CVE-2025-5994

A cache poisoning flaw was found in Unbound. Resolvers supporting EDNS Client Subnet (ECS) must segregate outgoing queries to accommodate different outgoing ECS information. This issue reopens resolvers to a birthday paradox attack, known as the Rebirthday Attack, which attempts to match the DNS transaction ID with cache non-ECS poisoned replies.

This vulnerability is considered Important rather than Moderate because it directly compromises the integrity of DNS caching mechanisms in resolvers supporting EDNS Client Subnet (ECS). The flaw allows an attacker to exploit the birthday paradox by generating a high volume of concurrent queries with different ECS values, thereby increasing the chance of a transaction ID collision with a spoofed response. If the resolver fails to properly segregate cache entries by ECS scope, it may accept and cache a malicious non-ECS response, effectively leading to DNS cache poisoning. Unlike typical poisoning attempts that require precise timing or privileged network positions, this attack can be carried out remotely with a high success rate, especially in resolvers that do not correctly isolate ECS queries.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.