mpg123 is a MPEG Audio Player.
mpg123 contains a buffer overflow in the code that handles layer2 decoding of media files.
An attacker can possibly exploit this bug with a specially-crafted mp3 or mp2 file to execute arbitrary code with the permissions of the user running mpg123.
There is no known workaround at this time.
All mpg123 users should upgrade to the latest version:
# emerge sync
# emerge -pv ">=media-sound/mpg123-0.59s-r4"
# emerge ">=media-sound/mpg123-0.59s-r4"