time-limited-regular-expressions

Why?

You want to let end users enter their own regular expressions. But regular expressions can lead to catastrophic backtracking. This can take up hours of CPU time. In Node.js this means no other code can execute. It is a Denial of Service (DOS) attack vector, whether intentionally or by accident.

This module lets you test regular expressions with a time limit to mitigate the pain.

Usage

// Set a 1-second limit. Default is 0.25 seconds
const regExp = require('time-limited-regular-expressions')({ limit: 1 });

// A common email address validator with potentially evil characteristics
// (catastrophic backtracking)
const evil = /^([a-zA-Z0-9])(([\-.]|[_]+)?([a-zA-Z0-9]+))*(@){1}[a-z0-9]+[.]{1}(([a-z]{2,3})|([a-z]{2,3}[.]{1}[a-z]{2,3}))$/;

(async () => {
  // Run a potentially slow regular expression on short, matching input
  const realEmail = 'test@test.com';
  const realEmailResult = await regExp.match(evil, realEmail);
  // Normal behavior, may be truthy or falsy according to match,
  // returns the same array result as regular regexp match() calls
  console.log(realEmailResult);
  // This input is long enough to trigger catastrophic backtracking and
  // could take hours to evaluate
  const evilEmail = 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA';
  try {
    const evilEmailResult = await regExp.match(evil, evilEmail);
    // We will not get here, exception will be thrown
  } catch (e) {
    console.log(e.name); // Will be 'timeout'
  }
})();

Notes

"Why is match an async function?" It runs in a separate process because that is the only way to avoid starving the Node.js application and implement a portable timeout on the regular expression.

"How bad is the performance overhead?" Communication with a separate worker process makes it slower of course, but the process is reused by later calls, so the hit is not serious.

Flags, for instance the g flag, are supported.

You can pass the regular expression as a string, but regular expression literals (what you are used to typing) are easier to get right because you don't have to double-escape anything.