Fix unrestricted file upload in Python and Flask
================================================

Decide the type from the bytes, and never let the client pick the name. Cap
request.max_content_length, check the file's exact size, require the
extension, file.mimetype and the PNG or JPEG signature to agree, save under
secrets.token_hex() outside the static folder, and serve with
send_from_directory(as_attachment=True) plus X-Content-Type-Options: nosniff.

Page: https://softwaresecurity.com/fix/file-upload/python-flask/

What is in this archive
-----------------------
file-upload/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 file-upload/python-flask folder, install the dependencies once (this
  needs network access):

    python3 -m venv .venv
    .venv/bin/python -m pip install -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 file-upload/python-flask
    node run-all.mjs file-upload/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, 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.
