You're receiving this bug because the package in Summary has produced _FORTIFY_SOURCE related warnings indicating the presence of a sure overflow in a static buffer.
Even though this is not always an indication of a security problem it might even be. So please check this out ASAP.
By the way, _FORTIFY_SOURCE is disabled when you disable optimisation, so don't try finding out the cause using -O0.
Your friendly neighborhood tinderboxer
Created attachment 250013 [details]
Created attachment 261035 [details, diff]
Fix buffer overflow on ustar magic field
Created attachment 261036 [details]
ebuild to apply the magic overflow patch
In encode.c there was an attempt to strncpy the 8 character string "ustar " into a 6 character field in the ustar header. The ustar format calls for a 6 character field that contains "ustar" terminated by a null, so I've changed it to that.
applied in -r4