It changes all the time (once per year) if you finally managed to remember it, it changes again.

Not in my experience. But OK, if this is the case, don't use exact IPv4 address, lookup the routing database and use the sub-net. Or whatever. This is belt & suspenders style of defense in depth, just another layer of security if all others fail. Not core functionality.

mobile devices change IP addresses all the time

I never noticed this. Yes, switch between mobile and WiFi, but this is only two addresses. In case of IPv4 this seems not problem. In case of IPv6, use /64 or /48 (or whatever is now recommended for residential end users) prefix instead of the entire 128bits. I'm not proposing to log-out the suer after IP change, I'm proposing multiple sessions to be accessible at the same time.

this might even be a solved problem by now

Yes, it is called HttpOnly and is decided by the server who is sending the cookie to you in HTTP response header. I believe there are also HTTPS-only cookies that when received via HTTPS, cannot be used from HTTP, but I cannot find it right now.

Oh I forgot another line of defense / basic security mitigation. If a server produces an access token (such as JWT or any other old school cookie / session ID), pair it with an IP address. So in case of cookie theft, the attacker cannot use this cookie from his computer (IP address). If the IP changes (mobile / WiFi / ADSL / whatever), the legitimate user should log-in again, now storing two auth cookies. In case of another IP change, no problemo, one of the stored cookies will work. Of course limit validity of the cookie in time (lets, say, keep it valid only for a day or for a week or so).

So what happened:

• Someone posted a post.
• The post contained some instruction to display custom emoji.
• So far so good.
• There is a bug in JavaScript (TypeScript) that runs on client's machine (arbitrary code execution?).
• The attacker leveraged the bug to grab victim's JWT (cookie) when the victim visited the page with that post.
• The attacker used the grabbed JWTs to log-in as victim (some of them were admins) and do bad stuff on the server.

Am I right?

I'm old-school developer/programmer and it seems that web is peace of sheet. Basic security stuff violated:

• User provided content (post using custom emojis) caused havoc when processing (doesn't matter if on server or on client). This is lack of sanitization of user-provided-data.
• JavaScript (TypeScript) has access to cookies (and thus JWT). This should be handled by web browser, not JS. In case of log-in, in HTTPS POST request and in case of response of successful log-in, in HTTPS POST response. Then, in case of requesting web page, again, it should be handled in HTTPS GET request. This is lack of using least permissions as possible, JS should not have access to cookies.
• How the attacker got those JWTs? JavaScript sent them to him? Web browser sent them to him when requesting resources form his server? This is lack of site isolation, one web page should not have access to other domains, requesting data form them or sending data to them.
• The attacker logged-in as admin and caused havoc. Again, this should not be possible, admins should have normal level of access to the site, exactly the same as normal users do. Then, if they want to administer something, they should log-in using separate username + password into separate log-in form and display completely different web page, not allowing them to do the actions normal users can do. You know, separate UI/applications for users and for admins.

Am I right? Correct me if I'm wrong.

Again, web is peace of sheet. This would never happen in desktop/server application. Any of the bullet points above would prevent this from happening. Even if the previous bullet point failed to do its job. Am I too naïve? Maybe.

Marek.