{"id":"GHSA-3hhw-38pf-pxj6","summary":"NLTK: Symlink-based arbitrary file read in IPIPANCorpusReader, bypasses nltk.pathsec entirely","details":"## Summary\n\n`IPIPANCorpusReader` (`nltk/corpus/reader/ipipan.py`) exposes public methods, `channels()`, `domains()`, `categories()`, and `fileids(channels=...)`, that accept a caller supplied `fileids` list and read a file via a completely unprotected builtin `open()` call, with no `nltk.pathsec` involvement at all. A symlink placed inside the corpus root, with a name containing no separators or `..`, passes NLTK's existing traversal checks and is opened directly, reading a file from anywhere on the filesystem the process can access.\n\n## Root cause\n\nAll four methods route through `_get_tag()`:\n\n```python\ndef _get_tag(self, f, tag):\n    tags = []\n    with open(f) as infile:   # builtin open(), no pathsec involvement\n        header = infile.read()\n    ...\n```\n\n`f` arrives via `_list_header_files()` / `_list_morph_files_by()`, both of which call:\n\n```python\nf.replace(\"morph.xml\", \"header.xml\")\n```\n\non the result of `self.abspath(...)` or `self.abspaths(...)`. `FileSystemPathPointer` subclasses `str`, so `.replace()` returns a plain Python string, silently discarding the `PathPointer` wrapper. That plain string is handed straight to builtin `open()`.\n\nThis is a more severe variant of the same CWE-59 class already fixed elsewhere in this codebase (`CorpusReader.open()`, `NKJPCorpusReader.add_root()`, and the recent `FramenetCorpusReader` fix): those route file access through `nltk.pathsec.validate_path()`, at minimum the global, non-scoped check, before opening. Here, converting the `PathPointer` to a plain string before calling `open()` skips `pathsec` completely, not just the corpus-root-scoped check, so the symlink target does not even need to land under a registered `nltk.data.path` root.\n\nPlain literal `../` traversal in the fileid is still blocked by `FileSystemPathPointer.join()`, so this is specifically the symlink variant, not a regression of the older, simpler traversal class.\n\n## Proof of concept\n\nConstructed the normal, documented way, `fileids` as a regex over file paths, so the reader auto-discovers whatever `.xml` files exist in its root with no special knowledge of the planted symlink.\n\n```python\nimport os\nimport tempfile\n\nfrom nltk.corpus.reader.ipipan import IPIPANCorpusReader\n\nroot = tempfile.mkdtemp()\ncorpus_root = os.path.join(root, \"ipipan\")\nos.makedirs(corpus_root)\n\nwith open(os.path.join(corpus_root, \"real_morph.xml\"), \"w\") as f:\n    f.write(\"\u003cchannel\u003elegit\u003c/channel\u003e\")\n\nsecret_dir = os.path.join(root, \"outside_ipipan_root\")\nos.makedirs(secret_dir)\nsecret_path = os.path.join(secret_dir, \"stolen.xml\")\nwith open(secret_path, \"w\") as f:\n    f.write(\"\u003cchannel\u003eTOP-SECRET-CHANNEL-DATA-FROM-OUTSIDE-CORPUS-ROOT\u003c/channel\u003e\")\n\nos.symlink(secret_path, os.path.join(corpus_root, \"evil_link.xml\"))\n\nreader = IPIPANCorpusReader(corpus_root, r\".*\\.xml\")\nprint(\"Auto-discovered fileids:\", sorted(reader.fileids()))\n\nresult = reader.channels(fileids=[\"evil_link.xml\"])\nprint(result)\n```\n\nActual output when run against current `develop`:\n\n```\nAuto-discovered fileids: ['evil_link.xml', 'real_morph.xml']\n['TOP-SECRET-CHANNEL-DATA-FROM-OUTSIDE-CORPUS-ROOT']\n```\n\nThat content was read from `secret_path`, a file entirely outside `corpus_root`. No exception raised anywhere. The planted symlink even surfaces naturally in the reader's own `fileids()` listing, exactly as a real file would.\n\nVerified separately that literal `../` traversal in the fileid is still rejected (`ValueError: Traversal blocked`), confirming this is specifically the symlink gap, not a broader regression.\n\n## Why this is in scope\n\n- No malicious file for a victim to open, no special user interaction. Just a tampered or shared corpus directory (`SECURITY.md` names \"shared environments... multi-tenant pipelines\" as the project's own stated threat model) plus a completely normal API call.\n- Core corpus-reader code, reached through plain `import nltk` and documented, programmatic usage (`words()`, `sents()`, `channels()`, etc.), not a demo or GUI tool.\n- Same reader category, and same CWE-59 mechanism, already treated as CVE-worthy twice in this codebase for `FramenetCorpusReader` and `NKJPCorpusReader`.\n- Not a bypass of a claimed fix. `ipipan.py` has never had security hardening applied, and has no dedicated test coverage at all.\n\n## CVSS v3.1\n\n- **AV:L, AC:L**: exploitation is local filesystem symlink placement, then immediate and deterministic once triggered.\n- **PR:L**: the attacker needs some pre-existing ability to plant a symlink somewhere reachable, not zero privilege, but not elevated either.\n- **UI:N**: fires during routine, automated corpus processing, no separate victim action.\n- **S:U**: stays within the same process's existing privileges.\n- **C:H, I:N, A:N**: arbitrary file read only, no write, no crash.\n\n## Suggested fix\n\nRoute `_get_tag()` through `nltk.pathsec.validate_path()` with the corpus root as `required_root`, or through `CorpusReader.open()`, instead of converting the `PathPointer` to a plain string and calling builtin `open()` directly. The same fix pattern already applied to `FramenetCorpusReader` and `NKJPCorpusReader` applies directly here.","aliases":["CVE-2026-62383","PYSEC-2026-3726"],"modified":"2026-09-08T16:45:03.602432270Z","published":"2026-09-08T16:40:38Z","database_specific":{"cwe_ids":["CWE-22","CWE-59"],"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-09-08T16:40:38Z","nvd_published_at":null},"references":[{"type":"WEB","url":"https://github.com/nltk/nltk/security/advisories/GHSA-3hhw-38pf-pxj6"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-62383"},{"type":"WEB","url":"https://github.com/nltk/nltk/pull/3727"},{"type":"WEB","url":"https://github.com/nltk/nltk/commit/ee1a42e51982c4dce6ad3ee77ff1ac43894288ab"},{"type":"PACKAGE","url":"https://github.com/nltk/nltk"},{"type":"WEB","url":"https://github.com/nltk/nltk/releases/tag/v3.10.2"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3726.yaml"},{"type":"WEB","url":"https://www.vulncheck.com/advisories/nltk-ipipancorpusreader-symlink-arbitrary-file-read"}],"affected":[{"package":{"name":"nltk","ecosystem":"PyPI","purl":"pkg:pypi/nltk"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"3.10.0"},{"fixed":"3.10.2"}]}],"versions":["3.10.0","3.10.1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-3hhw-38pf-pxj6/GHSA-3hhw-38pf-pxj6.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N"},{"type":"CVSS_V4","score":"CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N"}]}