CVE-2022-40188 in Resolver
Summary
by MITRE • 09/23/2022
Knot Resolver before 5.5.3 allows remote attackers to cause a denial of service (CPU consumption) because of algorithmic complexity. During an attack, an authoritative server must return large NS sets or address sets.
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Analysis
by VulDB Data Team • 05/28/2025
Knot Resolver version 5.5.2 and earlier contains a critical vulnerability that enables remote attackers to execute denial of service attacks through excessive cpu consumption. This flaw stems from inadequate handling of algorithmic complexity during DNS query processing, specifically when authoritative servers return large NS sets or address sets. The vulnerability represents a classic example of a computational complexity attack where malicious inputs can cause disproportionate resource consumption. According to CWE-400, this falls under algorithmic complexity vulnerabilities that can lead to resource exhaustion attacks. The attack vector involves sending specially crafted DNS responses containing oversized resource records that trigger inefficient processing algorithms within the resolver.
The technical implementation of this vulnerability exploits the way Knot Resolver handles large DNS response sets during iterative resolution processes. When the resolver encounters authoritative responses containing numerous NS records or address records, it processes these sets using algorithms with suboptimal time complexity. This creates a scenario where the CPU consumption grows exponentially with the size of the response data rather than maintaining linear or logarithmic scaling. The flaw is particularly dangerous because it can be triggered through normal DNS operations without requiring authentication or privileged access. Attackers can craft malicious DNS responses that cause the resolver to spend excessive computational resources during record processing, effectively consuming available CPU cycles and potentially rendering the resolver unresponsive to legitimate queries. This vulnerability directly maps to ATT&CK technique T1499.004 which describes resource exhaustion attacks targeting network services.
The operational impact of CVE-2022-40188 extends beyond simple service disruption to potentially compromise entire DNS infrastructure reliability. Organizations relying on Knot Resolver for DNS resolution face significant risk of sustained denial of service conditions that can affect multiple services simultaneously. The vulnerability affects both recursive and authoritative resolution functions within the resolver, making it particularly dangerous for DNS infrastructure providers. Network administrators may observe gradual performance degradation followed by complete service unavailability as the CPU consumption reaches critical thresholds. The attack can be executed through various means including compromised authoritative servers, man-in-the-middle attacks, or by leveraging existing DNS infrastructure weaknesses. Organizations using older versions of Knot Resolver are particularly vulnerable as the fix implemented in version 5.5.3 includes algorithmic complexity mitigations and improved resource handling for large DNS response sets. The vulnerability highlights the importance of proper input validation and algorithmic complexity considerations in DNS resolution software, aligning with security best practices outlined in RFC 1035 and subsequent DNS security standards.