{"id":"CVE-2026-21872","aliases":["GHSA-m7j5-rq9j-6jj9","PYSEC-2026-1701"],"url":"https://o3.security/vulnerability/CVE-2026-21872","summary":"NiceGUI apps are vulnerable to XSS which uses `ui.sub_pages` and render arbitrary user-provided links","details":"### Summary\n\nAn 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. \n\n### Details\n\n1. On `click`, eventually `sub_pages_navigate` event is emitted.\nhttps://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L41-L63\n\n2. SubPagesRouter (used by ui.sub_pages), lisnening on `sub_pages_navigate`, `_handle_navigate` runs.\nhttps://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L18-L22\n\n3. `_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. \n\nhttps://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L73-L88\n\n### PoC\n\nThe minimal PoC boils down to this:\n\n```py\nfrom nicegui import ui\n\nui.sub_pages({'/': lambda: ui.link('Go to XSS', '/\"+alert(1)+\"')})\n\nui.run()\n```\n\nHowever, it is more likely that the attack takes place with attacker-controlled input, for which this shows it:\n\n```py\nfrom nicegui import app, ui\n\nui.sub_pages({'/': lambda: ui.label('Hello, World!')})\n\nui.textarea('Markdown content').bind_value(app.storage.general, 'markdown_content')\n\nui.markdown().bind_content_from(app.storage.general, 'markdown_content')\n\nui.run()\n```\n\nVulnerable input is `[XSS LINK](/\"+alert(document.domain)+\")` (causes double payload execution, though)\n\n**Both cases require someone to click on the link.** \n\n<img width=\"1428\" height=\"254\" alt=\"image\" src=\"https://github.com/user-attachments/assets/8be4f345-c0cf-4df2-9917-677a2ea72626\" />\n\n### Impact\n\nAny page which uses `ui.sub_pages` and renders arbitrary links on screen (common case of `ui.markdown`) is affected.\n\nThe 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. \n\n### Appendix\n\nAI is used safely to judge the CVSS scoring (input is not even provided, just the impact statement).\n\nPlease find the results in https://poe.com/s/y5DvyqgtszDGLUuHin1O\n\n### Scoring update after manual review\n\n- Scope **Changed** is more inline with other posted XSS vulnerabilities\n- Availability **None**: No DDoS is possible with this. Site remains performant as ever.","published":"2026-01-08T09:50:02.801Z","modified":"2026-10-28T02:31:14.835525466Z","cvss":{"score":6.1,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"nicegui","fixedVersion":"3.5.0"}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/zauberzeug/nicegui/releases/tag/v3.5.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/21xxx/CVE-2026-21872.json"},{"type":"ADVISORY","url":"https://github.com/zauberzeug/nicegui/security/advisories/GHSA-m7j5-rq9j-6jj9"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-21872"},{"type":"PACKAGE","url":"https://github.com/zauberzeug/nicegui"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:31:14.835525466Z"}}