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The recently patched CVE-2026-12841 addressed a JVM argument injection vulnerability by introducing the `_validate_java_options()` function, designed to block dangerous flags like -agentlib, -agentpath, -javaagent, -Xrunjdwp, and `@argfile` references. However, this validation was only applied to the global `config_java()` function. The `java()` function, which handles per-call options via an `options` parameter (added by PR 3683 for CVE-2026-12615), does not call this validation, leaving a significant bypass.
The root cause lies in the `java()` function within nltk/internals.py. When an `options` argument is provided, it’s directly converted into a list and prepended to the JVM command without any sanitization. In contrast, `config_java()` explicitly calls `_validate_java_options()` on the new configuration. Four critical Stanford wrapper classes—GenericStanfordParser, StanfordTagger, StanfordTokenizer, and StanfordSegmenter—all accept user-supplied `java_options` and pass them directly through this vulnerable `java()` path, inheriting the flaw.
This means an attacker who can control the `java_options` parameter of any of these wrapper classes can inject arbitrary JVM flags. This includes loading a native agent for arbitrary code execution (-agentpath), using a Java agent for bytecode manipulation (-javaagent), enabling remote debugging to potentially execute code (-agentlib:jdwp), or using an argument file to smuggle any of these flags (@argfile). This is especially severe in environments like web services where user input can reach these parameters, leading to full system compromise.
The fix is simple: add a single line to the `java()` function to call `_validate_java_options()` on the per-call `options` before it is used in the command. While AI tooling was used to assist in the audit and drafting, the findings were manually verified against the source code. The impact is critical due to the ease of exploitation and the potential for arbitrary code execution, effectively bypassing the intended security patch for CVE-2026-12841.
DailyCVE Form:
Platform: NLTK
Version: All
Vulnerability : JVM Injection
Severity: Critical
date: 2026-08-31
Prediction: 2026-09-15
What Undercode Say:
Analytics: The bypass is effective due to inconsistent validation across functions.
Check for unvalidated per-call path in source
grep -n "options is None" nltk/internals.py
Find Stanford wrapper usage of java()
grep -rn "java(" nltk/tag/stanford.py nltk/parse/stanford.py nltk/tokenize/stanford
Exploit: (Educational Purposes!)
Exploit PoC demonstrating bypass of CVE-2026-12841
from nltk.parse.stanford import StanfordParser
Malicious JVM options to load a native agent
malicious_options = "-agentpath:/tmp/evil.so"
Instantiate StanfordParser with the malicious options
parser = StanfordParser(java_options=malicious_options)
Trigger the java() function call; the malicious flag reaches JVM
This will attempt to load /tmp/evil.so on the target system
try:
parser.parse(["The quick brown fox jumps over the lazy dog."])
except Exception as e:
print(f"Exploit triggered: {e}")
Protection:
Patch java() to validate options.
Avoid using Stanford wrappers.
Use config_java() securely.
Impact:
Arbitrary code execution.
Full system compromise.
Data breach and exposure.
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Sources:
Reported By: github.com
Extra Source Hub:
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