CVE-2021-32677High· 8.2▾ TwilightCross-Site Request Forgery (CSRF) in FastAPI
▾ Twilight zone — High severity, or a signal on a lesser flaw
impact 45.1 · likelihood 0.1 · exploitation 0
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Exploit-prediction probability, daily snapshots since Jul 8.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via OSV
Last analysed / modified upstream
0.8%
0.8% → 0.8%
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.
This is fixed in FastAPI 0.65.2.
The request data is now parsed as JSON only if the content-type header is application/json or another JSON compatible media type like application/geo+json.
It's best to upgrade to the latest FastAPI.
But still, 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.
If you have any questions or comments, write to [email protected]
fastapi < 0.65.2Upgrade to a patched release:
fastapi 0.65.2