NLTK: JVM argument injection bypass via per-call options in the NLTK Stanford wrappers (incomplete fix of CVE-2026-12841)

Proof of Concept

from nltk.internals import config_java, java, _validate_java_options

# 1. The global config_java() path correctly blocks dangerous flags:
try:
    config_java(options=["-agentpath:/tmp/evil.so"])
except ValueError as e:
    print(f"config_java blocked: {e}")   # blocked as expected

# 2. The per-call options path does NOT block them:
# (Would execute if Java were installed)
# java(["SomeClass"], classpath=".", options=["-agentpath:/tmp/evil.so"])
# This passes "-agentpath:/tmp/evil.so" directly to subprocess.Popen

# 3. Stanford wrapper classes pass through without validation:
# from nltk.parse.stanford import StanfordParser
# parser = StanfordParser(java_options="-agentpath:/tmp/evil.so")
# parser.parse(...)  # dangerous flag reaches JVM

# Verify the gap directly:
dangerous_opts = ["-agentpath:/tmp/evil.so"]
try:
    _validate_java_options(dangerous_opts)
    print("Would have been caught")
except ValueError:
    print("Correctly rejected by _validate_java_options()")

# But java() itself never calls _validate_java_options():
import inspect
source = inspect.getsource(java)
assert "_validate_java_options" not in source, "java() does not validate options"
print("Confirmed: java() does not call _validate_java_options()")

Impact

An attacker who controls the java_options parameter to any NLTK Stanford wrapper class can inject arbitrary JVM flags, including:

• -agentpath:/path/to/malicious.so -- loads a native agent, achieving arbitrary code execution
• -javaagent:/path/to/malicious.jar -- loads a Java agent for bytecode manipulation
• -agentlib:jdwp=transport=dt_socket,server=y,address=*:5005 -- enables remote debugging, allowing remote code execution
• @/path/to/argfile -- expands an argument file, which can smuggle any of the above

This is exploitable in scenarios where NLTK is deployed as a service and java_options is derived from user input, configuration files, or environment variables. The PR #3647 commit message explicitly states the fix was intended to cover "StanfordSegmenter, and GenericStanfordParser" but the implementation only validates in config_java().

Suggested Fix

Add _validate_java_options() to the java() function's per-call options handling:

# nltk/internals.py, in the java() function
if options is None:
    java_options = _java_options
else:
    if isinstance(options, str):
        options = options.split()
    java_options = list(options)
    _validate_java_options(java_options)   # ADD THIS LINE
cmd = [_java_bin] + java_options + cmd

This single-line addition closes the bypass for all four Stanford wrapper classes and any future callers of java(options=...).

AI tooling

AI assistance was used for the code audit and for drafting this report. The finding were manually verified against the project's source at the location cited above before reporting it, and the severity and impact assessment are the reporters.