Fix unrestricted file upload in Node.js and Express
===================================================

Decide the type from the bytes, and never let the client pick the name. Use
multer memoryStorage with limits.fileSize, require the extension,
file.mimetype and the PNG or JPEG signature to agree, write under a random
name outside the web root, and serve through a route that sends
res.attachment() and X-Content-Type-Options: nosniff.

Page: https://softwaresecurity.com/fix/file-upload/node-express/

What is in this archive
-----------------------
file-upload/node-express/   the project: three implementations of the same code and one test suite
  impl/vulnerable.js   (vulnerable)
  impl/fixed.js   (fixed)
  impl/wrong.js   (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 Node.js 26.11.1; package.json allows Node.js ^22.23.2 || ^24.18.1
  || >=26.5.1. run-all.mjs needs Node.js 22 or later. Supported versions:
  https://softwaresecurity.com/fix/versions/

  From the file-upload/node-express folder, install the dependencies once (this
  needs network access):

    npm ci

  Then run the tests. IMPL chooses the implementation the tests load:

    # every test passes
    IMPL=fixed npm test --silent
    # an exploit test fails
    IMPL=vulnerable npm test --silent
    # a test fails
    IMPL=wrong npm test --silent

  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 file-upload/node-express
    node run-all.mjs file-upload/node-express

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with node 26.11.1, npm
  11.20.0, express 5.3.0, multer 2.4.0, busboy 1.6.0. 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.
