Affected: PyPI: nltk

### Summary A path-traversal vulnerability in `NKJPCorpusReader` allows an attacker who can influence the `fileids` argument of its public read methods (`header`, `raw`, `words`, `sents`, `tagged_words`) to read files outside the corpus root. The reader builds the file path with no containment check and opens it with the builtin `open()`, so it bypasses NLTK's `nltk.pathsec` sandbox — including the strict `ENFORCE = True` mode that `SECURITY.md` recommends for web/multi-tenant deployments. `header()` returns the parsed content of the out-of-root file to the caller (arbitrary file read).

### Details `SECURITY.md` promises that file access is "validated against allowed NLTK data directories" and that with `nltk.pathsec.ENFORCE = True` "unauthorized file access … will raise `PermissionError`." That guarantee is enforced via `FileSystemPathPointer.open()` / `CorpusReader.open()`, which call `nltk.pathsec.validate_path(...)`.

`NKJPCorpusReader` never uses that protected path. In `nltk/corpus/reader/nkjp.py`:

- `add_root()` builds the path by **plain string concatenation** with no normalization or containment check: ```python def add_root(self, fileid): # lines 96-102 if self.root in fileid: return fileid # attacker-controlled value returned unchanged return self.root + fileid # plain concat, '..' not stripped ``` - The header view appends a fi…

Affected: PyPI: nltk

### Summary A path-traversal vulnerability in `NKJPCorpusReader` allows an attacker who can influence the `fileids` argument of its public read methods (`header`, `raw`, `words`, `sents`, `tagged_words`) to read files outside the corpus root. The reader builds the file path with no containment check and opens it with the builtin `open()`, so it bypasses NLTK's `nltk.pathsec` sandbox — including the strict `ENFORCE = True` mode that `SECURITY.md` recommends for web/multi-tenant deployments. `header()` returns the parsed content of the out-of-root file to the caller (arbitrary file read).

### Details `SECURITY.md` promises that file access is "validated against allowed NLTK data directories" and that with `nltk.pathsec.ENFORCE = True` "unauthorized file access … will raise `PermissionError`." That guarantee is enforced via `FileSystemPathPointer.open()` / `CorpusReader.open()`, which call `nltk.pathsec.validate_path(...)`.

`NKJPCorpusReader` never uses that protected path. In `nltk/corpus/reader/nkjp.py`:

- `add_root()` builds the path by **plain string concatenation** with no normalization or containment check: ```python def add_root(self, fileid): # lines 96-102 if self.root in fileid: return fileid # attacker-controlled value returned unchanged return self.root + fileid # plain concat, '..' not stripped ``` - The header view appends a fi…