ubuntu-cve-2026-72818
Vulnerability from osv_ubuntu
Published
2026-08-25 00:00
Modified
2026-08-27 08:42
Summary
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.
Severity
7.5 (High)
7.5 (High)
N/A (UNKNOWN)
References
{
"affected": [
{
"ecosystem_specific": {
"binaries": [
{
"binary_name": "python-nltk",
"binary_version": "2.0~b9-0ubuntu4.1~esm6"
}
]
},
"package": {
"ecosystem": "Ubuntu:Pro:14.04:LTS",
"name": "nltk",
"purl": "pkg:deb/ubuntu/nltk@2.0~b9-0ubuntu4.1~esm6?arch=source\u0026distro=esm-infra-legacy/trusty"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"2.0~b9-0ubuntu4",
"2.0~b9-0ubuntu4.1~esm2",
"2.0~b9-0ubuntu4.1~esm4",
"2.0~b9-0ubuntu4.1~esm5",
"2.0~b9-0ubuntu4.1~esm6"
]
},
{
"ecosystem_specific": {
"binaries": [
{
"binary_name": "python-nltk",
"binary_version": "3.1-1ubuntu0.1+esm4"
},
{
"binary_name": "python3-nltk",
"binary_version": "3.1-1ubuntu0.1+esm4"
}
]
},
"package": {
"ecosystem": "Ubuntu:Pro:16.04:LTS",
"name": "nltk",
"purl": "pkg:deb/ubuntu/nltk@3.1-1ubuntu0.1+esm4?arch=source\u0026distro=esm-apps-legacy/xenial"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.0.4-1",
"3.0.5-1",
"3.1-1",
"3.1-1ubuntu0.1",
"3.1-1ubuntu0.1+esm1",
"3.1-1ubuntu0.1+esm2",
"3.1-1ubuntu0.1+esm3",
"3.1-1ubuntu0.1+esm4"
]
},
{
"ecosystem_specific": {
"binaries": [
{
"binary_name": "python-nltk",
"binary_version": "3.2.5-1ubuntu0.1+esm4"
},
{
"binary_name": "python3-nltk",
"binary_version": "3.2.5-1ubuntu0.1+esm4"
}
]
},
"package": {
"ecosystem": "Ubuntu:Pro:18.04:LTS",
"name": "nltk",
"purl": "pkg:deb/ubuntu/nltk@3.2.5-1ubuntu0.1+esm4?arch=source\u0026distro=esm-apps/bionic"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.2.4-1",
"3.2.5-1",
"3.2.5-1ubuntu0.1",
"3.2.5-1ubuntu0.1+esm1",
"3.2.5-1ubuntu0.1+esm2",
"3.2.5-1ubuntu0.1+esm3",
"3.2.5-1ubuntu0.1+esm4"
]
},
{
"ecosystem_specific": {
"binaries": [
{
"binary_name": "python3-nltk",
"binary_version": "3.4.5-2ubuntu0.1~esm4"
}
]
},
"package": {
"ecosystem": "Ubuntu:Pro:20.04:LTS",
"name": "nltk",
"purl": "pkg:deb/ubuntu/nltk@3.4.5-2ubuntu0.1~esm4?arch=source\u0026distro=esm-apps/focal"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.4.5-1",
"3.4.5-2",
"3.4.5-2ubuntu0.1~esm1",
"3.4.5-2ubuntu0.1~esm2",
"3.4.5-2ubuntu0.1~esm3",
"3.4.5-2ubuntu0.1~esm4"
]
},
{
"ecosystem_specific": {
"binaries": [
{
"binary_name": "python3-nltk",
"binary_version": "3.7-1ubuntu0.1~esm2"
}
]
},
"package": {
"ecosystem": "Ubuntu:Pro:22.04:LTS",
"name": "nltk",
"purl": "pkg:deb/ubuntu/nltk@3.7-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/jammy"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.5-1",
"3.6.5-1",
"3.6.7-1",
"3.7-1",
"3.7-1ubuntu0.1~esm1",
"3.7-1ubuntu0.1~esm2"
]
},
{
"ecosystem_specific": {
"binaries": [
{
"binary_name": "python3-nltk",
"binary_version": "3.8.1-1ubuntu0.1~esm2"
}
]
},
"package": {
"ecosystem": "Ubuntu:Pro:24.04:LTS",
"name": "nltk",
"purl": "pkg:deb/ubuntu/nltk@3.8.1-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/noble"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.8.1-1",
"3.8.1-1ubuntu0.1~esm1",
"3.8.1-1ubuntu0.1~esm2"
]
},
{
"ecosystem_specific": {
"binaries": [
{
"binary_name": "python3-nltk",
"binary_version": "3.9.2-1ubuntu0.1~esm2"
}
]
},
"package": {
"ecosystem": "Ubuntu:Pro:26.04:LTS",
"name": "nltk",
"purl": "pkg:deb/ubuntu/nltk@3.9.2-1ubuntu0.1~esm2?arch=source\u0026distro=esm-apps/resolute"
},
"ranges": [
{
"events": [
{
"introduced": "0"
}
],
"type": "ECOSYSTEM"
}
],
"versions": [
"3.9.1-2",
"3.9.1-2build1",
"3.9.2-1",
"3.9.2-1ubuntu0.1~esm1",
"3.9.2-1ubuntu0.1~esm2"
]
}
],
"aliases": [],
"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": "UBUNTU-CVE-2026-72818",
"modified": "2026-08-27T08:42:51Z",
"published": "2026-08-25T00:00:00Z",
"references": [
{
"type": "REPORT",
"url": "https://ubuntu.com/security/CVE-2026-72818"
},
{
"type": "REPORT",
"url": "https://www.cve.org/CVERecord?id=CVE-2026-72818"
},
{
"type": "REPORT",
"url": "https://github.com/nltk/nltk"
},
{
"type": "REPORT",
"url": "https://pypi.org"
},
{
"type": "REPORT",
"url": "https://github.com/cveproject/cvelistv5/tree/main/cves/2026/72xxx/cve-2026-72818.json"
},
{
"type": "REPORT",
"url": "https://github.com/nltk/nltk/blob/3.9.4/nltk/tokenize/casual.py"
},
{
"type": "REPORT",
"url": "https://github.com/nltk/nltk/issues/3704"
},
{
"type": "REPORT",
"url": "https://github.com/nltk/nltk/releases/tag/v3.10.1"
},
{
"type": "REPORT",
"url": "https://nvd.nist.gov/vuln/detail/cve-2026-72818"
},
{
"type": "REPORT",
"url": "https://www.vulncheck.com/advisories/nltk-tweettokenizer-url-pattern-backtracks-catastrophically-on-naked-domain-like-input"
},
{
"type": "REPORT",
"url": "https://github.com/nltk/nltk/commit/7808692d451b962711005d954859bb83aabcf8fa"
},
{
"type": "REPORT",
"url": "https://security-tracker.debian.org/tracker/cve-2026-72818"
}
],
"related": [],
"schema_version": "1.7.0",
"severity": [
{
"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"
},
{
"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"
},
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
},
{
"score": "medium",
"type": "Ubuntu"
}
],
"upstream": [
"CVE-2026-72818"
]
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
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