CVE-2026-4893 in dnsmasq
Summary
by MITRE • 05/11/2026
An information disclosure vulnerability in dnsmasq allows remote attackers to bypass source checks via a crafted DNS packet with RFC 7871 client subnet information.
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Analysis
by VulDB Data Team • 07/03/2026
This vulnerability represents a significant information disclosure flaw in dnsmasq, a widely deployed DNS forwarder and DHCP server used in numerous network environments including enterprise networks, home routers, and cloud deployments. The issue stems from insufficient validation of DNS packets containing RFC 7871 client subnet information, which allows remote attackers to bypass source address checks that are typically enforced to prevent unauthorized access to network resources. The vulnerability specifically affects the handling of EDNS Client Subnet (ECS) option data within DNS responses, where the server fails to properly validate the source address information provided by clients.
The technical implementation of this flaw occurs when dnsmasq processes DNS queries that include the ECS option as defined in RFC 7871, which is designed to provide location information to DNS servers to enable more efficient caching and routing decisions. When attackers craft malicious DNS packets with spoofed or malformed ECS data, the server's inadequate validation mechanisms allow these packets to be processed without proper source address verification, potentially exposing internal network topology information and enabling attackers to infer network structure and device placement. This vulnerability falls under CWE-200, which addresses information disclosure issues, and specifically relates to improper input validation of network protocol data.
The operational impact of this vulnerability extends beyond simple information disclosure, as it can enable attackers to perform reconnaissance activities that would otherwise be blocked by proper source address validation. Network administrators may inadvertently expose internal subnet ranges, device configurations, and network mapping information that could facilitate subsequent attacks. The vulnerability is particularly concerning in environments where dnsmasq serves as a central DNS resolver, as it could allow attackers to map internal network structures and identify potential targets for further exploitation. This weakness can be leveraged as part of broader attack campaigns, potentially enabling lateral movement and privilege escalation activities within compromised networks.
Mitigation strategies should focus on implementing proper input validation for DNS packets containing ECS data, ensuring that source address information is properly authenticated and verified before being processed. Network administrators should update dnsmasq to versions that address this specific vulnerability, typically through patches released by the maintainers or through operating system vendors. Additional protective measures include implementing network segmentation, deploying proper access controls, and monitoring DNS traffic for anomalous ECS data patterns. From an ATT&CK framework perspective, this vulnerability relates to techniques such as network sniffing and reconnaissance activities that can be used to gather intelligence about target networks. Organizations should also consider implementing DNS query logging and analysis to detect unusual patterns that may indicate exploitation attempts, while ensuring that DNS servers are properly configured to limit the exposure of internal network information to external entities.