Executive brief
A vulnerability in the BIND 9 DNS resolver can be exploited to cause severe resource exhaustion on the server. BIND 9 is widely used software that translates human-readable domain names into IP addresses; an exploit could lead to a denial-of-service, making websites and internal services unreachable. This issue allows an unauthenticated remote attacker to overwhelm the system by sending specially crafted DNS queries.
Technical details
An unbounded resend loop vulnerability (CWE-606) exists in the BIND 9 resolver state machine during the handling of 'bad' upstream servers. The flaw is triggered when specific retry conditions are met, causing the resolver to enter an infinite or excessive loop of resending queries. A remote, unauthenticated attacker can exploit this by sending DNS queries that force the resolver into this state, leading to CPU and memory exhaustion. The vulnerability specifically affects recursive resolvers; authoritative-only servers are generally not impacted. Patches have been released in versions 9.18.49, 9.20.23, and 9.21.22.
Affected products
- ISC BIND 9 9.18.36 through 9.18.48, 9.20.8 through 9.20.22, 9.21.7 through 9.21.21, 9.18.36-S1 through 9.18.48-S1, 9.20.9-S1 through 9.20.22-S1
Timeline
- 2026-05-13: other: Early notification provided to partners
- 2026-05-20: disclosed: Public disclosure of the vulnerability
- 2026-05-20: patched: Patched versions 9.18.49, 9.20.23, and 9.21.22 released