Fix open redirects in Python and FastAPI
========================================

Refuse backslashes 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. Return RedirectResponse(target, status_code=303) with the
value you checked; never trust startswith('/') or a host substring test.

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

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

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with python CPython
  3.14.8, fastapi 0.142.2, starlette 1.7.0, pythonMultipart 0.0.32, adaUrl
  4.0.0, pytest 9.1.1, 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.
