Fix path traversal in TypeScript and Next.js
============================================

Check where the path lands, not what it looks like: resolve base with
fs.realpath, resolve the request with realpath(path.resolve(base, name)),
which collapses ../ and follows symlinks, and serve it only if it starts with
base + path.sep. Answer 404 otherwise. Stripping "../" or comparing without
the separator does not work.

Page: https://softwaresecurity.com/fix/path-traversal/typescript-nextjs/

What is in this archive
-----------------------
path-traversal/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,
  which node:fs needs); 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 path-traversal/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 path-traversal/typescript-nextjs
    node run-all.mjs path-traversal/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 (Vite 8.3.4), typescript
  6.0.3, semgrep 1.180.0. 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.
