Fix CSRF in Python and Flask
============================

Register Flask-WTF's CSRFProtect(app). It requires the session's token on
every POST, PUT, PATCH and DELETE: render {{ csrf_token() }} in a hidden
csrf_token field, or send it from scripts in an X-CSRFToken header. Over HTTPS
it also requires a same-origin Referer. Change state only in POST views, and
set SESSION_COOKIE_SAMESITE = "Lax".

Page: https://softwaresecurity.com/fix/csrf/python-flask/

What is in this archive
-----------------------
csrf/python-flask/   the project: three implementations of the same code and one test suite
  impl/vulnerable.py   (vulnerable)
  impl/fixed.py   (fixed)
  impl/wrong.py   (a plausible fix that still fails)
run-all.mjs   runs the tests in all three modes and checks the results (Node.js only)
LICENSE-CODE.txt

Run it
------
  Tested with CPython 3.14.8. run-all.mjs needs Node.js 22 or later. Supported
  versions: https://softwaresecurity.com/fix/versions/

  From the csrf/python-flask folder, install the dependencies once (this needs
  network access):

    python3.14 -m venv .venv
    .venv/bin/python -m pip install --require-hashes -r requirements.txt

  Then run the tests. IMPL chooses the implementation the tests load:

    # every test passes
    IMPL=fixed .venv/bin/python -m pytest
    # an exploit test fails
    IMPL=vulnerable .venv/bin/python -m pytest
    # a test fails
    IMPL=wrong .venv/bin/python -m pytest

  The JUnit report is written to reports/junit.xml. To run all three modes and
  check the results in one step, run the harness from this top folder (it needs
  only Node.js; --install runs the install step first):

    node run-all.mjs --install csrf/python-flask
    node run-all.mjs csrf/python-flask

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with python 3.14.8, pip
  26.2.1, flask 3.1.3, flaskWtf 1.3.0, werkzeug 3.1.9, pytest 9.1.1. Versions
  and results hold for that date: run the tests with your own versions before
  relying on the fix.

Licence
-------
  The code is MIT licensed (LICENSE-CODE.txt). The page text is CC BY 4.0.
