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.

  • terribleplanEnglish
    13•1 year ago

    Holy shit holy shit holy shit. Serious vulnerability confirmed. Combined with the issue(s) I have tried to report this is insane. I just tested this (and purged it so as not to publicly disclose just yet). This is really bad.

      • @Dusty@l.dustybeer.comEnglish
        9•1 year ago

        If you find a way to disclose vulnerabilities without being ghosted by Lemmy developers: update me.

        How have you been “ghosted by Lemmy developers” especially if you “do not use GitHub”

        • terribleplanEnglish
          6•1 year ago

          Yeah, I just wrote this up as a bug on github and added in that I tried to email them and to please get in contact about the other thing. Hopefully they see it. I can understand checking that email being overlooked considering how busy they likely are given the sudden influx and scaling issues.

            • terribleplanEnglish
              2•1 year ago

              I tried to email that previously with a different issue and got no response. I was planning to post publicly (on github) about a different issue on Friday, but that other issue is now way too severe to do that now given how this can be leveraged to exploit what I found.

          • fooEnglish
            4•1 year ago

            It’s been a bit of a busy week for them. Maybe you can cut them some slack and try again?

      • terribleplanEnglish
        8•1 year ago

        Yeah, I found something that was “holy shit this is bad if someone finds a way to do X” and tried to report that but didn’t dig any deeper. This is X.