zlib is a widely used free and patent unencumbered data compression library.
zlib improperly handles invalid data streams which could lead to a buffer overflow.
By creating a specially crafted compressed data stream, attackers can overwrite data structures for applications that use zlib, resulting in arbitrary code execution or a Denial of Service.
There is no known workaround at this time.
All zlib users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=sys-libs/zlib-1.2.3"