Breaking Custom Cursor to p0wn the web

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.

A red mouse cursor with evil eyes grinning with its sharp teeth, next to it the text Custom Cursor
Image credits: Custom Cursor, palomaironique

The attack surface of Custom Cursor is unnecessarily large: it grants website excessive privileges while also disabling default Content Security Policy protection. The result: anybody controlling (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.

Data exfiltration in Keepa Price Tracker

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.

Usually, the vendor will claim to anonymize all data, a claim that can rarely be verified. Even if the anonymization actually happens, it’s really hard to do this right. If anonymization can be reversed and the data falls into the wrong hands, this can have severe consequences for a person’s life.

Meat grinder with the Keepa logo on its side is working on the Amazon logo, producing lots of prices and stars
Image credits: Keepa, palomaironique, Nikon1803

Today we will take a closer look at a browser extension called “Keepa – Amazon Price Tracker” which is used by at least two million users across different browsers. The extension is being brought out by a German company and the privacy policy is refreshingly short and concise, suggesting that no unexpected data collection is going on. The reality however is: not only will this extension extract data from your Amazon sessions, it will even use your bandwidth to load various Amazon pages in the background.

Having fun with CSS injection in a browser extension

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.

Website injects CSS code via G App Launcher browser extension into website. As a result, the malicious website displays the message: Your name is John Smith,
Image credits: Mozilla, G App Launcher

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!

Universal XSS in Ninja Cookie extension

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.

Now I am generally wary of extensions that necessarily need full access to every website. This is particularly true if these extensions have to interact with the websites in complicated ways. What are the chances that this is implemented securely? So I took a closer look at Ninja Cookie source code, and I wasn’t disappointed. I found several issues in the extension, one even allowing any website to execute JavaScript code in the context of any other website (Universal XSS).

The cookie ninja from the extension’s logo is lying dead instead of clicking on prompts

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 ( 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.

Print Friendly & PDF: Full compromise

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.

Print Friendly & PDF in Chrome Web Store: 800,000+ users

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 and isn’t affected. The problematic Firefox extension is being distributed from the vendor’s website directly.

Follow-up on Amazon Assistant’s data collection

In my previous article on Amazon Assistant, one sentence caused considerable irritation:

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.

Amazon Assistant log with a borg eye
Image credits: Amazon, nicubunu, OpenClipart

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.

DuckDuckGo Privacy Essentials vulnerabilities: Insecure communication and Universal XSS

A few months ago I looked into the inner workings of DuckDuckGo Privacy Essentials, a popular browser extension meant to protect the privacy of its users. I found some of the typical issues (mostly resolved since) but also two actual security vulnerabilities. First of all, the extension used insecure communication channels for some internal communication, which, quite ironically, caused some data leakage across domain boundaries. The second vulnerability gave a DuckDuckGo server way more privileges than intended: a Cross-site Scripting (XSS) vulnerability in the extension allowed this server to execute arbitrary JavaScript code on any domain.

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.

A very dirty and battered rubber duck
Image credits: RyanMcGuire

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.

How Amazon Assistant lets Amazon track your every move on the web

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.

Amazon Assistant log with a borg eye
Image credits: Amazon, nicubunu, OpenClipart

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.

Setup for testing Android app vulnerabilities

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.

Reverse engineering a Unity-based Android game

My child is playing an Android game that asks for microphone access. And while it doesn’t insist on it and the privacy policy says that no recordings are being kept, I thought that I would take a closer look. The process turned out rather complicated thanks to the fact that the game was built with the Unity framework. Since I have little experience with games in general and Android applications in particular, I thought that I would document the analysis steps here. And maybe this turns out useful for other people as well.