Fix unrestricted file upload in Next.js
=======================================

Decide the type from the bytes, not from file.type or the name. In the POST
handler require the extension, the declared Content-Type and the PNG or JPEG
signature to agree, cap the size, write under a random name outside the web
root, and serve through a GET handler that sends the verified Content-Type
with nosniff and Content-Disposition: attachment.

Page: https://softwaresecurity.com/fix/file-upload/typescript-nextjs/

What is in this archive
-----------------------
file-upload/typescript-nextjs/   the project: three implementations of the same code and one test suite
  lib/impl/vulnerable.ts   (vulnerable)
  lib/impl/fixed.ts   (fixed)
  lib/impl/wrong.ts   (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 (route handlers use the default Node.js runtime);
  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/typescript-nextjs 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/typescript-nextjs
    node run-all.mjs file-upload/typescript-nextjs

Verified
--------
  11 October 2026 on macOS 27.0 (Darwin 27.0.0, arm64), with node 26.11.1, npm
  11.20.0, next 16.4.0, react 19.3.0, vitest 5.0.3, typescript 6.0.3. 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.
