[pulseaudio-discuss] Vulnerability in Webkit-GTK and PulseAudio volume handling
Alexander E. Patrakov
patrakov at gmail.com
Tue Oct 8 12:33:42 CEST 2013
Note: this is not a CVE request yet! Before making a formal CVE request,
I would need to collect "official" information on the topic who needs to
do what with this bug (although I do have my own opinion, see below).
For now, I just want to start a discussion by posting this to the
relevant mailing lists, and also I want to avoid the situation where
each side blames the other. Please note that I am not an upstream
developer of any of the mentioned projects. I will attend the audio
mini-conference at LinuxCon Europe 2013, it is OK to discuss the issue
there if representatives from both parties intend to come.
The following combination of software has a nasty bug when used
together, that I personally consider to be a vulnerability:
* PulseAudio (any version, especially when used in flat-volume mode that
is the default everywhere except Ubuntu).
* Any browser based on Webkit-GTK 2.x (any version with HTML5
audio/video support based on GStreamer).
cause an audio file to play at an unexpectedly high volume, not obeying
the volume that the user has set for the web browser in pavucontrol or
gnome-volume-control, and effectively not letting the user move the
volume slider corresponding to the web browser. When flat volumes are in
effect, the web page can play that audio file at the full volume that
the sound card is capable of, which can in some cases damage
loudspeakers (especially tweeters) or the user's hearing.
The reproducer is already public at http://jsfiddle.net/bteam/FbkGD/ and
can be trivially enhanced to also prevent muting of the audio stream.
View that in Epiphany or Midori on any Linux distribution except Ubuntu.
My own opinion is that both parties are equally responsible for the
vulnerability. The salt of the bug is that PulseAudio's security model
is based on clients not sending malicious requests to change the stream
volume, while Webkit passes all volume-changing requests (including
malicious) to GStreamer, because it has no way of telling user-initiated
volume change requests from automated malicious ones.
changes to pulseaudio means that the user cannot drag the "Epiphany"
mote the Epiphany stream in pavucontrol. So, in my opinion, using a
matter, the volume visible to any other runtime that can execute
untrusted programs/scripts) is always wrong.
However, the fact that flat volumes are enabled by default in upstream
PulseAudio makes this small annoyance a real vulnerability. Given that
the "100% hardware volume" type of bug is still present in some
applications given the vast amount of time the feature exists, I think
(but understand that it is an extreme position) that flat-volume mode,
by its mere existence, is a bug that needs to be removed. At the very
minimum, there is a documentation bug: it is nowhere explained that you
should never use PulseAudio stream volumes (and for that matter,
gstreamer sink volumes) for things that are not guaranteed to directly
correspond to user-draggable volume sliders that no automated script can
Alexander E. Patrakov
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