CVE-2016-6172 in Authoritative Server
Summary
by MITRE
PowerDNS (aka pdns) Authoritative Server before 4.0.1 allows remote primary DNS servers to cause a denial of service (memory exhaustion and secondary DNS server crash) via a large (1) AXFR or (2) IXFR response.
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Analysis
by VulDB Data Team • 09/21/2022
PowerDNS authoritative server versions prior to 4.0.1 contained a critical vulnerability that enabled remote primary DNS servers to execute a denial of service attack against secondary servers through malformed zone transfer responses. This vulnerability specifically affected the handling of AXFR (zone transfer) and IXFR (incremental zone transfer) operations, where malicious primary servers could send oversized responses that would exhaust memory resources on secondary servers. The flaw occurred during the processing of DNS zone transfers when the secondary server attempted to parse and store the received data, leading to uncontrolled memory consumption that eventually caused the server process to crash and become unavailable to serve legitimate DNS queries. This vulnerability directly maps to CWE-400, which describes unchecked resource consumption, and represents a classic example of how DNS infrastructure can be exploited to disrupt network services through memory exhaustion attacks. The impact extends beyond simple service disruption as it affects the entire DNS resolution chain, potentially causing cascading failures across dependent systems that rely on stable DNS infrastructure. Organizations running affected PowerDNS versions face significant operational risks since the vulnerability can be exploited remotely without authentication, making it particularly dangerous in internet-facing environments. The attack vector demonstrates how DNS protocol implementations can be manipulated to consume excessive system resources, aligning with ATT&CK technique T1499.004 for network denial of service. The vulnerability highlights the importance of proper input validation and resource management in DNS server implementations, as the secondary server failed to implement adequate bounds checking on zone transfer data sizes. This flaw represents a fundamental security gap in the handling of DNS zone transfer operations, where the server did not properly limit memory allocation based on the size of incoming zone data. The issue was particularly concerning for DNS infrastructure operators as it could be exploited by attackers to render DNS services completely unavailable, affecting thousands of domains and potentially causing widespread network disruption. Security teams needed to prioritize patching this vulnerability since it provided a straightforward path to service disruption without requiring complex attack chains or privileged access. The memory exhaustion occurred during the parsing phase of zone transfer responses, where the server allocated memory based on the reported size of the zone data without proper validation of whether such allocation was reasonable or safe. This vulnerability demonstrates the critical importance of implementing proper resource limits and validation mechanisms in DNS infrastructure software, as the absence of such protections can lead to complete service outages. Organizations should have implemented monitoring for unusual memory consumption patterns and had emergency response procedures in place to handle such denial of service scenarios. The fix for this vulnerability required modifications to how PowerDNS handled incoming zone transfer data, ensuring that memory allocation was properly bounded and validated against reasonable limits. This represents a common pattern in DNS security where protocol implementations must account for potentially malicious input data, particularly in operations that involve data transfer and storage. The vulnerability also underscores the need for regular security assessments of DNS infrastructure, as the issue remained undetected for an extended period despite being a fundamental resource management problem. Network administrators should have maintained updated threat intelligence on DNS-related vulnerabilities and applied patches promptly to protect their DNS infrastructure from similar attacks. The incident highlighted the broader challenge of securing DNS infrastructure against attacks that exploit implementation flaws rather than cryptographic weaknesses, demonstrating that even well-established DNS protocols can contain security gaps when not properly implemented. This vulnerability serves as a reminder that DNS servers must be hardened against both external attacks and malicious input data, particularly in operations that involve data processing and storage. The fix for CVE-2016-6172 required careful consideration of the trade-off between allowing legitimate large zone transfers and preventing memory exhaustion attacks, ultimately requiring stricter bounds checking on zone transfer data sizes.