CVE-2026-21872

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.

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.

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.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-21872 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.5.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-21872? CVE-2026-21872 is a medium-severity cross-site scripting (XSS) vulnerability in nicegui (pip), affecting versions >= 2.22.0, <= 3.4.1. It is fixed in 3.5.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-21872? CVE-2026-21872 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nicegui are affected by CVE-2026-21872? nicegui (pip) versions >= 2.22.0, <= 3.4.1 is affected.
4. Is there a fix for CVE-2026-21872? Yes. CVE-2026-21872 is fixed in 3.5.0. Upgrade to this version or later.
5. Is CVE-2026-21872 exploitable, and should I be worried? Whether CVE-2026-21872 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-21872 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-21872? Upgrade nicegui to 3.5.0 or later.