Affected: npm: form-data

### Summary

form-data uses `Math.random()` to select a boundary value for multipart form-encoded data. This can lead to a security issue if an attacker: 1. can observe other values produced by Math.random in the target application, and 2. can control one field of a request made using form-data

Because the values of Math.random() are pseudo-random and predictable (see: https://blog.securityevaluators.com/hacking-the-javascript-lottery-80cc437e3b7f), an attacker who can observe a few sequential values can determine the state of the PRNG and predict future values, includes those used to generate form-data's boundary value. The allows the attacker to craft a value that contains a boundary value, allowing them to inject additional parameters into the request.

This is largely the same vulnerability as was [recently found in `undici`](https://hackerone.com/reports/2913312) by [`parrot409`](https://hackerone.com/parrot409?type=user) -- I'm not affiliated with that researcher but want to give credit where credit is due! My PoC is largely based on their work.

### Details

The culprit is this line here: https://github.com/form-data/form-data/blob/426ba9ac440f95d1998dac9a5cd8d738043b048f/lib/form_data.js#L347

An attacker who is able to predict the output of Math.random() can predict this boundary value, and craft a payload that contains the boundary value, followed by another, fully attacker-controlled field. This is roughly equivalent to any sort of improper escaping vulnerability…