CVE-2026-79675

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

# nltk/internals.py, lines 211-217 (HEAD)
if options is None:
    java_options = _java_options       # validated by config_java()
else:
    if isinstance(options, str):
        options = options.split()
    java_options = list(options)       # NO validation
cmd = [_java_bin] + java_options + cmd

# nltk/internals.py, lines 122-123
_validate_java_options(options)
_java_options[:] = options

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()")

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().

CVE-2026-79675 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.10.3); upgrading removes the vulnerable code path.

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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=...).

1. What is CVE-2026-79675? CVE-2026-79675 is a critical-severity security vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.
2. How severe is CVE-2026-79675? CVE-2026-79675 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nltk are affected by CVE-2026-79675? nltk (pip) versions <= 3.10.2 is affected.
4. Is there a fix for CVE-2026-79675? Yes. CVE-2026-79675 is fixed in 3.10.3. Upgrade to this version or later.
5. Is CVE-2026-79675 exploitable, and should I be worried? Whether CVE-2026-79675 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-79675 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-79675? Upgrade nltk to 3.10.3 or later.