Memcached version 1.5.5 contains an Insufficient Control of Network Message Volume (Network Amplification, CWE-406) vulnerability in the UDP support of the memcached server that can result in denial of service via network flood (traffic amplification of 1:50,000 has been reported by reliable sources). This attack appear to be exploitable via network connectivity to port 11211 UDP. This vulnerability appears to have been fixed in 1.5.6 due to the disabling of the UDP protocol by default.
access.redhat.com/errata/RHBA-2018:2140
access.redhat.com/errata/RHSA-2018:1593
access.redhat.com/errata/RHSA-2018:1627
access.redhat.com/errata/RHSA-2018:2331
access.redhat.com/errata/RHSA-2018:2857
blogs.akamai.com/2018/03/memcached-fueled-13-tbps-attacks.html
github.com/memcached/memcached/commit/dbb7a8af90054bf4ef51f5814ef7ceb17d83d974
github.com/memcached/memcached/issues/348
github.com/memcached/memcached/wiki/ReleaseNotes156
twitter.com/dormando/status/968579781729009664
usn.ubuntu.com/3588-1/
www.debian.org/security/2018/dsa-4218
www.exploit-db.com/exploits/44264/
www.exploit-db.com/exploits/44265/
www.synology.com/support/security/Synology_SA_18_07