Fix TLS certificate validation in Python and Flask
==================================================

Never pass verify=False. Pass verify= the path of the partner's private CA
bundle on every request: requests then checks the chain against that CA alone,
the dates and the host name, and a per-request value cannot be replaced by
REQUESTS_CA_BUNDLE. Never switch host checks off with assert_hostname=False.

Page: https://softwaresecurity.com/fix/tls-certificate-validation/python-flask/

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

    python3 -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 tls-certificate-validation/python-flask
    node run-all.mjs tls-certificate-validation/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, requests 2.34.2, urllib3 2.8.0,
  cryptography 50.0.2, 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.
