The issue is exploitable by using a specially constructed video file which masquerades as a HTTP Live Streaming (HLS) M3U8 file which can use the "concat" protocol to read local files and leak them via an external HTTP request.
More details can be found by reading CVE-2016-1897 and CVE-2016-1898.
The bug has since been fixed in the following versions of FFmpeg:
It is recommended to compile the most current FFmpeg from source, which our guide here should help with:
If you're unable to do that, you may instead wish to download the latest static build available here:
If you're unable to do either of the above it is recommended to disable all FFmpeg functionality in XenForo Media Gallery (or other code that uses FFmpeg), at least temporarily (see below). You can disable FFmpeg in XFMG by removing the path to FFmpeg in the Gallery Options > Video Options tab.
As an added layer of protection for users of XenForo Media Gallery, the next release of XFMG includes code which should automatically reject any video files which could be used to exploit this bug, but upgrading FFmpeg remains the recommended solution.