NiceGUI apps which use ui.sub_pages vulnerable to zero-click XSS

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-21873: 0.26% estimated exploitation activity in the next 30 days; percentile 17.80 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary An unsafe implementation in the pushstate event listener used by ui.subpages allows an attacker to manipulate the fragment identifier of the URL, which they can do despite being cross-site, using an iframe. Details The problem is traced as follows: 1. On pushstate, handleStateEvent is executed. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/subpages.js#L38-L39 2. handleStateEvent emits subpagesopen event. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/subpages.js#L22-L25 3. SubPagesRouter (used by ui.subpages), lisnening on subpagesopen, handleopen runs. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/subpagesrouter.py#L18-L22 4. handleopen finds any SubPages and runs show() on them https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/subpagesrouter.py#L63-L71 5. If the if-logic is…

Published by the source

8 Jan 2026, 20:16 UTC

Last modified by the source

7 Jul 2026, 17:57 UTC

Source last checked

26 Oct 2026, 13:36 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mhpg-c27v-6mxr

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-mhpg-c27v-6mxr

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# Patch Next advisory brief: CVE-2026-21873

Advisory: NiceGUI apps which use `ui.sub_pages` vulnerable to zero-click XSS
IDs: CVE-2026-21873, GHSA-mhpg-c27v-6mxr
Record: https://patchnext.com/advisories/cve-2026-21873 (revision 2, source checked 26 Oct 2026, 13:36 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: nicegui 3.5.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI nicegui: affected >= 2.22.0, <= 3.4.1; fixed in 3.5.0

## Suggested inspection (read-only)
- `python -m pip show nicegui`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on nicegui.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-mhpg-c27v-6mxr (OSV record GHSA-mhpg-c27v-6mxr lists affected ranges: PyPI nicegui >=2.22.0 <3.5.0 (fixed))
- Verified: https://github.com/advisories/GHSA-mhpg-c27v-6mxr (GitHub-reviewed advisory GHSA-mhpg-c27v-6mxr lists affected ranges: PyPI nicegui >=2.22.0 <=3.4.1)

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