Browser extensions make attractive attack targets. That’s not necessarily because of the data handled by the extension itself, but too often because of the privileges granted to the extension. Particularly extensions with access to all websites should better be careful and reduce the attack surface as much as possible. Today’s case study is Custom Cursor, a Chrome extension that more than 6 million users granted essentially full access to their browsing session.
The attack surface of Custom Cursor is unnecessarily large: it grants custom-cursor.com website excessive privileges while also disabling default Content Security Policy protection. The result: anybody controlling custom-cursor.com (e.g. via one of the very common cross-site scripting vulnerabilities) could take over the extension completely. As of Custom Cursor 3.0.1 this particular vulnerability has been resolved, the attack surface remains excessive however. I recommend uninstalling the extension, it isn’t worth the risk.
Update (2021-11-24): I’ve now looked into Cute Cursors, a competing extension with close to identical functionality. My conclusion is that Cute Cursors is secure, the attack surface is as minimal as it should be. So I recommend any Custom Cursor users to switch.
As readers of this blog might remember, shopping assistants aren’t exactly known for their respect of your privacy. They will typically use their privileged access to your browser in order to extract data. For them, this ability is a competitive advantage. You pay for a free product with a privacy hazard.
Normally, CSS injection vulnerabilities are fairly boring. With some luck, you can use them to assist a clickjacking attack. That is, unless the vulnerable party is a browser extension, and it lets you inject CSS code into high profile properties such as Google’s. I’ve now had some fun playing with this scenario, courtesy of G App Launcher browser extension.
The vulnerability has been resolved in G App Launcher 23.6.1 on the same day as I reported it. Version 23.6.5 then added more changes to further reduce the attack surface. This was a top notch communication experience, many thanks to Carlos Jeurissen!
The cookie consent screens are really annoying. They attempt to trick you into accepting all cookies, dismissing them without agreeing is made intentionally difficult. A while back I wrote on Twitter than I’m almost at the point of writing a private browser extension to automate the job. And somebody recommended Ninja Cookie extension to me, which from the description seemed perfect for the job.
As of Ninja Cookie 0.7.0, the Universal XSS vulnerability has been resolved. The other issues remain however, these are exploitable by anybody with access to the Ninja Cookie download server (ninja-cookie.gitlab.io). This seems to be the reason why Mozilla Add-ons currently only offers the rather dated Ninja Cookie 0.2.7 for download, newer versions have been disabled. Chrome Web Store still offers the problematic extension version however. I didn’t check whether extension versions offered for Edge, Safari and Opera browsers are affected.
I looked into the Print Friendly & PDF browser extension while helping someone figure out an issue they were having. The issue turned out unrelated to the extension, but I already noticed something that looked very odd. A quick investigation later I could confirm a massive vulnerability affecting all of its users (close to 1 million of them). Any website could easily gain complete control of the extension.
This particular issue has been resolved in Print Friendly & PDF 2.7.39 for Chrome. The underlying issues have not been addressed however, and the extension is still riddled with insecure coding practices. Hence my recommendation is still to uninstall it. Also, the Firefox variant of the extension (version 1.3) is still affected. I did not look at the Microsoft Edge variant but it hasn’t been updated recently and might also be vulnerable.
Note: To make the confusion complete, there is a browser extension called Print Friendly & PDF 2.1.0 on the Firefox Add-ons website. This one has no functionality beyond redirecting the user to printfriendly.com and isn’t affected. The problematic Firefox extension is being distributed from the vendor’s website directly.
Mind you, I’m not saying that Amazon is currently doing any of this.
Yes, when I wrote that article I didn’t actually know how Amazon was using the power they’ve given themselves. The mere potential here, what they could do with a minimal and undetectable change on one of their servers, that was scary enough for me. I can see that other people might prefer something more tangible however.
So this article now analyzes what data Amazon actually collects. Not the kind of data that necessarily flows to Amazon servers to make the product work. No, we’ll look at a component dedicated exclusively to “analytics,” collecting data without providing any functionality to the user.
The logic explained here applies to Amazon Assistant browser extension for Mozilla Firefox, Google Chrome and Microsoft Edge. It is also used by Amazon Assistant for Android, to a slightly limited extent however: Amazon Assistant can only access information from the Google Chrome browser here, and it has less information available to it. Since this logic resides on an Amazon web server, I can only show what is happening for me right now. It could change any time in either direction, for all Amazon Assistant users or only a selected few.
Both issues are resolved in DuckDuckGo Privacy Essentials 2021.2.3 and above. At the time of writing, this version is only available for Google Chrome however. Two releases have been skipped for Mozilla Firefox and Microsoft Edge for some reason, so that the latest version available here only fixes the first issue (insecure internal communication). Update (2021-03-16): An extension version with the fix is now available for both Firefox and Edge.
These vulnerabilities are very typical, I’ve seen similar mistakes in other extensions many times. This isn’t merely extension developers being clueless. The extension platform introduced by Google Chrome simply doesn’t provide secure and convenient alternatives. So most extension developers are bound to get it wrong on the first try. Update (2021-03-16): Linked to respective Chromium issues.
I recently noticed that Amazon is promoting their Amazon Assistant extension quite aggressively. With success: while not all browsers vendors provide usable extension statistics, it would appear that this extension has beyond 10 million users across Firefox, Chrome, Opera and Edge. Reason enough to look into what this extension is doing and how.
Here I must say that the privacy expectations for shopping assistants aren’t very high to start with. Still, I was astonished to discover that Amazon built the perfect machinery to let them track any Amazon Assistant user or all of them: what they view and for how long, what they search on the web, what accounts they are logged into and more. Amazon could also mess with the web experience at will and for example hijack competitors’ web shops.
Mind you, I’m not saying that Amazon is currently doing any of this. While I’m not done analyzing the code, so far everything suggests that Amazon Assistant is only transferring domain names of the web pages you visit rather than full addresses. And all website manipulations seem in line with the extension’s purpose.Update (2021-03-25): There is a follow-up article with details on what Amazon actually does. But since all extension privileges are delegated to Amazon web services, it’s impossible to make sure that it always works like this. If for some Amazon Assistant users the “hoover up all data” mode is switched on, nobody will notice.
In the previous article I documented my approach for reverse engineering an Android game. But getting my hands on the code is only one part of security research. Once a potential issue is identified, I need to verify that it is actually exploitable. So there is no way around messing with an actual live app. Ideally that has to happen in a controlled environment with emulated hardware. As before, this is mostly me writing things down for my future self, but it might come useful for other people as well.