CVE-2026-21964 is a vulnerability identified in Oracle MySQL Server's Thread Pooling component affecting multiple supported versions including 8.0.0 to 8.0.44, 8.4.0 to 8.4.7, and 9.0.0 to 9.5.0. The flaw allows an attacker with high-level privileges and network access via multiple protocols to cause the MySQL Server to hang or crash repeatedly, resulting in a denial of service (DoS) condition. The vulnerability does not compromise confidentiality or integrity but impacts availability significantly. The CVSS 3.1 base score is 4.9, reflecting a medium severity primarily due to the requirement of high privileges (PR:H), network attack vector (AV:N), low attack complexity (AC:L), no user interaction (UI:N), and the impact limited to availability (A:H). The vulnerability is exploitable remotely without user interaction but requires the attacker to already have high privileges on the system, which limits the attack surface. No known exploits have been reported in the wild as of the publication date. The vulnerability's root cause lies in how the server handles thread pooling, which can be manipulated to cause hangs or crashes. This can disrupt database availability, affecting applications and services dependent on MySQL. Oracle is expected to release patches to address this issue, but until then, affected systems remain vulnerable.

For European organizations, the primary impact of CVE-2026-21964 is the potential for denial of service against MySQL Server instances, which can disrupt critical business applications, web services, and internal systems relying on MySQL databases. This can lead to operational downtime, loss of productivity, and potential financial losses. Since the vulnerability requires high privileges, it is more likely to be exploited by insiders or attackers who have already compromised internal systems, making it a significant concern for organizations with complex network environments and multiple administrative users. Sectors such as finance, healthcare, telecommunications, and government agencies in Europe that rely heavily on MySQL for data storage and processing are at risk. The disruption of database services could also impact compliance with data availability regulations under GDPR if service outages affect customer-facing systems. Additionally, repeated crashes could complicate incident response and recovery efforts, increasing operational costs.

To mitigate CVE-2026-21964, European organizations should prioritize the following actions: 1) Apply official patches from Oracle as soon as they become available to remediate the vulnerability in affected MySQL Server versions. 2) Restrict network access to MySQL servers by implementing strict firewall rules and network segmentation to limit exposure to trusted hosts and administrative personnel only. 3) Enforce the principle of least privilege by reviewing and minimizing the number of users with high privileges on MySQL servers. 4) Monitor MySQL server logs and system behavior for signs of unusual hangs or crashes that could indicate exploitation attempts. 5) Implement robust authentication and access controls to prevent unauthorized privilege escalation. 6) Consider deploying failover and high availability configurations to reduce downtime impact in case of service disruption. 7) Conduct regular security audits and vulnerability assessments focusing on database infrastructure. 8) Educate system administrators about the risks and signs of exploitation related to this vulnerability to enhance detection and response capabilities.