CVE-2011-5055 in MaraDNS
Summary
by MITRE
MaraDNS 1.3.07.12 and 1.4.08 computes hash values for DNS data without properly restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted queries with the Recursion Desired (RD) bit set. NOTE: this issue exists because of an incomplete fix for CVE-2012-0024.
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Analysis
by VulDB Data Team • 04/24/2019
The vulnerability described in CVE-2011-5055 affects MaraDNS versions 1.3.07.12 and 1.4.08, representing a critical hash collision issue that fundamentally undermines the DNS server's ability to process incoming queries efficiently. This weakness specifically targets the hash table implementation used to store DNS data, where the hash computation algorithm fails to properly restrict conditions that enable attackers to predictably trigger hash collisions. The vulnerability operates through a sophisticated attack vector that leverages the Recursion Desired (RD) bit within DNS query packets, a standard field that indicates whether a resolver should attempt recursive queries. When attackers craft multiple DNS queries with the RD bit set, they can systematically generate hash collisions that force the DNS server to degrade performance significantly.
The technical flaw resides in the insufficient hash function design that fails to implement proper collision resistance mechanisms, creating a scenario where an attacker can predictably construct input data that maps to the same hash bucket. This issue is particularly dangerous because it directly targets the core data structures that DNS servers use to cache and retrieve information efficiently. The incomplete fix for CVE-2012-0024 appears to have introduced a regression where the hash collision protection was weakened rather than strengthened, leaving the system vulnerable to the same class of attacks. This represents a classic example of a security regression where remediation efforts inadvertently introduced new weaknesses, a pattern commonly observed in complex cryptographic and data structure implementations.
The operational impact of this vulnerability manifests as a severe denial of service condition where the DNS server's CPU utilization spikes dramatically, effectively rendering the service unavailable to legitimate users. Attackers can consume system resources at an exponential rate by sending a relatively small number of crafted queries, making this attack particularly efficient and damaging. The resource exhaustion occurs because hash collisions force the server to handle queries in a linear fashion rather than the expected logarithmic time complexity, causing the system to spend enormous amounts of CPU cycles processing what should be simple hash table operations. This vulnerability particularly affects DNS servers that handle high volumes of queries and are deployed in environments where service availability is critical, such as internet service providers and enterprise networks.
Mitigation strategies for this vulnerability require immediate implementation of proper hash collision resistance measures, including the adoption of more robust hash functions that are resistant to predictable collision attacks. The recommended approach involves implementing a hash function with better distribution properties and ensuring that the hash table maintains proper load factors to prevent excessive collisions. System administrators should consider upgrading to patched versions of MaraDNS that properly address the hash collision issue, while also implementing rate limiting and query filtering mechanisms to prevent abuse of the affected functionality. The fix should align with industry standards such as those recommended in CWE-327 for cryptographic weaknesses and ATT&CK techniques related to resource exhaustion attacks. Additionally, network-level protections such as firewall rules that limit the rate of DNS queries or filtering based on query patterns can provide additional layers of defense against this specific attack vector, ensuring that the system maintains availability even when under attack.