Mining JavaScript Events for Fun & (Preventing Someone Else’s) Profit

For a deep dive analysis on a specific attack or suspicious transactions on a customer’s web application, one of my favorite things to examine is the interaction with the website. There is a wealth of JavaScript information that can be collected to figure out how someone or some bot behaved on the page. In this post, I’ll review some of the key items I investigate to get a sense of the interaction with the page, specifically for desktop web browser data.

What’s up with that mouse?

I look at mouse clicks and mouse movements. For mouse clicks, one of my favorite phenomenons is what we call a “Magic Mouse.” A Magic Mouse can click in all the right places, but never travels between them. It just magically appears in different locations without any trail. Of course it’s possible to miss some mouse movements depending on how often the JavaScript polls to track them. But if you know the polling conditions, you can easily rule that out as the cause of missing mouse movements.

Another interesting component to mouse clicks is where they occur. The best locations to see are negative coordinates, which means the click occurred off the visible screen. In case you are wondering how that’s humanly possible, the answer is - it’s a bot, not a human. If you’re a bot, the world is your oyster and you can click the right web elements, regardless of where they might be - on or off your screen.

For mouse movements, I like to look for straight-line tracks. Is it theoretically possible for humans to move their mouse in a straight line? Sure. But if you think this actually happens in real life, then I invite you to draw a “straight line” with your mouse and let me know how it turns out. Moreover, I’d like to know the last time you logged into your bank account while also moving your mouse in a straight line.

Beyond straight lines, I also look for identifiable patterns in tracks like fixed or otherwise predictable increases/decreases in the x or y direction. Polling frequency can matter a lot in terms of your ability to identify these patterns. They are generally more nuanced and difficult to spot but are very fun to watch. This process can also be generalized by considering the entropy of the movement. Human users typically have higher entropy in their movement data while bots may struggle to introduce the right amount of randomness in mouse moves.

How about those key events?

Under normal circumstances, every key movement should have three events: key down, key up, and key press. The first thing to check is whether every movement has all three of these events. It’s odd to find key down events with no key up events or key press events without either up or down events. Such anomalous key movements would be worth further investigation. Key events happening at the same time as mouse movements are also suspicious. It’s difficult for a human, but not for a bot.

The next thing to examine for key events is timing. I like to think I’m a quick typer, and I can hit around 85 words per minute (please hold your applause). Since the average English word is 4.7 characters, that puts me at about seven characters per second. So if I see transactions that have seven characters in 10 milliseconds, that seems just a little bit suspicious. Conversely, taking 30 seconds to type seven characters also seems a little odd. That would involve a slow hunt and peck process.

The timing between characters can also be interesting. Evenly spaced characters are typically suspicious because humans vary times between different keys based on typing strategy and keyboard layout.  Sometimes I also take it a step further and look at the timing between key down and key up events. Too-regular and too-quick timings on these events can also be a red flag, as they fail to align with the organic timings from human typing.

Along with general timing and cadence, copy and paste events are always of interest. Specifically, I mean the use of command/control-c and command/control-v, not a browser autofill. Depending on the particular webpage and the completed fields, the level of suspiciousness can vary significantly for copy and paste events.

Copy and paste events for a password, for example, don’t usually sway me one way or another because I envision a lot of people who need to retrieve their password from another application. Applications that are good and bad for this purpose is the subject of a different post. Copy and paste events for a username, although slightly less common perhaps, are probably also not too interesting. But copy and paste events for a social security number, or a home address, well now we are talking. All good things with context, as they say.

What are the screen dimensions?

When I consider screen dimensions, I look at the dimensions for the available real estate, meaning the screen itself, and the browser window. As a single piece of information, the screen dimensions aren’t wildly informative. A lot of real users have their screens set up for multitasking or multi-viewing. And there isn’t always a lot of variation between the screen dimensions used by millions of real humans. 

But sometimes the screen dimensions are useful for tracking attack traffic over time if they are sufficiently uncommon. In addition, browser windows that are small in relation to the available real estate might set off red flags, particularly if combined with mouse events like those negative-coordinate clicks. And multi-tasking setups, combined with visibility events, may also seem suspicious.

Speaking of visibility events, were there any?

A visibility event occurs when the user clicks off the main screen and allows another window to become active. If you envision your normal login process, you probably don’t venture off the login page. There are of course legitimate reasons you might move away based on a distraction or retrieving a password. Combined with keyboard events though, like those suspicious copy and paste events, visibility events can be significant.

Additional data as needed

These categories including mouse events, key events, screen dimensions and visibility events cover the main items I investigate. But there is a treasure trove of behavioral information that can be collected and explored over JavaScript.

Moreover, there is a lot more you can do once you consider groups of transactions. Semi-odd data seen once can typically be explained away. But if that same data is seen in multiple transactions, then more red flags may be raised.

I hope the items discussed here provide you with a springboard for diving into your own data to investigate suspicious traffic.

Published Oct 19, 2020
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