{"id":"PYSEC-2026-3870","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.","aliases":["CVE-2026-72818","GHSA-qx2g-xrx7-vfh8"],"modified":"2026-09-10T12:15:10.108062006Z","published":"2026-09-10T09:44:52.802393Z","references":[{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-72818"},{"type":"WEB","url":"https://github.com/nltk/nltk/issues/3704"},{"type":"WEB","url":"https://github.com/nltk/nltk/pull/3701"},{"type":"WEB","url":"https://github.com/nltk/nltk/commit/e092ed52eccae642304448bffc8d23cb301f85c1"},{"type":"PACKAGE","url":"https://github.com/nltk/nltk"},{"type":"WEB","url":"https://github.com/nltk/nltk/blob/3.9.4/nltk/tokenize/casual.py"},{"type":"WEB","url":"https://github.com/nltk/nltk/releases/tag/v3.10.1"},{"type":"WEB","url":"https://www.vulncheck.com/advisories/nltk-tweettokenizer-url-pattern-backtracks-catastrophically-on-naked-domain-like-input"},{"type":"PACKAGE","url":"https://pypi.org/project/nltk"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-qx2g-xrx7-vfh8"}],"affected":[{"package":{"name":"nltk","ecosystem":"PyPI","purl":"pkg:pypi/nltk"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.10.1"}]}],"versions":["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","2.0.1","2.0.1rc1","2.0.1rc2-git","2.0.1rc3","2.0.1rc4","2.0.2","2.0.3","2.0.4","2.0.5","2.0b4","2.0b5","2.0b6","2.0b7","2.0b8","2.0b9","3.0.0","3.0.0b1","3.0.0b2","3.0.1","3.0.2","3.0.3","3.0.4","3.0.5","3.1","3.10.0","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","3.5","3.5b1","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","3.9","3.9.1","3.9.2","3.9.3","3.9.4","3.9b1"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/nltk/PYSEC-2026-3870.yaml"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},{"type":"CVSS_V4","score":"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"}]}