Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') in nltk

Summary

nltk.app.wordnet_app contains a reflected cross-site scripting issue in the lookup_... route. A crafted lookup_<payload> URL can inject arbitrary HTML/JavaScript into the response page because attacker-controlled word data is reflected into HTML without escaping. This impacts users running the local WordNet Browser server and can lead to script execution in the browser origin of that application.

Details

The vulnerable flow is in nltk/app/wordnet_app.py:

• nltk/app/wordnet_app.py:144

  • Requests starting with lookup_ are handled as HTML responses:
  • page, word = page_from_href(sp)

• nltk/app/wordnet_app.py:755

  • page_from_href() calls page_from_reference(Reference.decode(href))

• nltk/app/wordnet_app.py:769

  • word = href.word

• nltk/app/wordnet_app.py:796

  • If no results are found, word is inserted directly into the HTML body:
  • body = "The word or words '%s' were not found in the dictionary." % word

This is inconsistent with the search route, which does escape user input:

• nltk/app/wordnet_app.py:136
  • word = html.escape(...)

As a result, a malicious lookup_... payload can inject script into the response page.

The issue is exploitable because:

• Reference.decode() accepts attacker-controlled base64-encoded pickle data for the URL state.
• The decoded word is reflected into HTML without html.escape().
• The server is started with HTTPServer(("", port), MyServerHandler), so it listens on all interfaces by default, not just localhost.

PoC

1. Start the WordNet Browser in an isolated Docker environment:

docker run -d --name nltk-wordnet-web -p 8002:8002 \
  nltk-sandbox \
  python -c "import nltk; nltk.download('wordnet', quiet=True); from nltk.app.wordnet_app import wnb; wnb(8002, False)"

2. Use the following crafted payload, which decodes to:

("<script>alert(1)</script>", {})

Encoded payload:

gAWVIQAAAAAAAACMGTxzY3JpcHQ-YWxlcnQoMSk8L3NjcmlwdD6UfZSGlC4=

3. Request the vulnerable route:

curl -s "http://127.0.0.1:8002/lookup_gAWVIQAAAAAAAACMGTxzY3JpcHQ-YWxlcnQoMSk8L3NjcmlwdD6UfZSGlC4="

4. Observed result:

The word or words '<script>alert(1)</script>' were not found in the dictionary.

<img width="867" height="208" alt="127" src="https://github.com/user-attachments/assets/ec09da08-09bc-4fc4-bfc1-c4489e9adaf6" />

I also validated the issue directly at function level in Docker:

import base64
import pickle

from nltk.app.wordnet_app import page_from_href

payload = base64.urlsafe_b64encode(
    pickle.dumps(("<script>alert(1)</script>", {}), -1)
).decode()

page, word = page_from_href(payload)
print(word)
print("<script>alert(1)</script>" in page)

Observed output:

WORD= <script>alert(1)</script>
HAS_SCRIPT= True

Impact

This is a reflected XSS issue in the NLTK WordNet Browser web UI.

An attacker who can convince a user to open a crafted lookup_... URL can execute arbitrary JavaScript in the origin of the local WordNet Browser application. This can be used to:

• run arbitrary script in the browser tab
• manipulate the page content shown to the user
• issue same-origin requests to other WordNet Browser routes
• potentially trigger available UI actions in that local app context

This primarily impacts users who run nltk.app.wordnet_app as a local or self-hosted HTTP service and open attacker-controlled links.