Hickory DNS 0.25.0-alpha.3 through 0.25.2 Query Amplification via Cyclic Referrals

Hickory DNS 0.25.0-alpha.3 through 0.25.2 contains a query amplification vulnerability in its recursive resolver, which mishandles cyclic sibling name server referrals that lack glue records. Attackers controlling two zones whose name servers reference each other without glue records can make the recursor loop, sending many upstream queries, amplified further by zero-TTL NS records.

Hickory DNS 0.25.0-alpha.3 through 0.25.2 contains a query amplification vulnerability in its recursive resolver, which mishandles cyclic sibling name server referrals that lack glue records. Attackers controlling two zones whose name servers reference each other without glue records can make the recursor loop, sending many upstream queries, amplified further by zero-TTL NS records.

Hickory DNS 0.25.0-alpha.3 through 0.25.2 contains a query amplification vulnerability in its recursive resolver, which mishandles cyclic sibling name server referrals that lack glue records. Attackers controlling two zones whose name servers reference each other without glue records can make the recursor loop, sending many upstream queries, amplified further by zero-TTL NS records.

Two zones can each delegate to name servers named only inside the other zone, with no glue records (the cyclic sibling case in RFC 9471 section 2.3). Such delegations can never be resolved, but the recursor did not stop when it came back to a zone it was already resolving. It kept sending queries until the per-request limits ran out, and zero TTLs on the referral NS records made this worse. Someone who controls two such zones can make a recursor send many upstream queries for each client query. This affects the recursive resolver, which is in hickory-resolver behind the opt-in `recursor` feature from 0.26.0-beta.1. Earlier releases shipped it as the separate hickory-recursor crate. From 0.26.2 each client request tracks the zones on its resolution stack and refuses to enter a zone that is already on it.

Two zones can each delegate to name servers named only inside the other zone, with no glue records (RFC 9471 section 2.3). The recursor did not stop when it came back to a zone it was already resolving, so someone who controls two such zones can make it send many upstream queries for each client query. The recursor code moved into hickory-resolver (opt-in `recursor` feature) in 0.26.0-beta.1, and the fix is in hickory-resolver 0.26.2. No hickory-recursor release contains a fix, so users should move to hickory-resolver 0.26.2 or later with the `recursor` feature.