== How to fix it in FastAPI

=== Code examples

The following code is vulnerable to cross-site scripting because it returns an HTML response that contains user input.

If you do not intend to send HTML code to clients, the vulnerability can be fixed by specifying the type of data returned in the response.
For example, you can use the `JsonResponse` class to return JSON messages securely.

It is also possible to set the `Content-Type` manually by using the `media_type` parameter of the `Response` constructor.

==== Noncompliant code example

[source,python,diff-id=41,diff-type=noncompliant]
----
from fastapi import FastAPI, Response
import json

app = FastAPI()

@app.get("/example")
def example(input: str):
    json = json.dumps({"data": input})
    return Response(json)  # Noncompliant
----

[source,python,diff-id=42,diff-type=noncompliant]
----
from fastapi import FastAPI, Response

app = FastAPI()

@app.get("/example")
def example(input: str):
    return Response(input)  # Noncompliant
----

==== Compliant solution

[source,python,diff-id=41,diff-type=compliant]
----
from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

@app.get("/example")
def example(input: str):
    return JSONResponse({"data": input})
----

[source,python,diff-id=42,diff-type=compliant]
----
from fastapi import FastAPI, Response

app = FastAPI()

@app.get("/example")
def example(input: str):
    return Response(input, media_type="text/plain")
----

=== How does this work?

If the HTTP response is HTML code, it is highly recommended to use a template engine like https://jinja.palletsprojects.com/[Jinja] to generate it.
This template engine separates the view from the business logic and automatically encodes the output of variables, drastically reducing the risk of cross-site scripting vulnerabilities.

If you do not intend to send HTML code to clients, the vulnerability can be fixed by correctly setting the `Content-Type` HTTP header.
This HTTP header defines which media type the browser can expect from the response, so the browser can parse it correctly. By specifying a type that is not HTML, the browser does not interpret the response as HTML, which in turn prevents cross-site scripting.

For example, when setting the `Content-Type` header to `text/plain`, browsers will interpret the HTTP response as plaintext and will not process it any further. This allows user input to be reflected safely.

=== Pitfalls

include::../../common/pitfalls/content-types.adoc[]

include::../../common/pitfalls/validation.adoc[]

=== Going the extra mile

include::../../common/extra-mile/csp.adoc[]

