CVE-2017-6519 in avahi
Summary
by MITRE
avahi-daemon in Avahi through 0.6.32 and 0.7 inadvertently responds to IPv6 unicast queries with source addresses that are not on-link, which allows remote attackers to cause a denial of service (traffic amplification) and may cause information leakage by obtaining potentially sensitive information from the responding device via port-5353 UDP packets. NOTE: this may overlap CVE-2015-2809.
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Analysis
by VulDB Data Team • 12/04/2025
The vulnerability identified as CVE-2017-6519 affects the avahi-daemon service within the Avahi mDNS/DNS-SD implementation across versions 0.6.32 and 0.7. This issue represents a significant security flaw in network discovery protocols that operates at the application layer of the network stack. The vulnerability specifically manifests when the daemon processes IPv6 unicast queries that originate from source addresses which are not properly on-link, creating an unexpected behavior in the service's response mechanism. This flaw was initially discovered and reported as part of the broader Avahi ecosystem security concerns, with particular attention drawn to its potential for abuse in denial of service scenarios.
The technical implementation of this vulnerability stems from the avahi-daemon's failure to properly validate the network topology of incoming IPv6 unicast queries. When such queries arrive with source addresses that do not correspond to directly connected network segments, the daemon still responds to them with full DNS packet responses containing service information. This behavior violates fundamental network protocol expectations where devices should only respond to queries from on-link addresses to prevent routing loops and unnecessary network traffic. The daemon essentially becomes a traffic amplifier by responding to queries from remote networks, causing network congestion and potentially exposing sensitive information through the DNS responses that contain service discovery data, including hostnames, service types, and other identifying information. This issue is classified under CWE-200 as "Information Exposure" and can be categorized under ATT&CK technique T1046 for "Network Service Scanning' and T1499 for 'Network Denial of Service'.
The operational impact of CVE-2017-6519 extends beyond simple denial of service conditions to include potential information leakage that could compromise network security. Attackers can exploit this vulnerability to perform traffic amplification attacks, where a single query can generate multiple response packets that are significantly larger than the original request, leading to network congestion and resource exhaustion. Additionally, the information leakage aspect means that attackers can gather sensitive data about network services and devices that may not be intended for public exposure. The vulnerability is particularly concerning because it affects the fundamental network discovery mechanism that many systems rely upon for automatic service detection, making it a vector for reconnaissance and further attacks. The overlap with CVE-2015-2809 indicates this represents a persistent weakness in the Avahi implementation's handling of network address validation and response protocols.
Mitigation strategies for CVE-2017-6519 should focus on both immediate patching and network-level defensive measures. Organizations should immediately upgrade to Avahi versions that contain the fix for this vulnerability, which typically involves implementing proper validation of source address topology before responding to queries. Network administrators should also consider implementing firewall rules that restrict access to port 5353 UDP traffic from unauthorized sources, particularly when dealing with IPv6 traffic. The implementation of ingress filtering and proper routing controls can help prevent the exploitation of this vulnerability by ensuring that only legitimate on-link addresses can trigger responses. Additionally, monitoring network traffic for unusual patterns of DNS queries and responses on port 5353 can help detect potential exploitation attempts. Security teams should also review their network discovery protocols and consider disabling unnecessary mDNS services when they are not actively required, reducing the attack surface for such vulnerabilities. The fix typically involves implementing proper IPv6 address validation checks that ensure source addresses are properly on-link before processing and responding to queries, which aligns with standard network security practices and helps prevent similar issues in other network services.