CVE-2021-32677: Cross-Site Request Forgery (CSRF) in FastAPI
Impact
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.
Patches
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.
Workarounds
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.
References
CORS on Mozilla web docs This answer on StackExchange OWASP CSRF Fixed in PR #2118
For more information
If you have any questions or comments, write to security@tiangolo.com
Other sources
FastAPI is a web framework for building APIs with Python 3.6+ based on standard Python type hints. 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). A request with a content type of text/plain containing JSON data would be accepted and the JSON data would be extracted. Requests with content type text/plain are exempt from CORS preflights, for being considered Simple requests. The browser will 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 if updating is not possible then 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 can act as a mitigating workaround.
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2021-32677?
CVE-2021-32677 is classified as a critical vulnerability due to its potential for Cross-Site Request Forgery (CSRF) attacks.
How do I fix CVE-2021-32677?
To fix CVE-2021-32677, upgrade FastAPI to version 0.65.2 or later.
Which versions of FastAPI are affected by CVE-2021-32677?
FastAPI versions lower than 0.65.2 are affected by CVE-2021-32677.
What type of vulnerability is CVE-2021-32677?
CVE-2021-32677 is a Cross-Site Request Forgery (CSRF) vulnerability.
What impact does CVE-2021-32677 have on applications?
CVE-2021-32677 can allow attackers to execute unauthorized actions on behalf of authenticated users.