Question about lies

[z0ccc]

Hi,
What is meant by:

• failed undefined properties
• does not match worker scope
• How does tampering with objects cause these detections?

Thanks

[abrahamjuliot]

These are great questions.

For example, navigator.hardwareConcurrency:

failed undefined properties

How is still a mystery to me, but both novice and advanced object tampering techniques leak subtle differences from the object's native behavior.

// This returns undefined in native engines
Object.getOwnPropertyDescriptor(navigator, 'hardwareConcurrency')
// => undefined

// Let's tamper with the value using a basic method
Object.defineProperty(navigator, 'hardwareConcurrency', { value: 2 })

// Now this returns the descriptor object
Object.getOwnPropertyDescriptor(navigator, 'hardwareConcurrency')
// => {value: 2, writable: false, enumerable: false, configurable: false}

does not match worker scope

Workers have their own scope separate from the window scope, so tampering with an object in one scope does not affect the other scope. Above, we changed navigator.hardwareConcurrency so it now returns 2, and if we now call navigator.hardwareConcurrency in the worker scope, it will return the real value.

Not all objects in the window are available to workers, and support varies per engine:

https://developer.mozilla.org/en-US/docs/Web/API/WorkerGlobalScope

[z0ccc]

Very interesting. Thanks for the reply.

[z0ccc]

Hi,
I got around 'failed undefined properties' by doing this:

Object.defineProperty(Object.getPrototypeOf(navigator), 'hardwareConcurrency', { value: 2 })

But instead of the 'failed undefined properties' I now get 'failed descriptor.value undefined'. What's the difference between these two lies?

Thanks

[abrahamjuliot]

• "failed undefined properties" detects the added descriptor object
• "failed descriptor.value undefined" detects the added value property to the descriptor

// Native return undefined
Object.getOwnPropertyDescriptor(Navigator.prototype, 'hardwareConcurrency').value

// tamper
Object.defineProperty(Object.getPrototypeOf(navigator), 'hardwareConcurrency', { value: 2 })

// Now we get 2
Object.getOwnPropertyDescriptor(Navigator.prototype, 'hardwareConcurrency').value

This will bypass both tests. Idea from: https://github.com/duckduckgo/duckduckgo-privacy-extension/blob/ec09b691eee1945ee80fedac5f7f8b1cf9527008/shared/js/content-scope/utils.js#L92

Object.defineProperty(
    Object.getPrototypeOf(navigator),
    'hardwareConcurrency', {
        get: (function hardwareConcurrency() { return 2 }).bind(null)
    }
)

But, then that creates a new set of leaks. For example:

// this should return an error and not "Number"
Navigator.prototype.hardwareConcurrency.constructor.name

There's a few interesting articles highlighting some of these leaks and potential solutions, but I'm pretty sure the only way to 100% bypass leaks is to change functions at engine level.

• https://adtechmadness.wordpress.com/2019/03/23/javascript-tampering-detection-and-stealth/
• https://palant.info/2020/12/10/how-anti-fingerprinting-extensions-tend-to-make-fingerprinting-easier/