CVE-2026-72818

CVE-2026-72818 · NLTK TweetTokenizer URL Pattern Backtracks Catastrophically on Naked-Domain-Like Input

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The URLS regular expression in nltk/tokenize/casual.py, compiled into TweetTokenizer.WORD_RE and applied by TweetTokenizer.tokenize, contains a naked-domain branch whose domain-label prefix [a-z0-9]+(?:[.\-][a-z0-9]+)* is unbounded. Input consisting of many alternating label separators can be partitioned in exponentially many ways, and because the branch also requires a trailing top-level domain that such input never supplies, the engine explores those partitions before failing at each offset. A few kilobytes of input therefore consumes seconds to minutes of single-threaded CPU, and the HANG_RE substitution performed before matching does not collapse the pattern. TweetTokenizer is intended for tokenizing untrusted social-media text, so any service that applies it, or the module-level casual_tokenize, to submitted text can be stalled per request without authentication. Version 3.10.1 bounds the label repetition.

CVSS 8.7 · High · EPSS 0.7% · Fix available

Published

2026-08-20T21:57:34.230Z

Updated

2026-09-24T14:18:57.476Z

Stack: system · python

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