CVE-2025-5994: Cache poisoning via the ECS-enabled Rebirthday Attack

Severity: high (CVSS 8.7 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-07-16 · updated 2026-06-17
Weakness: CWE-349

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.

Affected products

NLnet Labs Unbound: from 1.6.2 before 1.23.0 — fixed in 1.23.0

How to fix

Upgrade to a fixed version: NLnet Labs Unbound 1.23.0. Check the vendor advisory for mitigations.

References

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