Fix CRLF and log injection in Python and Flask
==============================================

Log events as structured data: pass request values to the logger as fields and
format each record as one JSON object, so CR, LF and escape characters are
encoded and cannot end it. For download names, keep only letters, digits and a
few separators, then let send_file(download_name=...) write
Content-Disposition. Never interpolate request data into log lines or headers.

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

What is in this archive
-----------------------
crlf-log-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 crlf-log-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 crlf-log-injection/python-flask
    node run-all.mjs crlf-log-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, werkzeug 3.1.9, 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.
