allow capturing restricted video (captureStream API is disabled for DRM with Widevine / EME) #2650

obunga commented 2025年08月26日 02:18:44 +02:00

(mostly lifted from https://github.com/BrowserWorks/Waterfox/issues/1895, since the Waterfox developers refused to implement this with no real explanation. 5th most 👍'd issue by the way)

​

DESCRIBE THE BUG
It is not possible to capture / record a restricted video because it is disabled in the official Firefox source code (captureStream API is disabled for DRM with Widevine / EME). The restriction / censorship source code seems to be currently located at line 4075 in the function HTMLMediaElement::CanBeCaptured of the file https://searchfox.org/firefox-main/source/dom/media/mediaelement/HTMLMediaElement.cpp#4075:

 // Prevent capturing restricted video
 if (aCaptureType == StreamCaptureType::CAPTURE_ALL_TRACKS &&
 ContainsRestrictedContent()) {
 return false;
 }
 return true;

Removing the if code block and recompiling should fix the issue.

Chromium has a similar restriction here ("Avoid capturing from EME-protected Media Elements" / "Stream capture not supported with EME"): https://chromium.googlesource.com/chromium/src/third_party/+/master/blink/renderer/modules/mediacapturefromelement/html_media_element_capture.cc#303

TO REPRODUCE

1. With Widevine DRM enabled, go to https://bitmovin.com/demos/drm to test a DRM demo video.
2. Play the video.
3. Open the web console (Ctrl+Shift+K shortcut).
4. Paste a JavaScript code using captureStream API to record a video, like the one on https://gist.github.com/lthrhx/610e94b03bed193f4a2e486baf20e477 and let the video end to trigger the file saving.
   You can also just use the following test code which will produce an error with the name NS_ERROR_FAILURE if there is a restriction:

var video = document.getElementsByTagName("video")[0];
var stream = video.captureStream ? video.captureStream() : video.mozCaptureStream();

The code will not produce an error in the demo page when "Play DRM-controlled content" is OFF.

Since LibreWolf is designed with user freedom in mind, it should not prevent capturing restricted video. Like it could be done if people would customize and compile the Firefox source code.

_(mostly lifted from <https://github.com/BrowserWorks/Waterfox/issues/1895>, since the Waterfox developers refused to implement this with no real explanation. 5th most 👍'd issue by the way)_ ​ **DESCRIBE THE BUG** It is not possible to capture / record a restricted video because it is disabled in the official Firefox source code (captureStream API is disabled for DRM with Widevine / EME). The restriction / censorship source code seems to be currently located at line 4075 in the function `HTMLMediaElement::CanBeCaptured` of the file <https://searchfox.org/firefox-main/source/dom/media/mediaelement/HTMLMediaElement.cpp#4075>: ```cpp // Prevent capturing restricted video if (aCaptureType == StreamCaptureType::CAPTURE_ALL_TRACKS && ContainsRestrictedContent()) { return false; } return true; ``` Removing the `if` code block and recompiling should fix the issue. Chromium has a similar restriction here ("Avoid capturing from EME-protected Media Elements" / "Stream capture not supported with EME"): https://chromium.googlesource.com/chromium/src/third_party/+/master/blink/renderer/modules/mediacapturefromelement/html_media_element_capture.cc#303 **TO REPRODUCE** 1. With Widevine DRM enabled, go to https://bitmovin.com/demos/drm to test a DRM demo video. 2. Play the video. 3. Open the web console (Ctrl+Shift+K shortcut). 4. Paste a JavaScript code using captureStream API to record a video, like the one on https://gist.github.com/lthrhx/610e94b03bed193f4a2e486baf20e477 and let the video end to trigger the file saving. You can also just use the following test code which will produce an error with the name `NS_ERROR_FAILURE` if there is a restriction: ```js var video = document.getElementsByTagName("video")[0]; var stream = video.captureStream ? video.captureStream() : video.mozCaptureStream(); ``` The code will not produce an error in the demo page when "Play DRM-controlled content" is OFF. Since LibreWolf is designed with user freedom in mind, it should not prevent capturing restricted video. Like it could be done if people would customize and compile the Firefox source code.

sertonix commented 2025年09月13日 16:33:28 +02:00

We can't just remove that if. This would be against the web spec and (more importantly) would be fingerprintable by websites. One would need to come up with some reliably way to only allow recording in user approved scripts which seems rather difficult to me. Is there any reason to not do a screen recording of a full screen or kios mode librewolf window?

Ref https://bugzilla.mozilla.org/show_bug.cgi?id=1071482

We can't just remove that `if`. This would be against the web spec and (more importantly) would be fingerprintable by websites. One would need to come up with some reliably way to only allow recording in user approved scripts which seems rather difficult to me. Is there any reason to not do a screen recording of a full screen or kios mode librewolf window? Ref https://bugzilla.mozilla.org/show_bug.cgi?id=1071482

obunga commented 2025年09月13日 21:45:07 +02:00

Is there any reason to not do a screen recording of a full screen

This type of browser recording is able to cleverly auto-detect width/height/framerate, that's one reason.
Meaning that if the video being played is, say, 1440x1080@120FPS, it will record at 1440x1080@120FPS with no further setup (not losslessly though, it's still not a proper "rip") , even if your monitor supports a maximum of 1280x720@60FPS.

The web spec reasoning seems kinda flimsy to me, it's clear that no one except some mega corpos wanted video DRM to become a web standard in the first place. Mozilla added support reluctantly with an ON/OFF toggle, so if we can add another small way to "weaken" it, I'm all for it.

I don't know about the fingerprinting part, there are so many ways to abuse it that I feel Tor is the one and only valid prevention...

> Is there any reason to not do a screen recording of a full screen This type of browser recording is able to cleverly auto-detect width/height/framerate, that's one reason. Meaning that if the video being played is, say, `1440x1080@120FPS`, it will record at `1440x1080@120FPS` with no further setup (not losslessly though, it's still not a proper "rip") , even if your monitor supports a maximum of `1280x720@60FPS`. The web spec reasoning seems kinda flimsy to me, it's clear that no one except some mega corpos wanted video DRM to become a web standard in the first place. Mozilla added support [reluctantly](<https://blog.mozilla.org/en/mozilla/drm-and-the-challenge-of-serving-users/>) with an ON/OFF toggle, so if we can add another small way to "weaken" it, I'm all for it. I don't know about the fingerprinting part, there are so many ways to abuse it that I feel Tor is the one and only valid prevention...

alyx161 commented 2026年03月08日 14:08:38 +01:00

Removing this would be plain illegal.
No matter how ineffective this measure is, removing it would count as "bypassing copy protection", which is illegal in many countries.

Removing this would be plain illegal. No matter how ineffective this measure is, removing it would count as "bypassing copy protection", which is illegal in many countries.