FastAPI CSRF Protect

Features

FastAPI extension that provides stateless Cross-Site Request Forgery (XSRF) Protection support. Aimed to be easy to use and lightweight, we adopt Double Submit Cookie mitigation pattern. If you were familiar with flask-wtf library this extension suitable for you. This extension inspired by fastapi-jwt-auth 😀

• Storing fastapi-csrf-token in cookies or serve it in template's context

Installation

The easiest way to start working with this extension with pip

pip install fastapi-csrf-protect
# or
uv add fastapi-csrf-protect

Getting Started

The following examples show you how to integrate this extension to a FastAPI App

Example Login Form

from fastapi import FastAPI, Request, Depends
from fastapi.responses import JSONResponse
from fastapi.templating import Jinja2Templates
from fastapi_csrf_protect import CsrfProtect
from fastapi_csrf_protect.exceptions import CsrfProtectError
from pydantic_settings import BaseSettings

app = FastAPI()
templates = Jinja2Templates(directory="templates")

class CsrfSettings(BaseSettings):
  secret_key: str = "asecrettoeverybody"
  cookie_samesite: str = "none"

@CsrfProtect.load_config
def get_csrf_config():
  return CsrfSettings()

@app.get("/login")
def form(request: Request, csrf_protect: CsrfProtect = Depends()):
  """
  Returns form template.
  """
  csrf_token, signed_token = csrf_protect.generate_csrf_tokens()
  response = templates.TemplateResponse(
    "form.html", {"request": request, "csrf_token": csrf_token}
  )
  csrf_protect.set_csrf_cookie(signed_token, response)
  return response

@app.post("/login", response_class=JSONResponse)
async def create_post(request: Request, csrf_protect: CsrfProtect = Depends()):
  """
  Creates a new Post
  """
  await csrf_protect.validate_csrf(request)
  response: JSONResponse = JSONResponse(status_code=200, content={"detail": "OK"})
  csrf_protect.unset_csrf_cookie(response)  # prevent token reuse
  return response

@app.exception_handler(CsrfProtectError)
def csrf_protect_exception_handler(request: Request, exc: CsrfProtectError):
  return JSONResponse(status_code=exc.status_code, content={"detail": exc.message})

How to send the CSRF token in your client code

HTML Form (Server-side rendered)

<form method="post" action="/login">
  <input type="hidden" name="token_key" value="{{ csrf_token }}">
  <!-- other fields -->
</form>

AJAX (JavaScript)

fetch("/items/123", {
  method: "DELETE",
  headers: {
    "X-CSRFToken": getCookie("csrftoken")
  },
  credentials: "include"
});

Important

• The flexible sub-package ignores the token_location setting — tokens from either header or body are always accepted.
• CSRF token validation still requires a matching CSRF cookie as in the base package.
• Priority is given to header over body when both are present.

📌 Flexible Mode (fastapi_csrf_protect.flexible)

Some applications combine Server-Side Rendering (SSR) with API endpoints in the same project. For example:

• SSR pages rendered with Jinja2 templates that use HTML forms (CSRF token in form body)
• AJAX / API calls (e.g. DELETE, PUT, PATCH) that pass the CSRF token in the HTTP header

The main fastapi-csrf-protect package is opinionated and expects the CSRF token in one location only (either header or body). For hybrid apps, this can be inconvenient.

The flexible sub-package provides a drop-in replacement for CsrfProtect that always accepts CSRF tokens from either the header or the form body, with the following priority:

• Header: X-CSRFToken
• Body: token_key (form-data)

When to use flexible

Use fastapi_csrf_protect.flexible if:

• You have both SSR pages and API endpoints in the same project.
• Some requests (like DELETE) cannot send a body but still require CSRF validation.
• You want to avoid maintaining two different CSRF configurations.

If your app only uses one method to send CSRF tokens, stick to the core package for a stricter policy.

Contributions

Prerequisites

• python version 3.9 and above
• uv

Setting up

The following guide walks through setting up your local working environment using pyenv as Python version manager and uv as Python package manager. If you do not have pyenv installed, run the following command.

Install using Homebrew (Darwin)

brew install pyenv --head

Install using standalone installer (Darwin and Linux)

curl https://pyenv.run | bash

If you do not have uv installed, run the following command.

Install using Homebrew (Darwin)

brew install uv

Install using standalone installer (Darwin and Linux)

curl -LsSf https://astral.sh/uv/install.sh | sh

Once you have pyenv Python version manager installed, you can install any version of Python above version 3.9 for this project. The following commands help you set up and activate a Python virtual environment where uv can download project dependencies from the PyPI open-sourced registry defined under pyproject.toml file.

Set up environment and synchronize project dependencies

pyenv shell 3.11.9
uv venv  --python-preference system
source .venv/bin/activate

Getting started

To contribute to the project, fork the repository and clone to your local device and install preferred testing dependency pytest Alternatively, run the following command on your terminal to do so:

uv sync --dev

Testing can be done by the following command post-installation:

uv sync --dev --group=tests
pytest

Change-logs

🚧 Breaking Changes (0.3.0 -> 0.3.1) The double submit update

• The generate_csrf method has now been marked for deprecation
• The recommended method is now generate_csrf_tokens which returns a tuple of tokens, first unsigned and the latter signed
• Recommended pattern is for the first token is aimed for returning as part of context
• Recommended pattern is for the signed token to be set in client's cookie completing Double Submit Cookie
• To prevent token reuse, protected endpoint can unset the signed CSRF Token in client's cookies as per example code and recommended pattern.

🚧 Breaking Changes (0.3.1 -> 0.3.2) The anti-JavaScript update

• New keys are added at setup token_location (either body or header) and token_key is key where form-encoded keeps the csrf token stored, cross-checked with csrf secret in cookies.
• Asynchronous validate_csrf method now needs to be awaited therefore protected endpoints need to be asynchronous as well.

Error in version 0.3.5 after updating to Pydantic V2

• Made a blunder when updating from Pydantic V1 to Pydantic V2 and caused an error to occur when setting cookie_samesite in settings
• Fixed in version 0.3.6

Version 1.0

• Remove deprecated method generate_csrf, please use generate_csrf_tokens.
• Validate FormData value received for given token_key is in fact a string, not UploadFile

Version 1.0.1

• Fix cookie unsetting when configuring library with cookie Secure and / or SameSite=None
• Test cookie settings covering SameSite options and Secure usage
• Bypass https tests using manual test_client.base_url = 'https://testserver'

Version 1.0.2

• Improve boolean handling for LoadConfig

Version 1.0.3

• Attempted to make mypyc compilation; Failed due to dependency injection pattern
• Add py.typed to project

Version 1.0.4 (Failed rollout; please use 1.0.5)

• Add submodule flexible where CsrfProtect does not pre-determine token_key & token_location
• Test fastapi_csrf_protect.flexible.CsrfProtect with runtime variable token_location

Version 1.0.5

• Remove @dataclass code leftover from mypyc experiment
• Clarify failure reasons under tests/load_config.py and tests/flexible/load_config.py

Run Examples

To run the provided examples, first you must install extra dependencies uvicorn and jinja2 Alternatively, run the following command on your terminal to do so

uv sync --group=examples

Running the example utilizing form submission

uvicorn examples.body:app

Running the example utilizing headers via JavaScript

uvicorn examples.header:app

License

This project is licensed under the terms of the MIT license.