mpg123 is a real time audio player designed for the MPEG format.
In httpdget.c, a variable is assigned to the heap, and is supposed to receive a smaller allocation. As this variable was not terminated properly, strncpy() will overwrite the data assigned next in memory.
By enticing a user to visit a malicious URL, an attacker could possibly execute arbitrary code with the rights of the user running mpg123.
There is no known workaround at this time.
All mpg123 users should update to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=media-sound/mpg123-0.59s-r11"