CVE-2026-11622: BIND 9 DNSSEC Resolver Memory Exhaustion Vulnerability

CVE-2026-11622 details a critical vulnerability affecting multiple versions of ISC BIND 9, a widely used DNS software. This flaw impacts DNSSEC validating resolvers, allowing an unauthenticated attacker to trigger a denial-of-service (DoS) condition. The attack involves initiating a "random subdomain attack" against a DNSSEC-signed zone. By sending a high volume of queries for non-existent, random subdomains, the attacker can force the resolver to consume excessive memory. The vulnerability specifically targets BIND 9 versions from 9.11.0 through 9.18.50, 9.20.0 through 9.20.24, 9.21.0 through 9.21.23, and their respective S1 branches (9.11.3-S1 through 9.18.50-S1, and 9.20.9-S1 through 9.20.24-S1). This uncontrolled memory growth can occur even when max-cache-size limits are configured, rendering the resolver unresponsive and disrupting critical DNS resolution services.

Attack Chain

1. An attacker identifies a target BIND 9 DNSSEC validating resolver and a DNSSEC-signed zone that the resolver can query.
2. The attacker initiates a flood of DNS queries for non-existent, random subdomains (e.g., randomstring.example.com) within the chosen DNSSEC-signed zone.
3. The vulnerable BIND 9 resolver attempts to validate each unique non-existent subdomain query, a process that is computationally intensive for DNSSEC.
4. The rate at which the attacker sends queries for new, random subdomains exceeds the resolver's capacity to complete DNSSEC validation for each request.
5. As the resolver's cache attempts to store validation states for numerous unique, non-existent entries, its memory usage begins to increase rapidly.
6. The memory consumption escalates significantly, potentially by orders of magnitude beyond any configured max-cache-size parameter due to the nature of DNSSEC validation state storage.
7. The BIND 9 process exhausts available memory resources, leading to a crash or severe performance degradation, effectively causing a denial-of-service condition for all DNS queries.

Impact

Successful exploitation of CVE-2026-11622 leads to a complete denial-of-service for affected BIND 9 DNSSEC validating resolvers. This means that organizations relying on these resolvers will experience service outages for DNS resolution, preventing access to internal and external resources that depend on DNS. The impact can be severe for internet service providers, enterprises, and other entities running vulnerable BIND instances, causing widespread network disruptions and unavailability of critical applications. The memory exhaustion can persist even with cache size limits, making manual intervention necessary to restore service.

Recommendation

• Patch CVE-2026-11622 on all affected BIND 9 instances immediately by upgrading to a version where the vulnerability is resolved. Consult ISC advisories for specific patched versions.
• Monitor DNS server resource utilization (CPU, memory, network I/O) for anomalies. Sudden spikes in memory usage on BIND 9 servers could indicate an attempted or ongoing exploitation of CVE-2026-11622.
• Implement rate limiting on incoming DNS queries where feasible to mitigate the impact of high-volume attacks, though this may not fully prevent memory exhaustion for CVE-2026-11622.
• Review DNS query logs for unusual patterns, such as a high volume of queries for random, non-existent subdomains from a single source or a small set of sources, which could indicate a random subdomain attack.