CVE-2026-79675: NLTK before 3.10.3 JVM Argument Injection via Per-Call Options

Vulnerability

The fix for CVE-2026-12841 (CWE-88, JVM argument injection) added validatejavaoptions() to block dangerous JVM flags such as -agentlib, -agentpath, -javaagent, -Xrunjdwp, and @argfile references. However, the validation is only applied when setting global options via configjava(). The java() function's per-call options parameter -- added by PR #3683 (CVE-2026-12615 fix) -- passes options directly to subprocess.Popen without calling validatejavaoptions().

All four Stanford Java wrapper classes accept user-supplied javaoptions and route them through the unvalidated per-call path, bypassing the CVE-2026-12841 fix entirely.

Root Cause

In nltk/internals.py, the java() function (line 128) accepts an options keyword argument. When options is not None, it is converted to a list and prepended to the JVM command (lines 211-217) without any validation:

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

Compare with configjava() (line 92) which does validate:

python nltk/internals.py, lines 122-123 validatejavaoptions(options) javaoptions[:] = options

The four affected wrapper classes store user-supplied javaoptions without validation and pass them through the unvalidated per-call path:

1. GenericStanfordParser (nltk/parse/stanford.py): constructor parameter at line 39, stored at line 78, passed at lines 247 and 256 2. StanfordTagger (nltk/tag/stanford.py): constructor parameter at line 51, stored at line 79, passed at line 118 3. StanfordTokenizer (nltk/tokenize/stanford.py): constructor parameter at line 43, stored at line 66, passed at line 109 4. StanfordSegmenter (nltk/tokenize/stanfordsegmenter.py): constructor parameter at line 68, stored at line 117, passed at line 337

Proof of Concept

python from nltk.internals import configjava, java, validatejavaoptions

1. The global configjava() path correctly blocks dangerous flags: try: configjava(options=["-agentpath:/tmp/evil.so"]) except ValueError as e: print(f"configjava 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(javaoptions="-agentpath:/tmp/evil.so") parser.parse(...) # dangerous flag reaches JVM

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

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

Impact

An attacker who controls the javaoptions 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=dtsocket,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 javaoptions 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 configjava().

Suggested Fix

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

python nltk/internals.py, in the java() function if options is None: javaoptions = javaoptions else: if isinstance(options, str): options = options.split() javaoptions = list(options) validatejavaoptions(javaoptions) # ADD THIS LINE cmd = [javabin] + javaoptions + 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.