GCVE Workshop - 22 September 2026 (14:00-18:00), Luxembourg Before The Vulnopticon Conference - Registration

PYSEC-2026-1701

Vulnerability from pysec - Published: 2026-07-07 16:03 - Updated: 2026-07-07 17:24
VLAI
Details

Summary

An 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.

Details

  1. On click, eventually sub_pages_navigate event is emitted. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/elements/sub_pages.js#L41-L63

  2. SubPagesRouter (used by ui.sub_pages), lisnening on sub_pages_navigate, _handle_navigate runs. https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L18-L22

  3. _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.

https://github.com/zauberzeug/nicegui/blob/59fa9424c470f1b12c5d368985fa36e21fda706b/nicegui/sub_pages_router.py#L73-L88

PoC

The minimal PoC boils down to this:

from nicegui import ui

ui.sub_pages({'/': lambda: ui.link('Go to XSS', '/"+alert(1)+"')})

ui.run()

However, it is more likely that the attack takes place with attacker-controlled input, for which this shows it:

from nicegui import app, ui

ui.sub_pages({'/': lambda: ui.label('Hello, World!')})

ui.textarea('Markdown content').bind_value(app.storage.general, 'markdown_content')

ui.markdown().bind_content_from(app.storage.general, 'markdown_content')

ui.run()

Vulnerable input is [XSS LINK](/"+alert(document.domain)+") (causes double payload execution, though)

Both cases require someone to click on the link.

image

Impact

Any page which uses ui.sub_pages and renders arbitrary links on screen (common case of ui.markdown) is affected.

The 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.

Appendix

AI is used safely to judge the CVSS scoring (input is not even provided, just the impact statement).

Please find the results in https://poe.com/s/y5DvyqgtszDGLUuHin1O

Scoring update after manual review

  • Scope Changed is more inline with other posted XSS vulnerabilities
  • Availability None: No DDoS is possible with this. Site remains performant as ever.
Severity
6.1 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Impacted products
Name purl
nicegui pkg:pypi/nicegui
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "nicegui",
        "purl": "pkg:pypi/nicegui"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.22.0"
            },
            {
              "fixed": "3.5.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.22.0",
        "2.22.1",
        "2.22.2",
        "2.23.0",
        "2.23.1",
        "2.23.2",
        "2.23.3",
        "2.24.0",
        "2.24.1",
        "2.24.2",
        "3.0.0",
        "3.0.0rc1",
        "3.0.1",
        "3.0.2",
        "3.0.3",
        "3.0.4",
        "3.1.0",
        "3.2.0",
        "3.3.0",
        "3.3.1",
        "3.4.0",
        "3.4.1"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-21872",
    "GHSA-m7j5-rq9j-6jj9"
  ],
  "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({\u0027/\u0027: lambda: ui.link(\u0027Go to XSS\u0027, \u0027/\"+alert(1)+\"\u0027)})\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({\u0027/\u0027: lambda: ui.label(\u0027Hello, World!\u0027)})\n\nui.textarea(\u0027Markdown content\u0027).bind_value(app.storage.general, \u0027markdown_content\u0027)\n\nui.markdown().bind_content_from(app.storage.general, \u0027markdown_content\u0027)\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\u003cimg width=\"1428\" height=\"254\" alt=\"image\" src=\"https://github.com/user-attachments/assets/8be4f345-c0cf-4df2-9917-677a2ea72626\" /\u003e\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.",
  "id": "PYSEC-2026-1701",
  "modified": "2026-07-07T17:24:49.318786Z",
  "published": "2026-07-07T16:03:17.198320Z",
  "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"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/nicegui"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-m7j5-rq9j-6jj9"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "NiceGUI apps are vulnerable to XSS which uses `ui.sub_pages` and render arbitrary user-provided links"
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.