@jmondi/url-to-png enables capture screenshot of localhost web services (unauthenticated pages)

Summary

The maintainer been contemplating whether FTP or other protocols could serve as useful functionalities, but there may not be a practical reason for it since we are utilizing headless Chrome to capture screenshots. The argument is based on the assumption that this package can function as a service.

The package includes an ALLOW_LIST where the host can specify which services the user is permitted to capture screenshots of. By default, capturing screenshots of web services running on localhost, 127.0.0.1, or the [::] is allowed.

The maintainer is of the opinion that the package should also have a blacklist due to a potential vulnerability (or rather design oversight). If someone hosts this on a server, users could then capture screenshots of other web services running locally.

Unless this is strictly for web pages. Something similar here: https://github.com/follow-redirects/follow-redirects/issues/235 (localhost is intended for end users or hosts to deny, and the package is for HTTP/HTTPS.)

This is marked as a LOW since the maintainer is not sure if this is a vulnerability, but it's still best to highlight it. :)

PoC

Have a service like so running locally:

const http = require("http")

const server = http.createServer((req, res) => {
  console.log("Received headers:", req.headers)
  res.writeHead(200, { "Content-Type": "text/plain" })
  res.end("Something private! But Hello from Server 2 :)")
})

server.listen(3001, () => {
  console.log("Server two running on http://localhost:3001")
})

Run the package in dev mode, pnpm dev. Feed these URLs:

http://localhost:3089/?url=http://[::]:3001&width=4000
http://localhost:3089/?url=http://localhost:3001&width=4000
http://localhost:3089/?url=http://127.0.01:3001&width=4000

Impact

Disclose internal web services?