VcXsrv binds to localhost by default, so unless you already have malware on your system the added risk is quite small I'd say.

What about javascript code on a harmfull website that does requests to localhost?

It used to be possible to easily scan what sites are running locally, even if js code was from a public website. [0]

[0] https://defuse.ca/in-browser-port-scanning.htm

A JS based exploit that hijacks the X Window System Core Protocol running on your localhost to inject key presses into your X server or steal screenshots? I mean it's possible, but it seems quite far-fetched except maybe if someone specifically targets you.

The main impediment is that http request won't look anything like the x window requests, so the x server would likely reject it.

There are actually several (published and non-published) ways to exploit that type of configuration, here is one example:

https://samy.pl/slipstream/

If you're talking about the general concept of using js to spoof another protocol: that exploit involves middleboxes sniffing TCP connections at the packet level, rather than at connection/stream level. It certainly won't work for connections with a TCP server.

If you're talking about using that exploit to allow access to the victim's machine from the internet: that won't work because listening interface for the x11 server is localhost, not the LAN interface.

Thank you for posting this. I tried many times to google how to do this and saw the magic cookie explanation, but never found a good clear description of what was happening. Much appreciated!

Glad to help.

Btw there is a typo, you need to use "nextract", it's too late to edit it now

maybe something like...

    (cmd.exe -C xauth -extract - $Display) | xauth nmerge -

first part to run via windows environment, second to pipe that to the wsl environment?

Actual commands probably different, but should be something you could maybe trigger from inside .bashrc file.

Instruction unclear: on the host running the X server, I get

  PS C:\Users\user> _