this post was submitted on 10 Jul 2023
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FYI: Lemmy.world and other instances were hacked. Beehaw.org took itself down to mitigate risks (lemmy.world)
submitted 1 year ago* (last edited 1 year ago) by [email protected] to c/[email protected]
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Drawing attention on this instance so Admins are aware and can address the propagating exploit.

EDIT: Found more info about the patch.

A more thorough recap of the issue.

GitHub PR fixing the bug: https://github.com/LemmyNet/lemmy-ui/pull/1897/files

If your instance has custom emojis defined, this is exploitable everywhere Markdown is available. It is NOT restricted to admins, but can be used to steal an admin's JWT, which then lets the attacker get into that admin's account which can then spread the exploit further by putting it somewhere where it's rendered on every single page and then deface the site.

If your instance doesn't have any custom emojis, you are safe, the exploit requires custom emojis to trigger the bad code branch.

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[–] [email protected] 7 points 1 year ago (3 children)

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.

[–] [email protected] 6 points 1 year ago (1 children)

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.

100%. Always act as though user provided content is malicious.

JavaScript (TypeScript) has access to cookies (and thus JWT). This should be handled by web browser, not JS.

Uh... what? JavaScript is a client-side language (unless you're using NodeJS, which Lemmy is not). Which means JavaScript runs in the browser. And that JavaScript has access to cookies, that's just a basic part of how web browsers work. Lemmy can't do anything to prevent that.

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.

Again, Lemmy can't do anything about that. Once there's a vulnerability that allows an attacker to inject arbitrary JS into the site, Lemmy can't do anything to prevent that JS from making requests.

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

On the backend you'd still have a single system which kind of defeats the purpose. Unless you're proposing a completely independent backend? Because that would be a massive PITA to build and would drastically increase the system's complexity and reduce maintainability.

[–] [email protected] 3 points 1 year ago

And that JavaScript has access to cookies, that’s just a basic part of how web browsers work. Lemmy can’t do anything to prevent that.

Yes and No. Cookies could be accessed by JS on the client. BUT. When the cookie is sent by the server with additional HttpOnly header, then the cookie cannot be accessed from JS. Look at Lemmy GitHub issue, they discuss exactly this. Lemmy server absolutely has power to prevent this.

Again, Lemmy can’t do anything about that. Once there’s a vulnerability that allows an attacker to inject arbitrary JS into the site, Lemmy can’t do anything to prevent that JS from making requests.

I believe they can. But I'm not sure about this one. The server could send a response preventing the web browser to request content from other domains. Banks are using this. There was an attack years ago when attacker created a web page with i-frame in it. The i-frame was full screen to confuse the victim it is actually using the Banks site and not the attacker site. The bank web site was inside the inner i-frame, the code in the outer frame then had access to sensitive data in the inner frame. I believe there are HTTP response headers that instruct the web browser to not allow this. But I'm not sure I remember how exactly this works.

completely independent backend

Yes, it would be more costly, but more secure. It is trade-off, which one is more important to you? In case of chat/blog/forum app such as Lemmy I prefer cheap, in case of my Bank website I prefer secure.

[–] [email protected] 5 points 1 year ago (1 children)

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).

[–] [email protected] 6 points 1 year ago (1 children)

I'm sorry you want a mobile user to login every time their IP address changes? Why bother to keep them authenticated at all, just make them login for every web request. /s

[–] [email protected] 0 points 1 year ago

It is trade-off between convenience and security. With my approach stolen cookies are not usable from different computer / IP, the attacker needs additional work, he needs the victim computer to do the harm, his computer cannot do any harm. The downside is the user needs another log-in in case of his external IP changes. How often is it? Switch between mobile/WiFi. Otherwise ... almost never ... maybe 1x per day? I'm not proposing to log-out the user after IP change, I'm proposing to keep multiple sessions (on server) / auth cookies (on client) for each IPv4 or IPv6 prefix (let's say /56).

[–] [email protected] 4 points 1 year ago

This would never happen in desktop/server application

MOVEit would suggest otherwise.