Fix open redirects in Python and Flask
======================================

Refuse backslashes, which browsers read as slashes and urllib.parse does not,
and control characters; then split next with urlsplit: accept a path that
starts with exactly one slash, or an https URL whose hostname is in an exact
allowlist, with no port and no @. Otherwise redirect to a fixed page. Never
trust startswith('/') or a host substring test.

Page: https://softwaresecurity.com/fix/open-redirect/python-flask/

What is in this archive
-----------------------
open-redirect/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 open-redirect/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 open-redirect/python-flask
    node run-all.mjs open-redirect/python-flask

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with python 3.14.8,
  flask 3.1.3, werkzeug 3.1.9, pytest 9.1.1, adaUrl 4.0.0, semgrep 1.180.0,
  bandit 1.9.4. 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.
