CVE-2025-21500

What it actually does

A flaw in the MySQL Server optimizer component allows a low-privileged network attacker to trigger a complete denial of service by causing the server to hang or crash repeatedly.

Why it matters to the business

This can lead to unplanned downtime for database-dependent applications, interrupting business operations and requiring manual intervention to restore service.

How an attack unfolds

An attacker authenticates to the MySQL server using a low-privileged account over the network, then submits specially crafted queries that exploit weaknesses in the optimizer, resulting in a server crash or hang.

How to fix it

Upgrade MySQL Server to a version beyond the affected ranges (8.0.40, 8.4.3, 9.1.0). Restrict network access and limit user privileges as interim measures.

How to confirm the fix worked

Verify the MySQL server version is updated. Monitor server stability under load with typical queries. Review release notes for the specific patch.

Typical attack path

1. Obtain network connectivity to the MySQL instance
2. Authenticate using valid low-privileged credentials
3. Issue malicious queries designed to trigger the optimizer vulnerability
4. Repeat to maintain denial of service

How to detect it

Examine MySQL error logs and system logs for unexpected crashes or restarts. Look for patterns of queries preceding service failures.

What your security team should do

• Apply the latest MySQL patches immediately
• Implement network access controls to limit exposure
• Audit and minimize database user privileges
• Enable logging to detect anomalous query patterns