CVE-2025-5994, known as the 'Rebirthday Attack,' is a high-severity cache poisoning vulnerability affecting multiple DNS caching resolvers that support EDNS Client Subnet (ECS). Unbound is vulnerable if compiled with ECS support and configured to send ECS information with queries. The vulnerability arises because resolvers do not properly segregate outgoing queries by ECS data, enabling an attacker to exploit the birthday paradox to match DNS transaction IDs and inject malicious cache entries.