What is XSS?
Cross-site scripting (XSS) is an exploit where the attacker attaches code onto a legitimate website that will execute when the victim loads the website. That malicious code can be inserted in several ways. Most popularly, it is either added to the end of a url or posted directly onto a page that displays user-generated content. In more technical terms, cross-site scripting is a client-side code injection attack. https://www.cloudflare.com/learning/security/threats/cross-site-scripting/
Impact
One-click Lemmy account compromise by social engineering users to click your posts URL.
Reproduction
Lemmy does not properly sanitize URI’s on posts leading to cross-site scripting. You can see this working in action by clicking the “link” attached to this post on the web client.
To recreate, simply create a new post with the URL field set to: javascript:alert(1)//
Patching
Adding filtering to block javascript: and data: URI’s seems like the easiest approach.
The “responsibility” part of responsible disclosure goes both ways
It absolutely does, it also means following up, not “They didn’t reply in a week so instead of trying other ways to contact them, I’m just going to post about it”. They didn’t even try to open an issue because they “don’t use github” all while coming here talking about how bad the vulnerability is.
It’s poor (lack of) judgement on OP’s part.
