CVE-2023-26249 in Resolver
Summary
by MITRE • 02/21/2023
Knot Resolver before 5.6.0 enables attackers to consume its resources, launching amplification attacks and potentially causing a denial of service. Specifically, a single client query may lead to a hundred TCP connection attempts if a DNS server closes connections without providing a response.
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Analysis
by VulDB Data Team • 11/08/2025
CVE-2023-26249 represents a significant denial of service vulnerability affecting Knot Resolver versions prior to 5.6.0. This vulnerability stems from the resolver's inadequate handling of TCP connection management during DNS query processing, creating an exploitable condition that allows malicious actors to consume excessive system resources. The flaw manifests when a DNS server terminates TCP connections without providing a response to client queries, enabling attackers to craft specific query patterns that trigger cascading connection attempts. The vulnerability operates under CWE-400, specifically addressing unchecked resource consumption, where the system fails to properly validate or limit the number of TCP connection attempts initiated in response to a single client request. This resource exhaustion condition directly maps to the ATT&CK technique T1499.004, which encompasses network denial of service attacks through resource consumption.
The technical implementation of this vulnerability exploits the TCP connection handling logic within Knot Resolver's DNS processing pipeline. When a client sends a query to a vulnerable resolver, the system's response handling mechanism fails to properly manage connection lifecycle states, particularly when connections are closed abruptly without providing a definitive response. This misconfiguration causes the resolver to attempt multiple TCP connection retries for what should be a single query, with each failed connection attempt potentially triggering additional connection attempts from the attacker's perspective. The amplification factor occurs because a single malicious query can generate numerous TCP connection attempts, creating a disproportionate resource consumption ratio that can overwhelm the target system's connection handling capabilities.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it enables attackers to perform resource exhaustion attacks that can effectively render the DNS resolver inoperable. Systems running vulnerable versions of Knot Resolver become susceptible to sustained amplification attacks where minimal input generates massive connection overhead, potentially leading to complete service disruption. The vulnerability affects the resolver's ability to maintain stable operations under normal traffic conditions, as the connection management logic becomes overwhelmed by the cascading connection attempts triggered by the flawed response handling. This creates a persistent threat vector that can be exploited continuously without requiring authentication or advanced privileges.
Mitigation strategies for CVE-2023-26249 focus on upgrading to Knot Resolver version 5.6.0 or later, which includes improved connection handling mechanisms and proper resource limits. Administrators should implement connection rate limiting and TCP connection tracking to prevent the amplification effects from escalating beyond acceptable thresholds. Network-level protections such as firewall rules and rate limiting can help reduce the impact of such attacks by limiting the number of connection attempts that can be initiated from a single source. Additionally, monitoring systems should be configured to detect unusual connection patterns and trigger alerts when connection attempts exceed predefined thresholds, providing early warning of potential exploitation attempts. The vulnerability demonstrates the importance of proper connection state management and resource validation in DNS resolver implementations, aligning with security best practices outlined in RFC 1035 and subsequent DNS security standards.