CVE-2026-21948: MySQL Server DOS Vulnerability

CVE-2026-21948 Overview

CVE-2026-21948 is a Denial of Service vulnerability in the MySQL Server product of Oracle MySQL, specifically affecting the Server: Optimizer component. This vulnerability allows a high-privileged attacker with network access to cause a complete denial of service condition, resulting in a hang or frequently repeatable crash of the MySQL Server.

Critical Impact

Successful exploitation enables attackers with administrative privileges to completely disrupt MySQL Server availability through repeated crashes or system hangs, impacting database-dependent applications and services.

Affected Products

• MySQL Server versions 8.0.0 through 8.0.44
• MySQL Server versions 8.4.0 through 8.4.7
• MySQL Server versions 9.0.0 through 9.5.0

Discovery Timeline

• 2026-01-20 - CVE-2026-21948 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2026-21948

Vulnerability Analysis

This vulnerability resides in the Server: Optimizer component of MySQL Server. The Optimizer is responsible for query execution planning and determining the most efficient method to execute SQL statements. The flaw is classified under CWE-400 (Uncontrolled Resource Consumption), indicating that the vulnerability involves improper handling of resource consumption that can be triggered through specially crafted queries or optimizer operations.

The vulnerability is easily exploitable, requiring no user interaction, though it does require the attacker to possess high privileges (such as administrative access) on the MySQL Server. When exploited, the attack results in availability impacts only—there is no compromise of data confidentiality or integrity. The attack can be executed remotely via multiple network protocols supported by MySQL Server.

Root Cause

The root cause stems from uncontrolled resource consumption (CWE-400) within the MySQL Server Optimizer component. When processing certain query optimization operations, the Optimizer fails to properly manage resource utilization, allowing an authenticated administrative user to trigger conditions that exhaust server resources or cause the server to enter an unrecoverable state, resulting in a denial of service.

Attack Vector

The attack is network-based and can be executed via multiple protocols that MySQL Server supports (such as the MySQL protocol on port 3306). An attacker must first authenticate with high-privileged credentials (e.g., a DBA or administrative account). Once authenticated, the attacker can submit malicious queries or operations that exploit the vulnerability in the Optimizer component to cause the server to hang or crash repeatedly.

The vulnerability mechanism involves sending specially crafted queries that trigger resource exhaustion in the query optimization phase. Due to the nature of the Optimizer component, these operations may involve complex query patterns that cause the optimizer to consume excessive CPU or memory resources, or enter infinite processing loops.

Detection Methods for CVE-2026-21948

Indicators of Compromise

• Unexpected MySQL Server crashes or service restarts without apparent cause
• MySQL error logs showing optimizer-related errors or abnormal query terminations
• Unusual query patterns from administrative accounts targeting complex optimization scenarios
• Elevated resource consumption (CPU/memory) during query optimization phases

Detection Strategies

• Monitor MySQL error logs for repeated crash events or optimizer-related exceptions
• Implement audit logging for all administrative account activities and query executions
• Set up alerts for abnormal database service restarts or availability interruptions
• Review query logs for suspicious patterns that may indicate exploitation attempts

Monitoring Recommendations

• Enable MySQL Enterprise Audit or equivalent logging for privileged operations
• Configure database activity monitoring (DAM) solutions to track administrative queries
• Implement real-time alerting on MySQL service availability and crash events
• Monitor system resources (CPU, memory) for anomalies during database operations

How to Mitigate CVE-2026-21948

Immediate Actions Required

• Apply the latest Oracle Critical Patch Update (CPU) for MySQL Server
• Review and restrict administrative account access to only essential personnel
• Implement network segmentation to limit access to MySQL Server ports
• Enable enhanced logging to detect potential exploitation attempts

Patch Information

Oracle has addressed this vulnerability in the January 2026 Critical Patch Update. Organizations should upgrade MySQL Server to patched versions as documented in the Oracle Security Alert January 2026. Consult the Oracle security advisory for specific version information and upgrade paths.

Workarounds

• Restrict network access to MySQL Server to trusted hosts only using firewall rules
• Review and minimize the number of accounts with high-privilege access
• Implement connection rate limiting to reduce the impact of potential attacks
• Consider using MySQL Proxy or connection poolers with query filtering capabilities

# Example: Restrict MySQL access to trusted networks
# Add to MySQL configuration (my.cnf)
[mysqld]
bind-address = 127.0.0.1
# Or use firewall rules to restrict access
# iptables -A INPUT -p tcp --dport 3306 -s trusted_ip_range -j ACCEPT
# iptables -A INPUT -p tcp --dport 3306 -j DROP