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CVE-2021-32677 — Cross-Site Request Forgery (CSRF) in FastAPI

Affected: tiangolo fastapi

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.

GHSA-8h2j-cgx8-6xv7 — Cross-Site Request Forgery (CSRF) in FastAPI

Affected: PyPI: 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](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) preflights, for being considered [Simple requests](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#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 we…

PYSEC-2021-100

Affected: PyPI: fastapi