CVE-2026-42004 in DNSdist
Summary
by MITRE • 06/25/2026
An attacker can send a crafted EDNS OPT record that will be ignored by DNSdist’s filtering rules, but will be rewritten as a valid OPT record when EDNS Client Subnet is inserted, causing the backend to see the EDNS option(s) that DNSdist did not filter.
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Analysis
by VulDB Data Team • 06/25/2026
This vulnerability represents a sophisticated bypass mechanism within DNSdist's filtering architecture that exploits the interaction between EDNS OPT records and EDNS Client Subnet functionality. The flaw occurs when DNSdist receives a maliciously crafted EDNS OPT record that appears to pass initial filtering checks due to its structure or content. However, during the processing pipeline where EDNS Client Subnet information is inserted into the query, the originally ignored record gets rewritten as a legitimate OPT record that reaches backend servers. This creates a critical gap in the security model where filtered traffic can bypass detection and reach downstream infrastructure.
The technical execution of this vulnerability leverages the specific ordering and transformation behavior within DNSdist's processing stack. When an attacker crafts an EDNS OPT record with malicious options that would normally be filtered out by DNSdist's rule set, these records are initially accepted due to their compliance with basic DNS protocol requirements. The vulnerability becomes apparent when the system processes EDNS Client Subnet insertion, which rewrites and restructures the original OPT record. This transformation creates a new valid OPT record that contains the malicious options previously filtered by DNSdist, allowing them to reach backend servers unimpeded.
The operational impact of this vulnerability extends beyond simple bypass of filtering rules to potentially enable advanced attack vectors targeting backend DNS infrastructure. Backend servers receive OPT records with options that were intended to be filtered but are now visible and actionable, creating opportunities for attacks such as cache poisoning, resource exhaustion through malformed options, or exploitation of backend server vulnerabilities that may not be present in the front-end filtering layer. This vulnerability essentially creates a tunnel through which malicious DNS options can traverse the security boundary established by DNSdist's filtering capabilities.
Security implications of this vulnerability align with CWE-284 access control weaknesses and specifically relate to improper privilege management in network protocol processing. The attack pattern corresponds to techniques described in MITRE ATT&CK framework under T1071.004 Application Layer Protocol: DNS, where attackers manipulate DNS records to bypass security controls. The vulnerability demonstrates a classic case of protocol processing inconsistency where different components of the system handle identical data differently, creating exploitable gaps in security architecture. Organizations using DNSdist may experience unauthorized access to backend services through this mechanism, potentially leading to complete DNS infrastructure compromise if not addressed promptly.
Mitigation strategies must address both immediate operational fixes and architectural improvements to prevent similar vulnerabilities. The most direct approach involves implementing additional validation layers that ensure OPT record processing consistency between filtering rules and transformation operations. Organizations should consider updating DNSdist configurations to disable EDNS Client Subnet functionality when it creates security gaps, or implement more robust validation mechanisms that prevent the rewriting of filtered records into valid formats. Network segmentation and additional monitoring of backend server DNS traffic can provide detection capabilities for this specific attack pattern while architectural fixes are implemented. The vulnerability highlights the importance of thorough testing across all processing pipeline stages to ensure security controls remain effective throughout the complete data transformation process.