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Not in CISA KEV
MEDIUM severity

CVE-2026-21872 — nicegui

MEDIUM

CVE-2026-21872 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in nicegui. A fix is available for nicegui — see the affected versions and patch details below.

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

Also known asGHSA-m7j5-rq9j-6jj9PYSEC-2026-1701
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 5, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21872.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs19th percentile — riskier than 19% of all scored CVEsHighest risk
0.00%0.26%0.52%0.78%0.0%0.3%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected
🐍nicegui

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

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:

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPInicegui≥ 2.22.0&&< 3.5.03.5.0pip install --upgrade 'nicegui==3.5.0'

Affected Products

1 product · 1 configurations
Application
niceguizauberzeug
≥ 2.22.0 && < 3.5.0
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nicegui, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update nicegui to 3.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21872 is resolved across your whole dependency graph.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### 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/
O3 Security · Impact-Aware SCA

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CVE-2026-21872: nicegui XSS — Fixed in 3.5.0