CVE-2021-32677 log

Severity Medium
Remote Yes
Type Cross-site request forgery
FastAPI versions lower than 0.65.2 that used cookies for authentication in path operations that received JSON payloads sent by browsers were vulnerable to a Cross-Site Request Forgery (CSRF) attack.

In versions lower than 0.65.2, FastAPI would try to read the request payload as JSON even if the content-type header sent was not set to application/json or a compatible JSON media type (e.g. application/geo+json).

So, a request with a content type of text/plain containing JSON data would be accepted and the JSON data would be extracted.

But requests with content type text/plain are exempt from CORS preflights, for being considered Simple requests. So, the browser would execute them right away including cookies, and the text content could be a JSON string that would be parsed and accepted by the FastAPI application.
Group Package Affected Fixed Severity Status Ticket
AVG-2060 python-fastapi 0.65.1-1 0.65.2-1 Medium Fixed
Date Advisory Group Package Severity Type
01 Jul 2021 ASA-202107-6 AVG-2060 python-fastapi Medium cross-site request forgery

To mitigate the issue, it would be possible to add a middleware or a dependency that checks the content-type header and aborts the request if it is not application/json or another JSON compatible content type.