CVE-2025-21566 is a vulnerability identified in Oracle MySQL Server, affecting versions 9.1.0 and earlier. The flaw resides in the Server Optimizer component, which is responsible for query optimization and execution planning. This vulnerability allows an attacker with low privileges and network access through multiple protocols to cause the MySQL Server to hang or crash repeatedly, resulting in a complete denial of service (DoS). The attack vector is network-based, requiring no user interaction, and the attacker needs only low privileges, which significantly lowers the barrier to exploitation. The vulnerability does not compromise confidentiality or integrity but severely impacts availability, potentially disrupting database services. The CVSS 3.1 base score is 6.5, reflecting a medium severity with the vector AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H, indicating network attack vector, low attack complexity, low privileges required, no user interaction, unchanged scope, and high impact on availability. No known exploits have been reported in the wild, and no official patches have been released at the time of this report. The CWE-732 classification suggests improper permissions or access control issues that allow the attacker to trigger the DoS condition. This vulnerability is critical for environments where MySQL Server uptime is essential, as repeated crashes can cause significant service interruptions and potential cascading failures in dependent applications.

For European organizations, the primary impact of CVE-2025-21566 is the potential disruption of critical database services due to denial of service conditions. Organizations relying heavily on MySQL Server for transactional systems, web applications, or data analytics may experience service outages, leading to operational downtime, loss of productivity, and potential financial losses. The availability impact could also affect customer-facing services, damaging reputation and trust. Since the vulnerability requires only low privileges and network access, attackers could exploit it from within the network or through exposed MySQL services, increasing the risk in environments with insufficient network segmentation or firewall protections. The lack of confidentiality or integrity impact reduces the risk of data breaches but does not mitigate the operational risks. European sectors such as finance, healthcare, e-commerce, and public administration, which often use MySQL for backend databases, are particularly vulnerable to service interruptions. Additionally, the absence of known exploits currently provides a window for proactive mitigation before widespread exploitation occurs.

European organizations should implement the following specific mitigations: 1) Restrict network access to MySQL Server instances by enforcing strict firewall rules and network segmentation, allowing only trusted hosts and applications to connect. 2) Monitor MySQL Server logs and system metrics for unusual crashes or hangs indicative of exploitation attempts. 3) Apply the principle of least privilege by ensuring MySQL user accounts have minimal permissions necessary for their function, reducing the risk posed by low privileged attackers. 4) Disable or restrict unused MySQL network protocols and services to reduce the attack surface. 5) Stay updated with Oracle's security advisories and apply patches promptly once they are released for this vulnerability. 6) Consider deploying intrusion detection/prevention systems (IDS/IPS) tuned to detect anomalous MySQL traffic patterns. 7) Conduct regular backups and have a tested recovery plan to minimize downtime impact in case of successful DoS attacks. 8) Evaluate the use of MySQL alternatives or containerization to isolate database services and limit blast radius.