CVE-2019-6469 in BIND
Summary
by MITRE
An error in the EDNS Client Subnet (ECS) feature for recursive resolvers can cause BIND to exit with an assertion failure when processing a response that has malformed RRSIGs. Versions affected: BIND 9.10.5-S1 -> 9.11.6-S1 of BIND 9 Supported Preview Edition.
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Analysis
by VulDB Data Team • 01/07/2024
The vulnerability identified as CVE-2019-6469 represents a critical assertion failure in the BIND DNS server implementation that specifically impacts the EDNS Client Subnet feature. This flaw exists within the recursive resolver functionality of BIND versions ranging from 9.10.5-S1 through 9.11.6-S1 in the Supported Preview Edition, creating a potential denial of service condition that can cause the entire DNS resolution process to terminate abruptly. The issue manifests when the resolver encounters DNS responses containing malformed RRSIG records, which are essential for DNS security validation within the DNS Security Extensions framework. The vulnerability demonstrates a classic software defect where insufficient input validation leads to an assertion failure, a condition that typically indicates a programming error in the software's logic flow.
The technical implementation flaw occurs within the processing logic of the EDNS Client Subnet functionality, which is designed to provide recursive resolvers with information about the client's subnet for load balancing and geographic routing purposes. When malformed RRSIG records are present in DNS responses, the resolver's internal state management fails to properly handle the malformed data structure, leading to an assertion failure that terminates the BIND process. This represents a failure in input validation and error handling mechanisms that should gracefully process malformed data rather than causing system termination. The vulnerability is classified under CWE-248, which addresses "Uncaught Exception" conditions, and demonstrates how improper exception handling can lead to complete service disruption. The flaw operates at the intersection of DNS security protocols and recursive resolver functionality, making it particularly dangerous in production environments where DNS availability is critical.
The operational impact of this vulnerability extends beyond simple service interruption, as it can be exploited to cause widespread denial of service across networks dependent on affected BIND versions. An attacker could craft malicious DNS responses containing malformed RRSIG records and send them to recursive resolvers, causing cascading failures that could disrupt DNS resolution for entire organizations or geographic regions. The vulnerability's exploitation requires minimal privileges and can be executed through standard DNS traffic, making it particularly dangerous in environments where recursive resolvers are exposed to untrusted networks. This attack vector aligns with ATT&CK technique T1499.004, which covers "Endpoint Denial of Service," and demonstrates how DNS infrastructure can be targeted for service disruption. Organizations using affected BIND versions face potential outages that could impact web browsing, email services, and other internet-dependent applications, with recovery requiring system restarts and potential configuration modifications.
Mitigation strategies for CVE-2019-6469 primarily involve immediate patching of affected BIND versions to the latest stable releases that contain the necessary code fixes for proper RRSIG validation and error handling. Organizations should also implement network-level protections such as DNS response filtering and rate limiting to reduce the impact of potential exploitation attempts. Configuration changes including disabling the EDNS Client Subnet feature where it is not essential can provide temporary workaround measures while patches are deployed. The vulnerability highlights the importance of proper input validation and error handling in security-critical infrastructure components, emphasizing that DNS servers must be resilient to malformed input data. Security monitoring should include detection of assertion failures and process termination events in DNS resolver logs, as these may indicate exploitation attempts. Regular vulnerability assessments and security updates are essential for maintaining DNS infrastructure integrity, particularly given the widespread deployment of BIND across internet infrastructure. Organizations should also consider implementing redundant DNS resolution mechanisms to ensure availability during patch deployment or remediation activities.