CVE-2021-2016 in MySQL Server
Summary
by MITRE • 01/20/2021
Vulnerability in the MySQL Server product of Oracle MySQL (component: Server: Optimizer). Supported versions that are affected are 8.0.19 and prior. Easily exploitable vulnerability allows high privileged attacker with network access via multiple protocols to compromise MySQL Server. Successful attacks of this vulnerability can result in unauthorized ability to cause a hang or frequently repeatable crash (complete DOS) of MySQL Server. CVSS 3.1 Base Score 4.9 (Availability impacts). CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H).
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Analysis
by VulDB Data Team • 08/10/2025
The vulnerability identified as CVE-2021-2016 represents a critical availability threat within Oracle MySQL Server's optimizer component, affecting versions 8.0.19 and earlier. This flaw exists within the server's query optimization logic where specific malformed queries can trigger unexpected behavior in the database engine's execution path. The vulnerability's classification as easily exploitable indicates that sophisticated attack techniques are not required, making it particularly dangerous in production environments where database availability is paramount. The attack vector requires only network access through multiple protocols, suggesting that the vulnerability can be leveraged from external networks without requiring physical access or complex reconnaissance phases.
The technical nature of this vulnerability stems from improper handling of certain query optimization scenarios within the MySQL Server's internal processing mechanisms. When specific query patterns are processed through the optimizer, the system enters into a state where it cannot properly manage memory allocation or execution flow, leading to system instability. The flaw manifests as a complete denial of service condition where the MySQL Server process becomes unresponsive or crashes repeatedly, effectively rendering the database service unavailable to legitimate users and applications. This behavior aligns with CWE-121, which describes heap-based buffer overflow conditions, though the specific manifestation here involves memory management issues within the query optimizer rather than direct buffer manipulation.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect business continuity and data availability. Organizations relying on MySQL for critical applications face significant risk of unplanned downtime when this vulnerability is exploited, particularly in environments where database servers are not regularly patched or monitored for security updates. The high privilege requirement for exploitation suggests that attackers must already possess elevated access rights to the database system, but this limitation does not mitigate the severity of the impact once achieved. The vulnerability's CVSS score of 4.9 indicates a moderate to high severity threat that requires immediate attention from security teams responsible for database infrastructure management.
Mitigation strategies for CVE-2021-2016 primarily focus on applying the official Oracle security patches and updates that address the specific optimizer logic flaw. Organizations should implement a comprehensive patch management process that includes testing new updates in non-production environments before deployment to prevent potential compatibility issues with existing applications. Network segmentation and access controls should be reviewed to limit the attack surface, ensuring that only authorized systems can establish connections to MySQL servers. Additionally, monitoring systems should be enhanced to detect unusual patterns of database connection failures or process restarts that might indicate exploitation attempts. The vulnerability's characteristics align with ATT&CK technique T1499.004, which covers network denial of service attacks targeting database services, making it essential for security teams to implement both preventive measures and detection capabilities. Regular vulnerability assessments and penetration testing should include checks for similar optimizer-related flaws that might exist in other database systems or components within the organization's infrastructure.