CVE-2020-8616 in BIND
Summary
by MITRE
A malicious actor who intentionally exploits this lack of effective limitation on the number of fetches performed when processing referrals can, through the use of specially crafted referrals, cause a recursing server to issue a very large number of fetches in an attempt to process the referral. This has at least two potential effects: The performance of the recursing server can potentially be degraded by the additional work required to perform these fetches, and The attacker can exploit this behavior to use the recursing server as a reflector in a reflection attack with a high amplification factor.
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Analysis
by VulDB Data Team • 10/18/2020
The vulnerability identified as CVE-2020-8616 represents a critical flaw in DNS recursive server implementations that enables attackers to exploit insufficient rate limiting mechanisms during referral processing. This vulnerability specifically targets the handling of DNS referrals where servers must perform additional queries to resolve names that cannot be answered directly. The flaw allows malicious actors to craft specially designed referrals that trigger excessive recursive lookup operations, creating a scenario where a single referral can spawn thousands or even millions of subsequent DNS queries. This behavior directly violates the fundamental security principle of resource limiting and demonstrates a clear weakness in the server's ability to handle malformed or maliciously constructed DNS responses. The vulnerability falls under CWE-770, which addresses the allocation of resources without proper limits, and represents a classic example of resource exhaustion that can be leveraged for both performance degradation and amplification attacks.
The technical exploitation of CVE-2020-8616 occurs when a DNS recursive server receives a referral that contains multiple or nested referrals that trigger cascading lookup operations. When the server processes these referrals without adequate safeguards, it can recursively follow chains of referrals that grow exponentially in the number of queries required to resolve them. This creates a scenario where an attacker can send a single DNS query that results in hundreds or thousands of additional queries being generated by the target server. The amplification factor can reach extreme levels, with a single packet potentially generating dozens or hundreds of times its original size in response traffic. This behavior aligns with ATT&CK technique T1499.004, which describes the use of resource exhaustion attacks, and demonstrates how DNS servers can be leveraged as reflectors in distributed denial-of-service attacks. The vulnerability exploits the inherent trust model in DNS resolution where recursive servers are expected to follow referrals without sufficient validation of the referral chain complexity.
The operational impact of CVE-2020-8616 extends beyond simple performance degradation to include significant security implications that can compromise entire DNS infrastructure. Servers affected by this vulnerability can experience complete service disruption as their resources become consumed by the excessive query processing, leading to legitimate queries being dropped or delayed. The amplification capabilities make this vulnerability particularly dangerous for attackers seeking to launch large-scale reflection attacks against other targets, as the compromised servers can generate massive amounts of traffic that appears to originate from the victim's IP address. Network operators may observe unusual traffic patterns and increased CPU utilization on affected servers, while the broader DNS ecosystem can experience cascading failures as multiple servers become compromised. The vulnerability affects the availability aspect of the CIA triad and can be classified under the broader category of denial-of-service attacks that exploit protocol implementation flaws rather than network-level congestion.
Mitigation strategies for CVE-2020-8616 require implementing robust rate limiting and recursion control mechanisms within DNS server configurations. Network administrators should configure servers to limit the number of referrals that can be processed during a single recursive query, typically setting thresholds that prevent excessive cascading lookups. Implementing DNS query rate limiting and establishing maximum recursion depth controls can effectively prevent the exploitation of this vulnerability. The solution aligns with security best practices outlined in RFC 1034 and RFC 1035, which provide guidelines for DNS server implementations to prevent resource exhaustion attacks. Additionally, deploying DNS security extensions such as DNSSEC can help validate the authenticity of referrals and prevent maliciously crafted responses from triggering excessive processing. Organizations should also consider implementing network-level controls such as firewall rules that limit outbound DNS traffic from recursive servers and monitor for unusual query patterns that may indicate exploitation attempts. The vulnerability highlights the importance of proper input validation and resource limiting in protocol implementations, as specified in the OWASP Top Ten security framework.