Cloudflare CDN Bug Outs User Locations on Signal, Discord

A flaw in the widely used Cloudflare content delivery network (CDN) can expose someone's location by sending them an image on platforms like Signal and Discord, deanonymizing them in seconds without their knowledge.

That's according to a 15-year-old security researcher who goes by only "Daniel," who published research on GitHub Gist about the flaw — which he discovered three months ago — as a warning for journalists, activists, and hackers, who could be at physical risk.

The flaw allows an attacker to grab the location of any target within a 250-mile radius when a vulnerable app is installed on a target's phone, or even as a background application on their laptop. Using either a one-click or zero-click approach, an attacker can use the app to "send a malicious payload and deanonymize you within seconds — and you wouldn't even know," Daniel wrote.

Cloudflare Content Caching Is the Cyber Culprit

The core of the flaw lies in one of Cloudflare's most used features: caching, Daniel explained. Cloudflare's cache stores copies of frequently accessed content, such as images, videos, or webpages, in its datacenters, ostensibly to reduce server load and improve website performance.

When a device sends a request for a resource that can be cached, Cloudflare retrieves the resource from its local data center storage, if possible, or from the origin server. It then caches it locally, and returns it. "By default, some file extensions are automatically cached but site operators can also configure new cache rules," Daniel explained.

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Because of this process flow, if an attacker can get a user's device to load a resource on a Cloudflare-backed site, causing it to be cached in their local datacenter, they can then enumerate all Cloudflare data centers to identify which one cached the resource. "This would provide an incredibly precise estimate of the user's location," Daniel explained.

Daniel did have to overcome a hurdle to this attack flow in that someone "can't simply send HTTP requests to individual Cloudflare datacenters," he wrote. However, he discovered a bug via a forum post that demonstrates how someone can send requests to specific Cloudflare datacenters with Cloudflare Workers, and created a tool called Cloudflare Teleport, a proxy powered by Cloudflare Workers that redirects HTTP requests to specific datacenters.

How to Exploit the Cloudflare Location Flaw

Daniel went on to demonstrate how he could send images via both Signal and Discord that would expose the recipient's location. For Signal, which is an app favored by journalists and activists due to its privacy features, a one-click attack allows someone to send either an attachment or an avatar to a user that exploits the cache geolocation method to pinpoint the recipient's location.

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An attacker also could use a zero-click attack in Signal by taking advantage of push notifications, which occur when a message is sent to a user while they are not actively using the app. In this case, the recipient doesn't even have to open the Signal conversation for their device to download the attachment, he said.

Attackers can exploit the flaw similarly in Discord, with potentially wider impact, using a custom emoji that's loaded from Discord's CDN and configured to be cached on Cloudflare, he explained.

"So, instead of sending an attachment in a Discord channel, an attacker can display a custom emoji in their user status and simply wait for the target to open their profile to run a deanonymization attack," Daniel wrote. A one-click attack vector also is possible in Discord by changing a user's avatar and sending a friend request to someone, which triggers a push notification, he added.

Signal, Discord, Cloudflare Response & Mitigation

Daniel contacted Signal, Discord, and Cloudflare about the bug. The first two companies did nothing to mitigate it, with Signal claiming users are responsible for protecting their own identities, and Discord claiming it was Cloudflare's responsibility.

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For its part, Cloudflare did fix the Cloudflare Workers bug that allowed Daniel to create the Teleport tool. The bug was reported to its HackerOne program a year ago by another researcher, but the company had not responded to the report. It reopened the case after Daniel's report and mitigated the issue, awarding him a $200 bug bounty in the process.

However, even after the mitigation, Daniel was able to exploit the flaw by reprogramming his Cloudflare Teleport tool to use a VPN instead, choosing a VPN provider with more than 3,000 servers located in various locations across 31 different countries worldwide. "Using this new method, I'm able to reach about 54% of all Cloudflare datacenters again," he explained.

At this time, "any app using a CDN for content delivery and caching can still be vulnerable if the proper precautions aren’t taken," Daniel wrote.

And this can be especially dangerous for people who need to protect their location for various reasons, such as a woman who may be hiding from a violent boyfriend or husband, or a political dissident who is being targeted by a hostile government, says Roger Grimes, data-driven defense evangelist at KnowBe4.

“At first glance, the flaw seems really innocuous and barely relevant, but there are scenarios … where it could be a problem," he tells Dark Reading. Moreover, Grimes suspects that Cloudflare CDN is not the only CDN affected by such a flaw, as "the attack is just generic enough that I think it can be applied to more CDNs," he says.

Daniel advised that people concerned about their privacy should limit their exposure on the affected apps, which "can make a significant difference" when it comes to protecting their location data.