Fix mass assignment in Python and FastAPI
=========================================

Give each write route its own Pydantic body model that declares only the
fields that caller may change (here displayName and bio), never the model that
describes the whole stored user, and copy model_dump(exclude_none=True) onto
the record. extra="forbid" turns any other key into a 422. Set role and
tenantId only in server-side code.

Page: https://softwaresecurity.com/fix/mass-assignment/python-fastapi/

What is in this archive
-----------------------
mass-assignment/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 mass-assignment/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 mass-assignment/python-fastapi
    node run-all.mjs mass-assignment/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, pydantic 2.13.5, starlette 1.7.0, 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.
