PYSEC-2026-3870

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Import Source

https://github.com/pypa/advisory-database/blob/main/vulns/nltk/PYSEC-2026-3870.yaml

JSON Data

https://api.osv.dev/v1/vulns/PYSEC-2026-3870

Aliases

Published

2026-09-10T09:44:52Z

Modified

2026-09-10T12:15:10Z

Severity

7.5 (High) CVSS_V3 - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H CVSS Calculator
8.7 (High) CVSS_V4 - 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 CVSS Calculator

Summary

NLTK TweetTokenizer vulnerable to denial of service through catastrophic regex backtracking

Details

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.

References

Affected packages

PyPI / nltk

Package

Name: nltk; View open source insights on deps.dev
Purl: pkg:pypi/nltk

Affected ranges

Type: ECOSYSTEM
Events: Introduced

0 Unknown introduced version / All previous versions are affected

Fixed

3.10.1

Affected versions

2.*

2.0.1rc2-git

2.0b4

2.0b5

2.0b6

2.0b7

2.0b8

2.0b9

2.0.1rc1

2.0.1rc3

2.0.1rc4

2.0.1

2.0.2

2.0.3

2.0.4

2.0.5

0.*

0.8

0.9

0.9.3

0.9.4

0.9.5

0.9.6

0.9.7

0.9.8

0.9.9

3.*

3.0.0b1

3.0.0b2

3.0.0

3.0.1

3.0.2

3.0.3

3.0.4

3.0.5

3.1

3.2

3.2.1

3.2.2

3.2.3

3.2.4

3.2.5

3.3

3.4

3.4.1

3.4.2

3.4.3

3.4.4

3.4.5

3.5b1

3.5

3.6

3.6.1

3.6.2

3.6.3

3.6.4

3.6.5

3.6.6

3.6.7

3.7

3.8

3.8.1

3.9b1

3.9

3.9.1

3.9.2

3.9.3

3.9.4

3.10.0

Database specific

source

"https://github.com/pypa/advisory-database/blob/main/vulns/nltk/PYSEC-2026-3870.yaml"