{"id":"CVE-2021-3828","aliases":["GHSA-2ww3-fxvq-293j","PYSEC-2021-356"],"url":"https://o3.security/vulnerability/CVE-2021-3828","summary":"NLTK Vulnerable to REDoS","details":"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.","published":"2021-09-27T13:15:07.993Z","modified":"2026-07-09T01:31:40.287799Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.01703,"percentile":0.75218,"asOf":"2026-08-28"},"cisaKev":null,"exploitsKnown":1,"affectedPackages":[{"ecosystem":"PyPI","name":"nltk","fixedVersion":"3.6.4"}],"fix":{"url":"https://github.com/nltk/nltk/commit/277711ab1dec729e626b27aab6fa35ea5efbd7e6","label":"nltk/nltk@277711a"},"references":[{"type":"FIX","url":"https://github.com/nltk/nltk/commit/277711ab1dec729e626b27aab6fa35ea5efbd7e6"},{"type":"FIX","url":"https://huntr.dev/bounties/d19aed43-75bc-4a03-91a0-4d0bb516bc32"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-3828"},{"type":"WEB","url":"https://github.com/nltk/nltk/pull/2816"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-2ww3-fxvq-293j"},{"type":"PACKAGE","url":"https://github.com/nltk/nltk"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2021-356.yaml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-09T01:31:40.287799Z"}}