CVE-2025-11411 affects NLnet Labs Unbound DNS resolver versions up to 1.24.0 and involves CWE-349, which is the acceptance of extraneous untrusted data alongside trusted data. The vulnerability allows attackers to inject malicious NS RRSets (Name Server Resource Record Sets) into DNS responses, specifically in the authority section, which Unbound uses to update its delegation information for DNS zones. Normally, these NS RRSets are legitimate data used to keep the resolver's knowledge of zone name servers current. However, due to insufficient validation, Unbound accepts unsolicited NS RRSets that complement positive DNS replies, trusting them as in-zone data. Attackers can exploit this by spoofing DNS packets or leveraging fragmentation attacks to insert malicious NS RRSets and possibly their corresponding address records. This causes Unbound to update its cache with attacker-controlled delegation data, effectively hijacking domain resolution. The impact is a poisoned DNS cache that can redirect users to malicious sites or intercept traffic. The vulnerability does not require authentication or user interaction, making it easier to exploit remotely. The fix implemented in Unbound 1.24.1 involves scrubbing unsolicited NS RRSets and their address records from DNS replies, preventing the poisoning effect. No known exploits are reported in the wild as of now, but the vulnerability poses a significant risk to DNS integrity and availability.

For European organizations, this vulnerability threatens the integrity and availability of DNS resolution services, which are foundational to network operations and security. Successful exploitation can lead to domain hijacking, enabling attackers to redirect traffic to malicious servers, intercept sensitive communications, or disrupt services. This can affect enterprises, government agencies, and critical infrastructure operators relying on Unbound as their DNS resolver. The attack could facilitate phishing, data exfiltration, or denial of service by manipulating DNS delegation data. Given the widespread use of Unbound in Europe, especially in academic, governmental, and ISP environments, the potential impact is significant. Disruption of DNS services can also affect compliance with EU data protection regulations if personal data is intercepted or redirected. The medium CVSS score reflects moderate ease of exploitation without authentication but with high impact on integrity and availability.

European organizations should immediately upgrade all Unbound DNS resolvers to version 1.24.1 or later, which includes the fix to scrub unsolicited NS RRSets. Network administrators should audit DNS resolver configurations to ensure they do not accept or cache unsolicited delegation data. Implementing DNSSEC validation can provide an additional layer of defense by verifying the authenticity of DNS data, reducing the risk of cache poisoning. Monitoring DNS resolver logs for unexpected delegation changes or unusual NS RRSet updates can help detect exploitation attempts. Network-level protections such as ingress filtering and anti-spoofing measures should be enforced to prevent attackers from sending spoofed DNS packets. Fragmentation attacks can be mitigated by configuring resolvers and firewalls to handle fragmented packets securely or by disabling IP fragmentation where feasible. Organizations should also consider segmenting DNS infrastructure and applying strict access controls to limit exposure. Finally, maintaining an incident response plan for DNS-related incidents will help quickly address any exploitation.