Advisoriesby IbisNLTKGHSA-2ww3-fxvq-293j
NLTK Vulnerable to REDoS
Fix: upgrade to 3.6.4 or later
The nltk package is vulnerable to ReDoS (regular expression denial of service). An attacker that is able to provide as an input to the [_read_comparison_block()(https://github.com/nltk/nltk/blob/23f4b1c4b4006b0cb3ec278e801029557cec4e82/nltk/corpus/reader/comparative_sents.py#L259) function in the file nltk/corpus/reader/comparative_sents.py may cause an application to consume an excessive amount of CPU.
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| nltk PyPI | < 3.6.4 | 3.6.4 |
Details and references
- CVSS 3.1
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Severity from
- GitHub (reviewed advisory)
- Weakness
- CWE-1333, CWE-697
- Also known as
- CVE-2021-3828, PYSEC-2021-356
- nvd.nist.gov/vuln/detail/CVE-2021-3828
- github.com/nltk/nltk/pull/2816
- github.com/nltk/nltk/commit/277711ab1dec729e626b27aab6fa35ea5efbd7e6
- github.com/advisories/GHSA-2ww3-fxvq-293j
- github.com/nltk/nltk
- github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2021-356.yaml
- huntr.dev/bounties/d19aed43-75bc-4a03-91a0-4d0bb516bc32
More NLTK advisories
All NLTK
NLTK has a Path Traversal issueCVE-2026-0847fixed in 3.9.3NLTK has a Zip Slip VulnerabilityCVE-2025-14009fixed in 3.9.3ntlk unsafe deserialization vulnerabilityCVE-2024-39705fixed in 3.9NLTK Vulnerable to REDoSCVE-2021-3842fixed in 3.6.6Inefficient Regular Expression Complexity in nltk (word_tokenize, sent_tokenize)CVE-2021-43854fixed in 3.6.6NLTK Vulnerable To Path TraversalCVE-2019-14751fixed in 3.4.5