CVE-2026-21872: Nicegui Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-m7j5-rq9j-6jj9",
  "modified" : "2026-01-08T20:08:42Z",
  "published" : "2026-01-08T20:08:42Z",
  "aliases" : [ "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "nicegui"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.22.0"
      }, {
        "fixed" : "3.5.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.4.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "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"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/zauberzeug/nicegui/releases/tag/v3.5.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-08T20:08:42Z",
    "nvd_published_at" : "2026-01-08T10:15:55Z"
  }
}