CVE-2023-5680: ISC BIND DoS Vulnerability

CVE-2023-5680 Overview

CVE-2023-5680 is a Denial of Service vulnerability affecting ISC BIND 9 DNS resolver software. When a resolver cache accumulates a very large number of EDNS Client Subnet (ECS) records stored for the same name, the process of cleaning the cache database node for this name can significantly impair query performance, effectively causing a denial of service condition.

Critical Impact

DNS resolvers running vulnerable BIND 9 versions may experience significant query performance degradation when cleaning cache nodes containing numerous ECS records, potentially disrupting DNS resolution services for dependent clients and applications.

Affected Products

• ISC BIND 9.11.3-S1 through 9.11.37-S1 (Supported Preview Edition)
• ISC BIND 9.16.8-S1 through 9.16.45-S1 (Supported Preview Edition)
• ISC BIND 9.18.11-S1 through 9.18.21-S1 (Supported Preview Edition)
• NetApp Active IQ Unified Manager for VMware vSphere

Discovery Timeline

• February 13, 2024 - CVE-2023-5680 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-5680

Vulnerability Analysis

This vulnerability exists in BIND 9's handling of EDNS Client Subnet (ECS) cache records. ECS is a DNS extension that allows DNS resolvers to provide geolocation-aware responses by including client subnet information in queries. When enabled, resolvers cache responses with ECS data to improve performance for subsequent queries from similar network locations.

The vulnerability arises when a single name accumulates an excessive number of ECS records in the cache database. The cache cleaning mechanism, which is designed to remove stale or expired records, becomes computationally expensive when processing nodes with large numbers of ECS entries. This inefficient cleanup process consumes excessive CPU resources and time, causing query processing to stall or significantly slow down.

The attack can be triggered remotely over the network without authentication. An attacker could craft DNS queries or influence authoritative DNS responses to cause the targeted resolver to cache numerous ECS records for specific names, then wait for or trigger cache cleanup operations to degrade service.

Root Cause

The root cause is an algorithmic complexity issue in BIND 9's cache database node cleanup routine. When iterating through and processing ECS records during cache maintenance, the algorithm exhibits poor performance characteristics with large datasets. This creates a resource exhaustion condition where the cleanup process monopolizes system resources, degrading the resolver's ability to process legitimate DNS queries.

Attack Vector

The vulnerability is exploitable via the network attack vector. An attacker can influence the resolver's cache contents by:

1. Sending crafted DNS queries with varying ECS client subnet values to the target resolver
2. Operating or compromising authoritative DNS servers to return responses that cause ECS record accumulation
3. Waiting for scheduled cache maintenance or triggering conditions that initiate cache cleanup

When the cache cleaning process encounters a database node with an unusually large number of ECS records, the cleanup operation consumes disproportionate resources, causing observable query latency increases or service degradation. No user interaction is required for exploitation.

Detection Methods for CVE-2023-5680

Indicators of Compromise

• Unusual spikes in DNS query latency or response times on BIND 9 resolvers
• Elevated CPU utilization during cache maintenance operations
• Log entries indicating slow or stalled cache cleaning processes
• Abnormally high memory usage in the named process related to cache structures

Detection Strategies

• Monitor BIND 9 resolver performance metrics for sudden degradation patterns
• Implement alerting on query response time thresholds and cache operation durations
• Analyze DNS query logs for patterns indicating ECS record flooding attempts
• Review BIND statistics for unusual cache growth rates on specific names

Monitoring Recommendations

• Enable BIND 9 statistics channels to track cache size and cleanup performance
• Deploy network monitoring to detect anomalous DNS query patterns targeting ECS functionality
• Configure log rotation and analysis for BIND 9 operational logs
• Establish baseline performance metrics to identify deviations indicative of exploitation attempts

How to Mitigate CVE-2023-5680

Immediate Actions Required

• Upgrade affected BIND 9 installations to patched versions as specified by ISC
• Review resolver configurations and consider disabling ECS if not required for operations
• Implement rate limiting on incoming DNS queries to reduce potential attack surface
• Monitor resolver performance closely during the patch deployment window

Patch Information

ISC has released patches addressing this vulnerability. Organizations should consult the ISC CVE-2023-5680 Knowledge Base article for specific version guidance and upgrade instructions. NetApp customers should also review the NetApp Security Advisory NTAP-20240503-0005 for guidance on Active IQ Unified Manager deployments.

Workarounds

• Disable EDNS Client Subnet (ECS) functionality if geolocation-aware DNS responses are not required
• Implement access control lists (ACLs) to restrict DNS query sources to trusted networks
• Configure cache size limits and cleanup intervals to reduce the impact of large cache accumulations
• Deploy DNS rate limiting to prevent rapid accumulation of ECS records from malicious sources

# Example: Disable ECS in BIND 9 configuration (named.conf)
options {
    // Disable sending ECS to authoritative servers
    send-client-subnet { none; };
    // Disable receiving ECS from clients  
    max-client-subnet-ipv4 0;
    max-client-subnet-ipv6 0;
};