Fix insecure deserialization in Python and FastAPI
==================================================

Do not rebuild objects from client data. Parse the token as JSON with a
Pydantic model (Profile.model_validate_json(raw)), which keeps only declared
fields of declared types; pickle.loads runs code chosen by the bytes. Load
YAML with yaml.safe_load and validate it the same way. Blocklisting module or
class names does not work.

Page: https://softwaresecurity.com/fix/deserialization/python-fastapi/

What is in this archive
-----------------------
deserialization/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 deserialization/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 deserialization/python-fastapi
    node run-all.mjs deserialization/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, pyyaml 6.0.3, 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.
