Inefficient Regular Expression Complexity Affecting nltk package, versions <3.10.3-1


Severity

Recommended
0.0
high
0
10

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Threat Intelligence

EPSS
0.51% (41st percentile)

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  • Snyk IDSNYK-DEBIANUNSTABLE-NLTK-19215384
  • published23 Aug 2026
  • disclosed20 Aug 2026

Introduced: 20 Aug 2026

NewCVE-2026-72818  (opens in a new tab)
CWE-1333  (opens in a new tab)

How to fix?

Upgrade Debian:unstable nltk to version 3.10.3-1 or higher.

NVD Description

Note: Versions mentioned in the description apply only to the upstream nltk package and not the nltk package as distributed by Debian. See How to fix? for Debian:unstable relevant fixed versions and status.

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 Base Scores

version 3.1