Fix NoSQL operator injection in Python and Flask
================================================

Never let a request value reach a PyMongo filter as a dict: accept str only
(isinstance(value, str)), compare with {"field": {"$eq": value}}, and check
secrets such as reset tokens in code with hmac.compare_digest after looking
the account up by email. Blocklisting $ne or $where fails, because dozens of
other operators match too.

Page: https://softwaresecurity.com/fix/nosql-injection/python-flask/

What is in this archive
-----------------------
nosql-injection/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 nosql-injection/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 nosql-injection/python-flask
    node run-all.mjs nosql-injection/python-flask

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with python CPython
  3.14.8, flask 3.1.3, pymongo 4.18.2, mongomock 4.3.0, pytest 9.1.1,
  parityCheck mongod 8.0.32 (arm64 macOS build) through PyMongo 4.18.2, 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.
