CVE-2025-5994

Description

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.