CVE-2019-19331 in knot-resolver
Summary
by MITRE
knot-resolver before version 4.3.0 is vulnerable to denial of service through high CPU utilization. DNS replies with very many resource records might be processed very inefficiently, in extreme cases taking even several CPU seconds for each such uncached message. For example, a few thousand A records can be squashed into one DNS message (limit is 64kB).
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Analysis
by VulDB Data Team • 10/02/2024
The vulnerability identified as CVE-2019-19331 affects knot-resolver versions prior to 4.3.0, presenting a significant denial of service risk through excessive CPU utilization. This flaw manifests when processing DNS replies containing an exceptionally high number of resource records, creating a scenario where the resolver's performance degrades dramatically. The issue stems from the inefficient handling of DNS messages that contain thousands of records compressed into a single response, with the system consuming disproportionate computational resources for each such message. When dealing with DNS replies containing several thousand A records that are compressed into a single 64kB message, the resolver exhibits extreme CPU consumption patterns, potentially requiring several seconds of processing time per uncached message. This performance degradation represents a critical operational risk for systems relying on knot-resolver for DNS resolution, as it can effectively render the service unavailable to legitimate users while consuming excessive computational resources.
The technical root cause of this vulnerability lies in the inefficient algorithmic processing of DNS resource records within the knot-resolver implementation. The flaw demonstrates characteristics consistent with CWE-778, which addresses insufficient logging of resource usage, and more specifically relates to CWE-400, concerning unspecified resource exhaustion. The system's inability to efficiently process DNS messages with large numbers of resource records creates a resource exhaustion scenario where CPU cycles are consumed disproportionately to the actual data size. This processing inefficiency becomes particularly problematic when dealing with DNS responses that have been intentionally crafted to maximize resource record density, as the resolver's internal parsing and caching mechanisms become overwhelmed by the volume of records that must be processed sequentially. The vulnerability is exacerbated by the fact that DNS responses can legitimately contain thousands of records, particularly in scenarios involving large DNS zones or when dealing with certain types of DNS amplification attacks.
The operational impact of this vulnerability extends beyond simple performance degradation to encompass complete service unavailability for legitimate users. When a malicious actor or even a misconfigured DNS server sends a response containing thousands of resource records, the knot-resolver system becomes effectively paralyzed, consuming CPU resources at an unsustainable rate. This creates a denial of service condition where the resolver cannot process legitimate DNS queries while simultaneously being overwhelmed by the resource-intensive processing of malicious responses. The time complexity of the processing algorithm increases exponentially with the number of resource records, making it particularly susceptible to exploitation through carefully crafted DNS responses. Organizations using knot-resolver versions before 4.3.0 face significant risk of service disruption, especially in environments where DNS resolution is critical for application availability and network operations. The vulnerability also aligns with ATT&CK technique T1499.004, which covers network disruption through resource exhaustion, as the attack vector specifically targets the CPU resource consumption patterns of the DNS resolver.
Mitigation strategies for CVE-2019-19331 primarily focus on upgrading to knot-resolver version 4.3.0 or later, which contains the necessary patches to address the inefficient resource record processing. Organizations should implement immediate version upgrades across all systems utilizing affected knot-resolver installations, as this represents the most direct and effective solution to the vulnerability. Additionally, implementing DNS response size limits and rate limiting mechanisms can provide additional protection layers, though these measures are considered temporary workarounds rather than permanent solutions. Network administrators should also consider implementing DNS filtering rules that can identify and block DNS responses containing unusually large numbers of resource records, particularly those exceeding typical DNS zone sizes. The patch for this vulnerability specifically addresses the inefficient processing algorithm by introducing more robust handling of large DNS responses and implementing proper resource limits to prevent CPU exhaustion during DNS message processing. Security monitoring should be enhanced to detect unusual CPU utilization patterns that may indicate exploitation attempts, and system administrators should establish baseline performance metrics to quickly identify when the vulnerability is being actively exploited.