A multi-vendor cache poisoning vulnerability named 'Rebirthday
Attack' has been discovered in caching resolvers that support
EDNS Client Subnet (ECS). Unbound is also vulnerable when compiled
with ECS support, i.e., '--enable-subnet', AND configured
to send ECS information along with queries to upstream name servers,
i.e., at least one of the 'send-client-subnet',
'client-subnet-zone' or 'client-subnet-always-forward'
options is used. Resolvers supporting ECS need to segregate outgoing
queries to accommodate for different outgoing ECS information. This
re-opens up resolvers to a birthday paradox attack (Rebirthday
Attack) that tries to match the DNS transaction ID in order to cache
non-ECS poisonous replies.