In NLnet Labs Unbound 1.18.0 up to and including 1.25.1, when Unbound listens on a 'proxy-protocol-port' interface with 'answer-cookie: yes', the RFC 9018 server-cookie SipHash is computed over the proxy's wire address instead of the PROXYv2-declared client. One server cookie obtained through a given proxy node therefore validates for every PROXYv2-declared source behind that node. On a UDP+proxy-protocol front, an off-path attacker can harvest one cookie with a single legitimate query, then replay it under any spoofed source and pass DNS Cookie checks that were deployed to defeat this in the first place.
This vulnerability carries a LOW severity rating with a CVSS v3.1 score of 3.7, indicating it can be exploited remotely over the network but requires specific conditions to be met without requiring user interaction and does not require pre-existing privileges . The vulnerability impacts limited integrity, for affected systems. Impacting 1 product from nlnetlabs organizations running these solutions should prioritize assessment and patching.
Reported in 2026, this vulnerability emerged during an era marked by increased sophistication in supply chain attacks, cloud infrastructure vulnerabilities, and software-as-a-service (SaaS) security challenges. Security practices during this period emphasized zero-trust architectures, container security, and API protection.
2026-07-22T14:17:21.177
2026-07-24T14:24:03.907
Analyzed
CVSSv3.1: 3.7 (LOW)
SecUtils normalizes and enriches National Vulnerability Database (NVD) records by standardizing vendor and product identifiers, aggregating vulnerability metadata from both NVD and MITRE sources, and providing structured context for security teams. For nlnetlabs's affected products, we extract Common Platform Enumeration (CPE) data, Common Weakness Enumeration (CWE) classifications, CVSS severity metrics, and reference data to enable rapid vulnerability prioritization and asset correlation. This record contains no exploit code, proof-of-concept instructions, or attack methodologies—only defensive intelligence necessary for patch management, risk assessment, and security operations.