Summary

The version parameter of the /docs endpoint is vulnerable to a Reflected XSS (Cross-Site Scripting) attack.

PoC

1. Setup a minimal app following the quickstart guide: https://docs.cadwyn.dev/quickstart/setup/
2. Click on the following PoC link: http://localhost:8000/docs?version=%27%2balert(document.domain)%2b%27

Impact

Refer to this security advisory for an example of the impact of a similar vulnerability that shares the same root cause.

This XSS would notably allow an attacker to execute JavaScript code on a user's session for any application based on Cadwyn via a one-click attack.

A CVSS for the average case may be: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:L

Details

The vulnerable code snippet can be found in the 2 functions swagger_dashboard and redoc_dashboard: https://github.com/zmievsa/cadwyn/blob/main/cadwyn/applications.py#L387-L413

The implementation uses the get_swagger_ui_html function from FastAPI. This function does not encode or sanitize its arguments before using them to generate the HTML for the swagger documentation page and is not intended to be used with user-controlled arguments.

    async def swagger_dashboard(self, req: Request) -> Response:
        version = req.query_params.get("version")

        if version:
            root_path = self._extract_root_path(req)
            openapi_url = root_path + f"{self.openapi_url}?version={version}"
            oauth2_redirect_url = self.swagger_ui_oauth2_redirect_url
            if oauth2_redirect_url:
                oauth2_redirect_url = root_path + oauth2_redirect_url
            return get_swagger_ui_html(
                openapi_url=openapi_url,
                title=f"{self.title} - Swagger UI",
                oauth2_redirect_url=oauth2_redirect_url,
                init_oauth=self.swagger_ui_init_oauth,
                swagger_ui_parameters=self.swagger_ui_parameters,
            )
        return self._render_docs_dashboard(req, cast("str", self.docs_url))

In this case, the openapi_url variable contains the version which comes from a user supplied query string without encoding or sanitisation. The user controlled injection ends up inside of a string in a <script> tag context: https://github.com/fastapi/fastapi/blob/master/fastapi/openapi/docs.py#L132

    f"""
    ...
    const ui = SwaggerUIBundle({{
        url: '{openapi_url}',
    """

By simply injecting a single quote we can escape from the string context and execute JavaScript like so '+alert(document.domain)+'

The resulting HTML sent back from the server contains the following injection:

  const ui = SwaggerUIBundle({
        url: '/openapi/flows.json?flows='+alert(document.domain)+'',

• https://github.com/zmievsa/cadwyn/security/advisories/GHSA-2gxp-6r36-m97r
• https://github.com/zmievsa/cadwyn/commit/b424ecd57cd8dabbc8fe39b8f8ccafea629c7728
• https://nvd.nist.gov/vuln/detail/CVE-2025-53528
• https://github.com/zmievsa/cadwyn/blob/5.4.3/CHANGELOG.md#543
• https://github.com/pypa/advisory-database/tree/main/vulns/cadwyn/PYSEC-2025-71.yaml
• https://github.com/advisories/GHSA-2gxp-6r36-m97r