Fix server-side template injection in Python and Flask
======================================================

Pass input as data, never as template text: render_template_string('Hello {{
name }}', name=name), not render_template_string('Hello ' + name). Where users
author templates, render them with a jinja2.sandbox.SandboxedEnvironment and
put only the data they need in scope, not config, request or os. Do not
denylist {{ or __class__; {{ config }} and |attr gadgets slip past it.

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

What is in this archive
-----------------------
template-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 template-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
    SSTI_SECRET=TMPL-INJECTION-PROOF-4d9f2a7c IMPL=fixed .venv/bin/python -m pytest
    # an exploit test fails
    SSTI_SECRET=TMPL-INJECTION-PROOF-4d9f2a7c IMPL=vulnerable .venv/bin/python -m pytest
    # a test fails
    SSTI_SECRET=TMPL-INJECTION-PROOF-4d9f2a7c 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 template-injection/python-flask
    node run-all.mjs template-injection/python-flask

Verified
--------
  11 October 2026, with python 3.14.8, flask 3.1.3, jinja2 3.1.6, markupsafe
  3.0.4, werkzeug 3.1.9, 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.
