GHSA-QX2G-XRX7-VFH8
Vulnerability from github – Published: 2026-08-21 00:31 – Updated: 2026-09-02 15:16The 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.
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "nltk"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.10.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-72818"
],
"database_specific": {
"cwe_ids": [
"CWE-1333"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-02T15:16:37Z",
"nvd_published_at": "2026-08-20T22:18:05Z",
"severity": "HIGH"
},
"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.",
"id": "GHSA-qx2g-xrx7-vfh8",
"modified": "2026-09-02T15:16:37Z",
"published": "2026-08-21T00:31:23Z",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
},
{
"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",
"type": "CVSS_V4"
}
],
"summary": " NLTK TweetTokenizer vulnerable to denial of service through catastrophic regex backtracking"
}
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