CVE-2026-21871: Nicegui Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-7grm-h62g-5m97",
  "modified" : "2026-01-08T20:00:29Z",
  "published" : "2026-01-08T20:00:29Z",
  "aliases" : [ "CVE-2026-21871" ],
  "summary" : "NiceGUI is vulnerable to XSS via Unescaped URL in ui.navigate.history.push() / replace()",
  "details" : "### Summary\nXSS risk exists in NiceGUI when developers pass attacker-controlled strings into `ui.navigate.history.push()` or `ui.navigate.history.replace()`. These helpers are documented as History API wrappers for updating the browser URL without page reload. However, if the URL argument is embedded into generated JavaScript without proper escaping, a crafted payload can break out of the intended string context and execute arbitrary JavaScript in the victim’s browser.\n\n**Applications that do not pass untrusted input into `ui.navigate.history.push/replace` are not affected.**\n\n### Details\nNiceGUI provides `ui.navigate.history.push(url)` and `ui.navigate.history.replace(url)` to update the URL using the browser History API. If an application forwards user-controlled data (e.g., URL path segments, query parameters like `next=...`, form values, etc.) into these methods, an attacker can inject characters such as quotes and statement terminators to escape the JavaScript string context and execute arbitrary code.\n\nA vulnerable pattern is:\n- attacker controls a value (e.g., via the request path),\n- the application passes it to `ui.navigate.history.push(payload)` (or `replace`).\n\nThis is similar in spirit to other NiceGUI XSS advisories:\n- `ui.html()`,`ui.chat_message()` can cause XSS when developers render untrusted input as HTML (XSS risk/footgun).\n- `ui.interactive_image` had XSS via unsanitized SVG content and was handled as a security advisory with a fix and severity rating.\n\nBecause `ui.navigate.history.*` is expected to accept a URL (data) rather than executable code, the library should escape/encode the argument before emitting JavaScript.\n\n### PoC\n#### Create a simple app\n\n```python\nfrom nicegui import ui\n\n@ui.page('/')\ndef index():\n    # A link/button a victim could click (attacker can also send the URL directly)\n    ui.button('open crafted path', on_click=lambda: ui.navigate.to('/%22);alert(document.domain);//'))\n\n@ui.page('/{payload:path}')\ndef victim(payload: str):\n    ui.label(f'payload = {payload!r}')\n\n    # Vulnerable use: forwarding attacker-controlled path to history.push\n    ui.button('trigger', on_click=lambda: ui.navigate.history.push(payload))\n\nui.run()\n```\n\n#### Run the app\n\n```bash\npython app.py\n```\n\n#### Trigger\n\n1. Open `http://localhost:8080/`\n2. Click `open crafted path`\n3. Click `trigger`\n\nExpected result: JavaScript executes (an alert showing `document.domain`).\n\n### Impact\n- Vulnerability type: DOM-based XSS\n- Attack vector: attacker-controlled input embedded into JavaScript via `ui.navigate.history.push/replace`\n- Affected users: any NiceGUI-based application that forwards untrusted input into `ui.navigate.history.push()` or `ui.navigate.history.replace()`\n- Potential outcomes: client-side code execution, phishing UI injection, and other typical XSS impacts",
  "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" : "2.13.0"
      }, {
        "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-7grm-h62g-5m97"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-21871"
  }, {
    "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" : "2026-01-08T20:00:29Z",
    "nvd_published_at" : "2026-01-08T10:15:55Z"
  }
}