sandboxed-regexp

Process untrusted regexes in JavaScript, with the power of Rust!

JavaScript's builtin RegExp class is not suitable for working with regular expressions obtained from untrusted sources, because it can easily fall victim to Catastrophic Backtracking.

Rust's regex crate, by contrast, is specifically designed to safely handle untrusted input without blowing up.

So let's use WebAssembly to bring this same safe handling of untrusted regexes from Rust to JavaScript, with the addition of extra sandboxing!

Building it

You'll need wasm-pack.

Build the package using make build. Test that it works using make test.

Using it

Like so:

const { SandboxedRegExp } = require('sandboxed-regexp');

// Problematic regex example from http://www.rexegg.com/regex-explosive-quantifiers.html
const FastRE = new SandboxedRegExp("^(A+)*B")

// This will run quickly to completion and output `false`.
console.log(FastRE.test("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAC"));

// But if you try the same thing with the builtin RegExp class.
const SlowRE = new RegExp("^(A+)*B");

// Then this will churn CPU for several seconds before outputting `false`.
console.log(SlowRE.test("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAC"));

// And this will churn CPU until you get sick of waiting and interrupt it.
console.log(SlowRE.test("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAC"));

Being Careful with it

Using the SandboxedRegExp class should feel fairly similar to using the builtin RegExp class for simple use-cases, but it is in no way intended to be a drop-in replacement.

It's missing the following features of the builtin class that might conceivably be added by someone who needed them:

• No support for any methods besides test (such as exec or match).
• No support for capturing groups.
• No support for updating the lastIndex property after a match.

It has the following differences from the builtin class that will probably always remain:

• It uses the Rust crate's regex syntax. This should be the same as the JS syntax for simple cases but is likely to be very different around the edges.
  • In particular, this means no support for look-ahead or backreferences, which are popular non-regular enhancements to reglar expression syntax that are resistant to safe execution of untrusted inputs.
• Thorough unicode handling is on by default. This is most likely to show up in practice as character classes like \w matching unicode character classes rather than ASCII.

It has the following non-functional differences that might matter to you at scale:

• You have to slurp in a few hundred kilobytes of wasm code.
• Testing against a SandboxedRegExp is cheap, but creating one is likely to be much more expensive than creating a builtin RegExp. If you're creating SandboxedRegExp instances in a loop, you're likely to have a bad time.
• Each SandboxedRegExp instance consumes more memory than a builtin RegExp instance. Significantly more. We might expose some knobs for tuning the memory use in future.
• Testing a string against a SandboxedRegExp involves copying that string into the wasm linear memory, which might get expensive if you're testing very large strings.
• If you're working with a particularly-badly-behaved regex, it might run out of memory or similar catastrophic failure behaviour that will be surfaced as an opaque wasm-related error. This could render the SandboxedRegExp object unusable, but the rest of your program should be fine.