CVE-2026-21872 is a medium severity vulnerability with a CVSS score of 6.1. No known exploits currently, and patches are available.
Very low probability of exploitation
EPSS predicts the probability of exploitation in the next 30 days based on real-world threat data, complementing CVSS severity scores with actual risk assessment.
An unsafe implementation in the click event listener used by ui.sub_pages, combined with attacker-controlled link rendering on the page, causes an XSS when the user actively clicks on the link.
On click, eventually sub_pages_navigate event is emitted.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L41-L63
SubPagesRouter (used by ui.sub_pages), lisnening on sub_pages_navigate, _handle_navigate runs.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L18-L22
_handle_navigate runs run_javascript with f-string substituting self.current_path which is simply surrounded by double-quotes. The string context can be broken out easily.
https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L73-L88
The minimal PoC boils down to this:
from nicegui import ui
ui.sub_pages({'/': lambda: ui.link('Go to XSS', '/"+alert(1)+"')})
ui.run()
However, it is more likely that the attack takes place with attacker-controlled input, for which this shows it:
from nicegui import app, ui
ui.sub_pages({'/': lambda: ui.label('Hello, World!')})
ui.textarea('Markdown content').bind_value(app.storage.general, 'markdown_content')
ui.markdown().bind_content_from(app.storage.general, 'markdown_content')
ui.run()
Vulnerable input is [XSS LINK](/"+alert(document.domain)+") (causes double payload execution, though)
Both cases require someone to click on the link.
<img width="1428" height="254" alt="image" src="https://github.com/user-attachments/assets/8be4f345-c0cf-4df2-9917-677a2ea72626" />Any page which uses ui.sub_pages and renders arbitrary links on screen (common case of ) is affected.
| Vendor | Product |
|---|---|
| Zauberzeug | Nicegui |
Please cite this page when referencing data from Strobes VI. Proper attribution helps support our vulnerability intelligence research.
ui.markdownThe impact is low since a click is always required from the user, who can on-hover to discover the sketchy content of the link and stop if well-trained.
AI is used safely to judge the CVSS scoring (input is not even provided, just the impact statement).
Please find the results in https://poe.com/s/y5DvyqgtszDGLUuHin1O