CVE-2026-21872

NiceGUI is a Python-based UI framework. From versions 2.22.0 to 3.4.1, an unsafe implementation in the click event listener used by ui.sub_pages, combined with attacker-controlled link rendering on the page, causes XSS when the user actively clicks on the link. This issue has been patched in version 3.5.0.

NiceGUI is a Python-based UI framework. From versions 2.22.0 to 3.4.1, an unsafe implementation in the click event listener used by ui.sub_pages, combined with attacker-controlled link rendering on the page, causes XSS when the user actively clicks on the link. This issue has been patched in version 3.5.0.

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:

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" />

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.

NiceGUI is a Python-based UI framework. From versions 2.22.0 to 3.4.1, an unsafe implementation in the click event listener used by ui.sub_pages, combined with attacker-controlled link rendering on the page, causes XSS when the user actively clicks on the link. This issue has been patched in version 3.5.0.

NiceGUI is a Python-based UI framework. From versions 2.22.0 to 3.4.1, an unsafe implementation in the click event listener used by ui.sub_pages, combined with attacker-controlled link rendering on the page, causes XSS when the user actively clicks on the link. This issue has been patched in version 3.5.0.

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:

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" />

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.

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:

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" />

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.