WebAssembly Fingerprinting: How a Browser Turns into a Virtual Machine and Begins to Recognize the User

A technology that almost no one talks about

Sometimes the most significant changes on the internet start quietly. While everyone is busy fighting over JavaScript blocking and “proper” privacy settings, a tool has been living in the browser for several years now that is pushing the boundaries of what’s possible. It’s WebAssembly (Wasm): a way to run code on the web at nearly native speed.

I call Wasm the “new beast” of the web, and not just for the sake of a nice metaphor. With it, websites gain a whole new class of capabilities, including something people usually don’t like to talk about openly: new ways to identify users.

Why JavaScript Is No Longer the Prime Suspect

For a long time, JavaScript was an easy target. Dozens of tracking and identification mechanisms rely on it, which is why Tor Browser blocks scripts by default, and NoScript and similar extensions have become symbols of “hardcore privacy.”

The problem is that Wasm shifts the balance. It doesn’t eliminate JavaScript, but it makes the browser significantly more “computational.” Old notions of a “secure browser” are starting to crack, because some aspects now fall into a realm where simple bans don’t work the way you’re used to.

What is WebAssembly and why is it important?

WebAssembly is a format and runtime environment that allows code compiled from C, C++, Rust, and other languages to run in the browser. To put it simply, a website gains access to lower-level computing than typical JavaScript.

Hence my favorite way of putting it: the browser gets the chance to become a “mini-computer inside a computer.” Ultimately, this feels like “almost a regular program” in a tab window: wallets, messengers, complex anti-fraud agents. Yes, comparisons to VirtualBox sound provocative. But the point is simple: the browser is getting closer to the hardware.

A New Fingerprint: When Speed Becomes an Identifier

The real fun begins when Wasm is used not for convenience, but for fingerprinting.

When I talk about a “huge number of fingerprints” via WebAssembly, I don’t mean good old Canvas or WebGL, which many have already learned to obfuscate. What comes into play here is what you usually perceive as “just speed”: performance and execution characteristics.

The diagram looks almost ridiculous because of its simplicity:

• JavaScript calls WebAssembly functions.
• WebAssembly calls JavaScript back.
• This “ping-pong” is repeated many times.
• You measure exactly how and how quickly identical operations are executed.

Next, you convert the statistics into a compact identifier. In practical implementation, this is easily hashed, for example, using SHA-256.

A New Fingerprint: When Speed Becomes an Identifier

The real fun begins when Wasm is used not for convenience, but for fingerprinting.

When I talk about a “huge number of fingerprints” via WebAssembly, I don’t mean good old Canvas or WebGL, which many have already learned to obfuscate. What comes into play here is what you usually perceive as “just speed”: performance and execution characteristics.

The diagram looks almost ridiculous because of its simplicity:

• JavaScript calls WebAssembly functions.
• WebAssembly calls JavaScript back.
• This “ping-pong” is repeated many times.
• You measure exactly how and how quickly identical operations are executed.

Next, you convert the statistics into a compact identifier. In practical implementation, this is easily hashed, for example, using SHA-256.

Where is this already being used?

It ceases to be an “experiment” the moment specific names start to surface.

From what I can see, it’s already part of the toolkit of major anti-fraud platforms, including:

• CyberSource
• PerimeterX

There are also signs that Microsoft was among the first to start implementing Wasm fingerprinting on its resources, including to filter registrations that look like automation or come from a virtualized environment.

Why will this become the “top” fingerprint?

The reason is simple and unpleasant:

• high accuracy,
• relative ease of execution in the browser,
• linking to real hardware characteristics.

If websites start collecting this kind of telemetry en masse, it will quickly become an identification standard that is difficult to ignore. And there’s no need to wait for “sometime in the future”: Wasm has been in browsers for a long time and is already in use.

What does this mean?

To sum it all up in one sentence: browser privacy depends less and less on unchecked boxes and more and more on how the hardware behaves under load.

WebAssembly was conceived as a web accelerator. In practice, it has also become an accelerator for anti-fraud systems that need to distinguish humans from bots and real devices from virtualized environments.

That’s why Wasm fingerprinting isn’t just another “scare story,” but a topic worth understanding at least at a conceptual level. You’ll encounter it more and more often, especially where the stakes are high: finance, major platforms, registration, and protection against abuse.