mpg321 is a GPL replacement for mpg123, a command line audio player with support for ID3. ID3 is a tagging system that allows metadata to be embedded within media files.
A routine security audit of the mpg321 package revealed a known security issue remained unpatched. The vulnerability is a result of mpg321 printing embedded ID3 data to the console in an unsafe manner.
Successful exploitation would require a victim to play a specially crafted audio file using mpg321, potentially resulting in the execution of arbitrary code.
There is no known workaround at this time.
All mpg321 users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=media-sound/mpg321-0.2.10-r2"