CVE-2025-21501: Oracle MySQL Server DoS Vulnerability

CVE-2025-21501 Overview

CVE-2025-21501 is a vulnerability in the MySQL Server product of Oracle MySQL, specifically affecting the Server: Optimizer component. This flaw allows a low-privileged attacker with network access to exploit the MySQL Server through multiple protocols, resulting in a complete denial of service condition. The vulnerability enables attackers to cause the MySQL Server to hang or experience frequently repeatable crashes, severely impacting database availability for organizations relying on MySQL infrastructure.

Critical Impact

Successful exploitation allows attackers to completely disrupt MySQL Server availability, causing system hangs or repeatable crashes that can take down critical database services.

Affected Products

• Oracle MySQL Server 8.0.40 and prior versions
• Oracle MySQL Server 8.4.3 and prior versions
• Oracle MySQL Server 9.1.0 and prior versions

Discovery Timeline

• 2025-01-21 - CVE-2025-21501 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-21501

Vulnerability Analysis

This vulnerability resides in the MySQL Server Optimizer component, which is responsible for query optimization and execution planning. The flaw is classified under CWE-770 (Allocation of Resources Without Limits or Throttling), indicating that the optimizer fails to properly limit resource allocation during certain operations. When exploited, this allows an attacker with minimal privileges to consume server resources in a way that causes the database engine to become unresponsive or crash entirely.

The attack can be initiated remotely via network access using multiple protocols supported by MySQL. The vulnerability is considered easily exploitable, requiring only low-privilege access to the MySQL Server—such as a basic database user account. No user interaction is required for exploitation, making it particularly dangerous in multi-tenant database environments where users with limited permissions could weaponize this flaw.

Root Cause

The root cause stems from improper resource allocation controls within the MySQL Server Optimizer component (CWE-770). The optimizer fails to implement adequate limits or throttling mechanisms when processing certain query patterns or optimization requests. This allows an attacker to craft queries that trigger unbounded resource consumption, exhausting server resources and leading to denial of service conditions. The lack of proper bounds checking in the resource allocation logic enables attackers to force the server into a hung state or trigger repeatable crashes.

Attack Vector

The attack vector is network-based, allowing remote exploitation through multiple protocols supported by MySQL Server. An attacker requires only low-privilege access—such as a standard database user account with basic query execution permissions—to exploit this vulnerability. The attack does not require any user interaction, and the scope is unchanged (confined to the MySQL Server itself).

The exploitation mechanism involves sending specially crafted requests to the MySQL Server that trigger the vulnerable code path in the Optimizer component. These requests cause the server to allocate resources without proper limits, leading to resource exhaustion and denial of service. Given the ease of exploitation and network accessibility, organizations should prioritize patching systems exposed to untrusted networks or users.

Detection Methods for CVE-2025-21501

Indicators of Compromise

• Unexpected MySQL Server crashes or hangs occurring repeatedly without clear cause
• Abnormally high CPU or memory utilization by the MySQL Server process during query processing
• Error logs showing optimizer-related failures or resource exhaustion messages
• Unusual query patterns from low-privilege user accounts targeting complex optimization scenarios

Detection Strategies

• Monitor MySQL error logs for optimizer component errors, resource exhaustion warnings, or unexpected server restarts
• Implement query monitoring to detect abnormal or malformed queries targeting the optimizer
• Set up alerts for MySQL Server process crashes or unresponsive states in production environments
• Use database activity monitoring (DAM) solutions to track query execution patterns from low-privilege accounts

Monitoring Recommendations

• Enable MySQL slow query log and general query log to capture potentially malicious query activity
• Configure system-level monitoring for MySQL process health, including CPU, memory, and thread utilization
• Implement network traffic analysis to detect unusual connection patterns to MySQL Server ports
• Set up automated alerting for MySQL Server availability drops or service interruptions

How to Mitigate CVE-2025-21501

Immediate Actions Required

• Apply the Oracle Critical Patch Update from January 2025 to all affected MySQL Server installations
• Audit database user accounts and revoke unnecessary privileges, especially for accounts that don't require optimizer access
• Implement network segmentation to limit MySQL Server exposure to untrusted networks
• Enable query timeout settings to prevent long-running queries from consuming excessive resources

Patch Information

Oracle has addressed this vulnerability in the January 2025 Critical Patch Update. Administrators should upgrade to the following patched versions:

• MySQL Server 8.0.41 or later (for 8.0.x branch)
• MySQL Server 8.4.4 or later (for 8.4.x branch)
• MySQL Server 9.1.1 or later (for 9.x branch)

The official security advisory is available at Oracle Critical Patch Update - January 2025. Additional vendor guidance is provided in the NetApp Security Advisory for environments utilizing NetApp storage with MySQL.

Workarounds

• Restrict network access to MySQL Server using firewall rules, allowing only trusted IP addresses and networks
• Implement strict privilege controls, limiting optimizer-related permissions to essential administrative accounts only
• Configure MySQL resource limits using max_execution_time and connection timeouts to mitigate resource exhaustion
• Consider deploying MySQL behind a database proxy or firewall that can filter malicious query patterns

# Example MySQL configuration to limit query execution time
# Add to my.cnf or my.ini configuration file
[mysqld]
max_execution_time=30000
max_connections=100
wait_timeout=300
interactive_timeout=300