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Sound problems?

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HowTos

A couple of times I’ve encountered problems with the sound card on Debian boxes. Some notes I’ve found that may help others having difficulties:

* Check the permissions on /dev/dsp, and make sure that the relevant user(s) are in the audio group in /etc/group. You may need to log out & back in again for this to take effect.

* If using the alsa sound daemon, install the alsa-utils package and run alsaconf.

Read More Here.

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Security Leftovers

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    “The attacker just shines an image of something on the road or injects a few frames into a digital billboard, and the car will apply the brakes or possibly swerve, and that’s dangerous,” Ben Gurion University researcher Yisroel Mirsky told the magazine. “The driver won’t even notice at all. So somebody’s car will just react, and they won’t understand why.”

  • File Exfiltration via Libreoffice in BigBlueButton and JODConverter

    BigBlueButton is a free web-based video conferencing software that lately got quite popular, largely due to Covid-19. Earlier this year I did a brief check on its security which led to an article on Golem.de (German). I want to share the most significant findings here. BigBlueButton has a feature that lets a presenter upload a presentation in a wide variety of file formats that gets then displayed in the web application. This looked like a huge attack surface. The conversion for many file formats is done with Libreoffice on the server. Looking for ways to exploit server-side Libreoffice rendering I found a blog post by Bret Buerhaus that discussed a number of ways of exploiting such setups. One of the methods described there is a feature in Opendocument Text (ODT) files that allows embedding a file from an external URL in a text section. This can be a web URL like https or a file url and include a local file. This directly worked in BigBlueButton. An ODT file that referenced a local file would display that local file. This allows displaying any file that the user running the BigBlueButton service could access on the server. A possible way to exploit this is to exfiltrate the configuration file that contains the API secret key, which then allows basically controlling the BigBlueButton instance. I have a video showing the exploit here. (I will publish the exploit later.) I reported this to the developers of BigBlueButton in May. Unfortunately my experience with their security process was not very good. At first I did not get an answer at all. After another mail they told me they plan to sandbox the Libreoffice process either via a chroot or a docker container. However that still has not happened yet. It is planned for the upcoming version 2.3 and independent of this bug this is a good idea, as Libreoffice just creates a lot of attack surface. Recently I looked a bit more into this. The functionality to include external files only happens after a manual user confirmation and if one uses Libreoffice on the command line it does not work at all by default. So in theory this exploit should not have worked, but it did. It turned out the reason for this was another piece of software that BigBlueButton uses called https://github.com/sbraconnier/jodconverter JODConverter. It provides a wrapper around the conversion functionality of Libreoffice. After contacting both the Libreoffice security team and the developer of JODConverter we figured out that it enables including external URLs by default.

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