GHSA-gpwc-27cw-rh9r (OSV record GHSA-gpwc-27cw-rh9r lists affected ranges: PyPI nltk >=3.6 <3.10.4 (fixed))

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