CVE-2025-71408

NLTK collocations eval injection allows code execution via CLI arguments.

• CVSS 8.5
• CWE-95
• Input Validation and Sanitization
• Local

NLTK (Natural Language Toolkit) before version 3.9.3 contains an eval injection vulnerability in the nltk.collocations module that allows an attacker who controls command-line arguments to execute arbitrary Python code. When collocations.py is invoked directly, the __main__ block passes command-line arguments directly to eval() as suffixes of BigramAssocMeasures without allowlist validation or sanitization, enabling an attacker to supply a Python expression that escapes the intended attribute lookup and executes arbitrary code including OS commands via the os module.

CVE

CVE-2025-71408

CWE

CWE-95

CVSS base score

8.5

Published

2026-07-24

OWASP

A03 Injection

Orthogonal defect classification

Checking

Code defect classification

Missing Check

Category

Input Validation and Sanitization

Subcategory

Command Injection

Accessibility scope

Local

Impact

Arbitrary Code Execution

Affected component

NLTK

Fixed by upgrading

Yes

Solution

Upgrade NLTK to version 3.9.3 or later.

import sys

from nltk.metrics import BigramAssocMeasures


def _measure_from_argv():
    """Resolve the association measure named on the command line."""
    suffix = sys.argv[1] if len(sys.argv) > 1 else "raw_freq"
    # VULNERABLE: command-line suffix is evaluated as Python code
    return eval("BigramAssocMeasures." + suffix)

import sys

from nltk.metrics import BigramAssocMeasures

ALLOWED_MEASURES = frozenset(
    ["raw_freq", "pmi", "student_t", "chi_sq", "likelihood_ratio"]
)


def _measure_from_argv():
    """Resolve the association measure named on the command line."""
    suffix = sys.argv[1] if len(sys.argv) > 1 else "raw_freq"
    if suffix not in ALLOWED_MEASURES:
        raise ValueError("unknown association measure: %s" % suffix)
    # FIX: allowlisted attribute lookup replaces eval of untrusted input
    return getattr(BigramAssocMeasures, suffix)

__import__('os').system('id')

@misc{vaitp:cve202571408,
  title        = {{NLTK collocations eval injection allows code execution via CLI arguments.}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2026},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2025-71408},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2025-71408/}}
}