CVE-2026-21872

MEDIUM6.1EPSS 0.02%

NiceGUI apps are vulnerable to XSS which uses `ui.sub_pages` and render arbitrary user-provided links

Published: 1/8/2026Modified: 2/3/2026

Description

### Summary 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. ### Details 1. On `click`, eventually `sub_pages_navigate` event is emitted. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L41-L63 2. 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 3. `_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 ### PoC The minimal PoC boils down to this: ```py 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: ```py 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" /> ### Impact Any page which uses `ui.sub_pages` and renders arbitrary links on screen (common case of `ui.markdown`) is affected. The 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. ### Appendix 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 ### Scoring update after manual review - Scope **Changed** is more inline with other posted XSS vulnerabilities - Availability **None**: No DDoS is possible with this. Site remains performant as ever.

Affected packages (1)

CVSS scores

SourceVersionSeverityVector
osvCVSS 3.1MEDIUM6.1CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

References (4)