CVE-2025-5994: Nlnet Labs Unbound

High severity, CVSS 8.7. EPSS: 0.2% chance of exploitation in the next 30 days.

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.

Affected products

• Nlnet Labs Unbound: from 1.6.2, before 1.23.0 (fixed in 1.23.0)

Published 2025-07-16. Last modified 2026-06-17.