Tor browser rovina le immagini a causa dell'ottimizzazione locale

Falco · 17 Febbraio 2022, 1:25pm

On my test the Tor browser blocked this call here:

discourse/discourse/blob/1c73b97d508fd79e6c8eff0cc8a44bdc5084f4a7/app/assets/javascripts/discourse/app/lib/media-optimization-utils.js#L52


      
            const canvas = document.createElement("canvas");
            canvas.width = width;
            canvas.height = height;
          
            // Draw image onto canvas
            const ctx = canvas.getContext("2d");
            if (!ctx) {
              throw "Could not create canvas context";
            }
            ctx.drawImage(drawable, sx, sy, sw, sh, 0, 0, width, height);
            const imageData = ctx.getImageData(0, 0, width, height);
            canvas.remove();
            return imageData;
          }
          
          function isTransparent(type, imageData) {
            if (!/(\.|\/)(png|webp)$/i.test(type)) {
              return false;
            }
          
            for (let i = 0; i < imageData.data.length; i += 4) {

with

Blocked https://try.discourse.org/ from extracting canvas data because no user input was detected.

which is obviously a lie, since the File Picker is user input.

That was detect by our failsafe mechanism:

Resize failed: Image corrupted during resize. Falling back to the original for encode

Which prompts Discourse to only re-encode the file, which appears to be successful, but then fails with a 422 when finishing the multi-part upload…

Tor Browser should block the canvas creation API instead of silently corrupting return values

Anyway, looks like users can disable the silently failure of canvas operations by toggling privacy.resistFingerprinting.autoDeclineNoUserInputCanvasPrompts on about:config which will allow users to see this prompt:

I just tested and that fixes the upload.

Argomento		Risposte	Visualizzazioni
How to disable use of HTML5 canvas image data? Support	9	2335	Novembre 17, 2021
Client side image optimization enabled by default Announcements	4	1588	Settembre 4, 2023
Testing composer image optimization Feature	33	4338	Settembre 28, 2021
Image upload stuck with "Processing" message Support	11	1664	Novembre 14, 2021
Faster (and smaller) uploads in Discourse with Rust, WebAssembly and MozJPEG \| Blog Blog	36	4852	Maggio 13, 2025

Tor browser rovina le immagini a causa dell'ottimizzazione locale

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