--- id: CVE-2026-21873 aliases: - GHSA-mhpg-c27v-6mxr - PYSEC-2026-1702 title: NiceGUI apps which use `ui.sub_pages` vulnerable to zero-click XSS summary: NiceGUI apps which use `ui.sub_pages` vulnerable to zero-click XSS severity: high cvss: 7.2 cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N' vendor: nicegui product: nicegui ecosystem: pip affected: - 'nicegui >= 2.22.0, < 3.5.0' patched: - nicegui 3.5.0 published: '2026-01-08' updated: '2026-07-07' source: OSV sourceUrl: 'https://osv.dev/vulnerability/GHSA-mhpg-c27v-6mxr' references: - url: >- https://github.com/zauberzeug/nicegui/security/advisories/GHSA-mhpg-c27v-6mxr - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-21873' - url: 'https://github.com/zauberzeug/nicegui' - url: 'https://github.com/zauberzeug/nicegui/releases/tag/v3.5.0' tags: - osv - pip epss: 0.00264 epssPercentile: 0.16337 ingestedAt: '2026-07-08T18:25:51.228Z' --- ## Overview ### Summary An unsafe implementation in the `pushstate` event listener used by `ui.sub_pages` 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/sub_pages.js#L38-L39 2. `handleStateEvent` emits `sub_pages_open` event. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L22-L25 3. `SubPagesRouter` (used by `ui.sub_pages`), lisnening on `sub_pages_open`, `_handle_open` runs. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L18-L22 4. `_handle_open` finds any `SubPages` and runs `_show()` on them https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L63-L71 5. If the if-logic is followed or debug prints are added, it can be found that it calls `self._handle_scrolling(match, behavior='smooth')` directly https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.py#L76-L100 6. **CULPRIT** `_handle_scrolling` runs `_scroll_to_fragment` as there is a fragment, which runs vulnerable JS if the `fragment` (attacker-controlled) escapes out of the quotes. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.py#L206-L217 ### PoC Just visiting this page (no click required), consistently triggers XSS in https://nicegui.io domain. ```html ``` image ### Impact Any page which uses `ui.sub_pages` and does not actively prevent itself from being put in an iframe is affected. The impact is high since by-default NiceGUI pages are iframe-embeddable with no native opt-out functionalities except by manipulating the underlying `app` via FastAPI methods, and that `ui.sub_pages` is actively promoted as the new modern way to create Single-Page Applications (SPA). ### Patch 1. Not use `ui.sub_pages` 2. Block iframe with the following code ```py @app.middleware('http') async def iframe_blocking_middleware(request, call_next): response = await call_next(request) response.headers['X-Frame-Options'] = 'DENY' return response ``` ### Appendix AI is used safely to judge the CVSS scoring (input is censored). Please find the results in https://poe.com/s/3FXuwp7TAYxqLomARXma ### Scoring update after manual review The scoring done by AI was quite biased. Upon further review it is less dramatic. - User Interaction **None**: There's _almost_ no interaction required, and none of the interaction is with the vulnerable system. - Confidentiality & Integrity **Low**: The extent of data confidentiality & integrity loss is bounded by the highest priviledged user in the entire NiceGUI application. There does not exist a means of performing data manipulating tasks that said admin cannot already do. - Availability **None**: No DDoS is possible with this. Site remains performant as ever. ## Affected packages - `nicegui >= 2.22.0, < 3.5.0` ## Remediation Upgrade to a patched release: - `nicegui 3.5.0`