CVE-2025-53528

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

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)+'',

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

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2025-53528 has a CVSS score of 7.6 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.4.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-53528? CVE-2025-53528 is a high-severity cross-site scripting (XSS) vulnerability in cadwyn (pip), affecting versions < 5.4.3. It is fixed in 5.4.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2025-53528? CVE-2025-53528 has a CVSS score of 7.6 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of cadwyn are affected by CVE-2025-53528? cadwyn (pip) versions < 5.4.3 is affected.
4. Is there a fix for CVE-2025-53528? Yes. CVE-2025-53528 is fixed in 5.4.3. Upgrade to this version or later.
5. Is CVE-2025-53528 exploitable, and should I be worried? Whether CVE-2025-53528 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-53528 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-53528? Upgrade cadwyn to 5.4.3 or later.