Executive brief
Unbound is a widely used DNS resolver that helps translate human-readable domain names into IP addresses. A vulnerability in how it handles secure DNS-over-TLS connections can allow an attacker to crash the service by sending specific queries under high-load conditions. This results in a denial of service, preventing users and systems from resolving domain names and potentially disrupting internet connectivity for the affected network.
Technical details
A use-after-free (CWE-416) vulnerability exists in Unbound's DNS-over-TLS (DoT) forwarding logic. The TLS server name string is owned by the 'serviced_query' struct but is also referenced by the 'waiting_tcp' struct. If a query is 'jostled' (evicted) from the internal mesh during a DoT TCP handshake, the memory for the server name is freed; a subsequent TLS error causes a read-only dereference of this freed pointer, crashing the daemon. Exploitation requires the Unbound instance to have a stub or forward zone configured for DoT with an '#authname' suffix. An attacker must time queries to coincide with high server pressure and transient connection failures to trigger the vulnerable path. The issue is fixed in version 1.25.2.
Affected products
- NLnet Labs Unbound 1.15.0 up to and including 1.25.1
Timeline
- 2026-07-22: disclosed
- 2026-07-22: advisory
- 2026-07-22: patched: Fixed in Unbound 1.25.2