# CVE-2021-3828: python3-nltk: nltk is vulnerable to Inefficient Regular Expression Complexity

CVE-2021-3828 is fixed in 2 products on 1 platform, first by python-nltk 3.2.5-1ubuntu0.1+esm1, released September 27, 2021. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2021-3828
- Priority: Routine
- CVSS: n/a (Medium)
- EPSS: 0.0159
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 27, 2021

## Description

nltk is vulnerable to Inefficient Regular Expression Complexity

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | python-nltk Ubuntu 18.04.6 LTS (bionic) | 3.2.5-1ubuntu0.1+esm1 | [python-nltk 3.2.5-1ubuntu0.1+esm1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-python-nltk-3.2.5-1ubuntu0.1-esm1.md) | Sep 27, 2021 |
| Linux | python3-nltk Ubuntu 18.04.6 LTS (bionic) | 3.2.5-1ubuntu0.1+esm1 | [python3-nltk 3.2.5-1ubuntu0.1+esm1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-python3-nltk-3.2.5-1ubuntu0.1-esm1.md) | Sep 27, 2021 |
| Linux | python3-nltk Ubuntu 20.04 LTS (focal) | 3.4.5-2ubuntu0.1~esm1 | [python3-nltk 3.4.5-2ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-python3-nltk-3.4.5-2ubuntu0.1-esm1.md) | Sep 27, 2021 |
| Linux | python3-nltk Ubuntu 22.04.1 LTS (jammy) | 3.6.5-1 | [python3-nltk 3.6.5-1](https://patchblox.com/security/data/fix/linux/ubuntu-jammy-python3-nltk-3.6.5-1.md) | Sep 27, 2021 |
| Linux | python3-nltk Ubuntu 24.10 (oracular) | 3.6.5-1 | [python3-nltk 3.6.5-1](https://patchblox.com/security/data/fix/linux/ubuntu-oracular-python3-nltk-3.6.5-1.md) | Jan 16, 2026 |
| Linux | python3-nltk Ubuntu 25.04 (plucky) | 3.6.5-1 | [python3-nltk 3.6.5-1](https://patchblox.com/security/data/fix/linux/ubuntu-plucky-python3-nltk-3.6.5-1.md) | Jan 16, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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