Fix CSP IndieAuth redirection issue

app/indieauth.py

@@ -10,7 +10,6 @@ from fastapi import Form
 from fastapi import HTTPException
 from fastapi import Request
 from fastapi.responses import JSONResponse
-from fastapi.responses import RedirectResponse
 from loguru import logger
 from sqlalchemy import select
 
@@ -21,6 +20,7 @@ from app.admin import user_session_or_redirect
 from app.config import verify_csrf_token
 from app.database import AsyncSession
 from app.database import get_db_session
+from app.redirect import redirect
 from app.utils import indieauth
 from app.utils.datetime import now
 
@@ -80,7 +80,7 @@ async def indieauth_flow(
     db_session: AsyncSession = Depends(get_db_session),
     csrf_check: None = Depends(verify_csrf_token),
     _: None = Depends(user_session_or_redirect),
-) -> RedirectResponse:
+) -> templates.TemplateResponse:
     form_data = await request.form()
     logger.info(f"{form_data=}")
 
@@ -114,9 +114,8 @@ async def indieauth_flow(
     db_session.add(auth_request)
     await db_session.commit()
 
-    return RedirectResponse(
-        redirect_uri + f"?code={code}&state={state}&iss={iss}",
-        status_code=302,
+    return await redirect(
+        request, db_session, redirect_uri + f"?code={code}&state={state}&iss={iss}"
     )
 
 

app/redirect.py

@@ -0,0 +1,28 @@
+from fastapi import Request
+
+from app import templates
+from app.database import AsyncSession
+
+
+async def redirect(
+    request: Request,
+    db_session: AsyncSession,
+    url: str,
+) -> templates.TemplateResponse:
+    """
+    Similar to RedirectResponse, but uses a 200 response with HTML.
+
+    Needed for remote redirects on form submission endpoints,
+    since our CSP policy disallows remote form submission.
+    https://github.com/w3c/webappsec-csp/issues/8#issuecomment-810108984
+    """
+    return await templates.render_template(
+        db_session,
+        request,
+        "redirect.html",
+        {
+            "request": request,
+            "url": url,
+        },
+        headers={"Refresh": "0;url=" + url},
+    )

app/templates/redirect.html

@@ -0,0 +1,15 @@
+{%- import "utils.html" as utils with context -%}
+{% extends "layout.html" %}
+
+{% block head %}
+<title>{{ local_actor.display_name }}'s microblog - Redirect</title>
+{% endblock %}
+
+{% block content %}
+{% include "header.html" %}
+
+<div class="box">
+    <p>You are being redirected to: <a href="{{ url }}">{{ url }}</a></p>
+</div>
+
+{% endblock %}