CVE-2025-61430 is a vulnerability identified in Simple DNS Plus version 9, involving improper handling of DNS over TCP connections. The core issue stems from the server's processing of the TCP length prefix that precedes DNS payloads. When this length prefix is malformed—either smaller or differing from the actual packet length—or due to concurrency and buffering race conditions, the server may erroneously return DNS request payloads originating from other clients. This behavior results in unintended information leakage, allowing a remote attacker with the ability to send DNS queries to the server to observe queries made by other clients. The vulnerability is rooted in CWE-350 (Improper Handling of Exceptional Conditions), indicating a failure to correctly manage unexpected or malformed input data. The CVSS v3.1 base score is 6.5 (medium severity), reflecting that the attack vector is network-based with low attack complexity, no privileges or user interaction required, and impacts confidentiality and integrity but not availability. No authentication is needed, making the attack feasible for any entity able to query the DNS server over TCP. The vulnerability does not currently have known exploits in the wild, and no official patches have been released at the time of publication. This flaw could be exploited to compromise the privacy of DNS queries, potentially exposing sensitive domain lookups and user behavior patterns.

For European organizations, this vulnerability poses a significant privacy risk as DNS queries often reveal sensitive information about internal network activities, user behavior, and accessed resources. Leakage of DNS queries could facilitate further reconnaissance by attackers, enabling targeted phishing, malware distribution, or lateral movement within networks. Organizations in sectors such as finance, healthcare, government, and critical infrastructure are particularly vulnerable due to the sensitivity of their DNS traffic. The integrity impact, while limited, could allow attackers to infer or manipulate query responses indirectly by observing query patterns. Although availability is not affected, the confidentiality breach could lead to regulatory compliance issues under GDPR and other data protection laws, potentially resulting in legal and financial penalties. The medium severity rating reflects that while the vulnerability is not immediately catastrophic, it undermines trust in DNS infrastructure and could be leveraged as part of a broader attack chain.

European organizations should implement several targeted mitigations beyond generic advice: 1) Restrict DNS over TCP query access to trusted clients and networks using firewall rules and access control lists to minimize exposure. 2) Monitor DNS server logs and network traffic for anomalies indicative of malformed TCP length prefixes or unusual query patterns that may signal exploitation attempts. 3) Employ network segmentation to isolate DNS servers from untrusted networks and reduce the attack surface. 4) Use DNS encryption technologies such as DNS over HTTPS (DoH) or DNS over TLS (DoT) where possible to protect query confidentiality and integrity. 5) Engage with the Simple DNS Plus vendor to obtain timely patches or workarounds once available and prioritize their deployment. 6) Conduct regular security assessments and penetration testing focusing on DNS infrastructure to detect similar concurrency or buffering issues. 7) Educate network administrators about this vulnerability to ensure prompt detection and response.