CVE-2025-40775 is a high-severity vulnerability affecting ISC BIND 9 versions 9.20.0 through 9.20.8 and 9.21.0 through 9.21.7. The issue arises from improper handling of undefined values in the Transaction Signature (TSIG) algorithm field within DNS protocol messages. Specifically, when BIND receives a DNS message containing a TSIG, it always attempts to validate the signature. However, if the TSIG's algorithm field contains an invalid or undefined value, BIND triggers an assertion failure and aborts immediately. This behavior corresponds to CWE-232, which relates to improper handling of undefined values. The vulnerability can be exploited remotely without authentication or user interaction by sending a specially crafted DNS message with an invalid TSIG algorithm value. The consequence is a denial of service (DoS) condition due to the abrupt termination of the BIND process, impacting DNS availability. The CVSS v3.1 base score is 7.5, reflecting a high severity with network attack vector, low attack complexity, no privileges required, no user interaction, and impact limited to availability. No known exploits are currently reported in the wild, and no patches have been linked yet, indicating that remediation may still be pending or in progress. Given BIND's widespread use as a DNS server software, this vulnerability poses a significant risk to DNS infrastructure stability.

For European organizations, the impact of this vulnerability can be substantial. BIND is widely deployed across various sectors including government, telecommunications, financial institutions, and internet service providers throughout Europe. A successful exploitation could lead to denial of service on critical DNS servers, disrupting domain name resolution services essential for internal and external communications, web services, and other network-dependent operations. This disruption could affect business continuity, cause loss of productivity, and potentially impact critical infrastructure services. Since DNS is foundational to internet operations, outages could cascade to affect email delivery, VPN access, cloud services, and other dependent systems. The lack of required authentication and user interaction makes exploitation easier for remote attackers, increasing the risk of widespread attacks. Although no exploits are currently known in the wild, the vulnerability's nature and severity warrant urgent attention to prevent potential future attacks targeting European DNS infrastructure.

Organizations should immediately inventory their DNS infrastructure to identify BIND 9 versions 9.20.0 through 9.20.8 and 9.21.0 through 9.21.7 deployments. Until patches are released, consider implementing network-level mitigations such as filtering or rate-limiting DNS traffic containing TSIG signatures from untrusted sources to reduce exposure. Deploying intrusion detection/prevention systems (IDS/IPS) with signatures tuned to detect malformed TSIG algorithm fields can help identify and block exploit attempts. Administrators should monitor DNS server logs for abnormal crashes or assertion failures indicative of exploitation attempts. Where possible, isolate DNS servers behind firewalls and restrict access to trusted clients only. Once ISC releases patches, prioritize prompt application of updates to affected BIND versions. Additionally, consider implementing DNS redundancy and failover mechanisms to maintain availability during potential attacks. Regularly review and test incident response plans for DNS service disruptions to minimize operational impact.