Affected: nltk

NLTK from 3.6 before 3.10.4 contains an algorithmic complexity vulnerability in LegalitySyllableTokenizer.tokenize that allows remote attackers to exhaust CPU by supplying long all-vowel tokens. Attackers can submit a token such as a long run of vowels, causing the unguarded onset reversal loop to perform quadratic work and deny service to syllabification components.

Affected: PyPI: nltk

## 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 match, `onset` never becomes `True`, and the reset branch that would clear `current_onset` is never reached. As a result `current_onset` grows by one character on every iteration. Building `current_onset[::-1]` and hashing the resulting string costs O(k) on iteration k, and this is repeated for k from 1 to n, giving total cost O(n^2). The security invariant that is violated is bounded per-token work: there is no length cap on this code path, so untrusted token length translates directly into quad…