[Desktop] FP blocking: OffscreenCanvas not protected in WorkerGlobalScope

[abrahamjuliot]

OffscreenCanvas context is not protected in WorkerGlobalScope

Possible contexts parameters include:

• 2d (tested)
• webgl
• webgl2
• bitmaprenderer

Related issue #9186

Steps to Reproduce

• turn on fingerprint blocking (standard and strict) and generate OffscreenCanvas data URI from worker scope
• exit/restart browser and repeat
• compare results

Tested in Nightly 1.14.44 desktop

Example code:

// worker.js
(async () => {
  let canvas2d
  try {
    const canvas = new OffscreenCanvas(256, 256)
    const context = canvas.getContext('2d')
    const str = 'sP6V85pee$5pRs5yH8&VyGJ3NFSo'
    context.font = '20px Arial'
    context.fillText(str, 100, 100)
    context.fillStyle = 'red'
    context.fillRect(100, 30, 80, 50)
    context.font = '32px Times New Roman'
    context.fillStyle = 'blue'
    context.fillText(str, 20, 70)
    context.font = '20px Arial'
    context.fillStyle = 'green'
    context.fillText(str, 10, 50)
    const getDataURI = async () => {
      const blob = await canvas.convertToBlob()
      const reader = new FileReader()
      reader.readAsDataURL(blob)
      return new Promise(resolve => {
        reader.onloadend = () => resolve(reader.result)
      })
    }
    canvas2d = await getDataURI()
  } catch (error) {}

  postMessage({
    canvas2d
  })
  close()
})()

// script.js
(async () => {
  const getWorkerOffscreenCanvas = () => {
    return new Promise(resolve => {
      try {
        const worker = new Worker('worker.js')
        worker.addEventListener('message', async event => {
          const {
            data: {
              canvas2d
            }
          } = event
          return resolve(canvas2d)
        }, false)
      } catch (error) {
        return resolve(undefined)
      }
    })
  }

  const result = await getWorkerOffscreenCanvas()
  console.log(result)
})()

[pes10k]

Hi @abrahamjuliot , thank you for the issue! Both this and #11253 have the same underlying issue, being tracked in #11368.

I am going to close these issues in favor of the general issue / solution, just to keep things tidy. Thank you again for the report!

[abrahamjuliot]

@pes10k awesome. Thank you too :) ☕