CVE-2025-5994: Cache poisoning via the ECS-enabled Rebirthday Attack
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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Unboundto a version that resolves this vulnerability.Fixed in 1.23.1 - Configuration
To mitigate the Rebirthday Attack on Unbound compiled with ECS support (--enable-subnet), configure Unbound so it does not send ECS information to upstream name servers—set at least one of the following options used for ECS forwarding to disabled: 'send-client-subnet', 'client-subnet-zone', or 'client-subnet-always-forward'.
Unbound send-client-subnet / client-subnet-zone / client-subnet-always-forward = disable (do not send ECS to upstream) - Configuration
For resolvers that support ECS, segregate outgoing queries to accommodate different outgoing ECS information to prevent ECS-enabled Rebirthday Attack cache poisoning.
DNS resolvers supporting EDNS Client Subnet (ECS) outgoing query segregation by ECS = segregate queries - Configuration
As a mitigation for affected versions, do not use EDNS Client Subnet (ECS) (i.e., disable ECS support/usage in affected caching resolvers).
DNS resolvers (general) EDNS Client Subnet (ECS) usage = not using ECS
Event History
Frequently Asked Questions
What is the severity of CVE-2025-5994?
CVE-2025-5994 is classified as a moderate severity vulnerability affecting caching resolvers that support EDNS Client Subnet.
How do I fix CVE-2025-5994?
To mitigate CVE-2025-5994, ensure that your Unbound installation is compiled without EDNS Client Subnet support by avoiding the '--enable-subnet' configuration.
Which versions of Unbound are affected by CVE-2025-5994?
CVE-2025-5994 affects any version of Unbound that is compiled with EDNS Client Subnet support enabled.
What is a 'Rebirthday Attack' related to CVE-2025-5994?
A 'Rebirthday Attack' is a cache poisoning vulnerability that exploits the EDNS Client Subnet feature in caching resolvers.
How can I determine if my Unbound configuration is vulnerable to CVE-2025-5994?
Check your Unbound configuration for the presence of the '--enable-subnet' option and confirm if ECS information is being sent.