Imgur has patched a serious vulnerability that allowed attackers to attach malicious code to files uploaded to the image sharing service.
Attackers have leveraged the flaw to launch sort of a distributed denial-of-service (DDoS) attack against the imageboard website 8chan.
According to Imgur, the flaw affecting its systems was exploited to upload what seemed like images, but were actually HTML files containing malicious JavaScript code. Then, links to these crafted image files were posted to the 8chan section on Reddit.
When the links were clicked, the malicious JavaScript code created an iframe containing a specially crafted Flash file hosted on 8chan. The Flash file, which could not be seen by users during this process, injected another piece of JavaScript into the victim’s local storage. This was possible due to a couple of security issues in 8chan, experts said.
Once this was done, every time victims accessed an 8chan page, the JavaScript code in local storage caused their browser to communicate with a command and control (C&C) server. The JavaScript also loaded an image from 8chan hundreds of times, which resulted in the website becoming unavailable.
Imgur has patched the vulnerability and has taken steps to ensure that files other than valid images cannot be uploaded to the company’s servers, and JavaScript code can no longer be served from i.imgur.com. The image sharing website has also clarified that the malicious images had not made it to the gallery.
On its end, 8chan disabled access and upload permissions for Flash files, and rolled out a patch to clear users’ local storage the next time they visit the website.
Despite the fact that Imgur patched the vulnerability and removed the malicious files, the JavaScript code from the local storage can still communicate with the C&C server, which is why users have been advised to clear their browsing data, cookies and localStorage.
Some believe that if Imgur and 8chan had not taken measures, the individual or group behind the attack might have done more than just disrupt 8chan.