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Fencer Vulns
high 7.5

CVE-2021-3842 json

Inefficient Regular Expression Complexity in nltk/nltk

published 2022-01-04 4 y ago modified 2026-06-17 4 mo ago also GHSA-rqjh-jp2r-59cj , PYSEC-2022-5 , EUVD-2022-0162

cvss
7.5
version 3.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.6%
EPSS 1.6% chance of exploitation in 30 days, percentile 75%
public exploits
0
none known

Summary

nltk is vulnerable to Inefficient Regular Expression Complexity

Patch / fix

  • pypi nltk 3.6.6
  • nltk nltk/nltk 3.6.6
  • nltk Debian 14, Debian 13, Debian 12 3.6.7-1

debian advisories DEBIAN-CVE-2021-3842

  • nltk Ubuntu Pro 14.04 LTS 2.0~b9-0ubuntu4.1~esm4
  • nltk Ubuntu Pro 16.04 LTS 3.1-1ubuntu0.1+esm2
  • nltk Ubuntu Pro 18.04 LTS 3.2.5-1ubuntu0.1+esm2
  • nltk Ubuntu Pro 20.04 LTS 3.4.5-2ubuntu0.1~esm2

ubuntu advisories UBUNTU-CVE-2021-3842

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-3842 updated cvss 5.9

python-nltk: ReDoS in function get_pos_tagger and malt_regex_tagger

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.57% chance of exploitation in the next 30 days, higher than 75% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • huntr.dev https://huntr.dev/bounties/761a761e-2be2-430a-8d92-6f74ffe9866a referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.0 7.5 CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:N/A:P
GitHub 3.0 7.5 CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
pypi nltk
<3.6.6
3.6.6

Affected products

  • nltk nltk/nltk

Classification

Weakness
  • CWE-1333 Inefficient Regular Expression Complexity

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.