Fix ReDoS in Node.js and Express
================================

A validation regex with a nested quantifier backtracks catastrophically: a
40-character string pins a CPU for minutes and, on Node's single event loop,
blocks every request. Match on RE2, a finite-automaton engine whose run time
is linear for any pattern; cap input length before matching; never run
untrusted input through a backtracking regex on the request thread.

Page: https://softwaresecurity.com/fix/regex-dos/node-express/

What is in this archive
-----------------------
regex-dos/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 regex-dos/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 regex-dos/node-express
    node run-all.mjs regex-dos/node-express

Verified
--------
  11 October 2026, with node 26.11.1, npm 11.20.0, express 5.3.0, re2 1.26.1, os
  macOS 27.0, Apple M5 Max. 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.
