CVE-2020-8618 in BIND
Summary
by MITRE
An attacker who is permitted to send zone data to a server via zone transfer can exploit this to intentionally trigger the assertion failure with a specially constructed zone, denying service to clients.
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Analysis
by VulDB Data Team • 10/25/2020
This vulnerability exists within DNS server implementations that handle zone transfer operations, specifically affecting the handling of malformed or specially constructed zone data during the zone transfer process. The flaw manifests as an assertion failure that occurs when an attacker with permission to send zone data to a DNS server can craft malicious zone records that cause the server to terminate unexpectedly. This represents a classic denial of service vulnerability that leverages the legitimate zone transfer functionality to disrupt normal service operations. The vulnerability is particularly concerning because it requires only minimal privileges to exploit, as the attacker merely needs permission to send zone data to the target server, which is often granted to authorized secondary DNS servers or during normal operational procedures.
The technical implementation of this vulnerability stems from inadequate input validation and error handling within the DNS server's zone transfer processing logic. When a server receives zone data through a zone transfer operation, it typically validates the incoming records against established DNS standards and performs various consistency checks. However, the specific assertion failure indicates that the server's validation routines contain a condition that is not properly handled when encountering certain malformed data patterns. This assertion failure essentially causes the DNS server process to crash or terminate, preventing it from serving legitimate DNS queries to clients. The vulnerability is classified under CWE-248 as an "Uncaught Exception" and can be mapped to ATT&CK technique T1499.004 for "Network Denial of Service" within the context of DNS service disruption.
The operational impact of this vulnerability extends beyond simple service interruption, as DNS servers form the backbone of internet infrastructure and their disruption can cascade across multiple dependent services. When a DNS server becomes unavailable due to this assertion failure, clients attempting to resolve domain names experience complete resolution failures until the server is restarted or the vulnerable process is recovered. This vulnerability can be particularly devastating in environments where DNS servers are critical to business operations, such as enterprise networks, hosting providers, or internet service providers where multiple services depend on continuous DNS availability. The attack vector is relatively straightforward for an authorized user to exploit, making it a significant risk in environments where zone transfer permissions are not properly restricted or monitored.
Mitigation strategies for this vulnerability should focus on both immediate defensive measures and long-term architectural improvements. Organizations should implement strict access controls to limit zone transfer permissions to only trusted entities and regularly audit these permissions to ensure they remain appropriate. Network segmentation and monitoring can help detect unusual zone transfer activities that might indicate exploitation attempts. Additionally, implementing proper input validation and exception handling within DNS server software can prevent the assertion failures from occurring in the first place. Regular security updates and patches from vendors should be applied promptly to address known vulnerabilities in DNS server implementations. The vulnerability also highlights the importance of robust error handling in critical infrastructure software, as proper exception management can prevent exploitation of similar assertion failures in other components of the DNS infrastructure. Organizations should also consider implementing redundant DNS server configurations to minimize the impact of individual server failures and ensure continued service availability during exploitation attempts.