Date: June 2024
Author: [Your Name]


A new weakness in the MySQL Server software, tracked as CVE-2025-21536, is making waves in the cybersecurity community. This post explains in simple terms what this bug is, what versions are at risk, what attackers can do with it, and even how it can be triggered in the real world.

What Is CVE-2025-21536?

CVE-2025-21536 is a Denial of Service (DoS) vulnerability found in the Optimizer component of Oracle's MySQL Server. If an attacker has high-level privileges (such as a database administrator account) and access via the network, they can send a special database query that makes your MySQL database hang or crash—over and over, if they want.

9.: Versions up to and including 9..1

If you aren’t running any of these versions or below, your server could be vulnerable.

User Interaction Needed: None

- Impact: Total loss of availability (DoS – server hangs/crashes)

How Does the Attack Work?

The bug exists in the way MySQL’s Optimizer handles certain SQL queries. While Oracle hasn’t shared exact details (for security), the pattern is often:

The server gets stuck or crashes outright.

This results in downtime: users can’t access your website, app, or service until someone restarts the service (and maybe not even then, if the query keeps being run).

Code Example: What Does an Exploit Look Like?

While exact payloads are not public yet, based on similar bugs, an attacker might use subqueries with specific joins and unions. Here’s an illustrative, theoretical example:

-- WARNING: This could crash an affected MySQL Server! DO NOT RUN IN PRODUCTION!

SELECT t1.a, (SELECT MAX(t2.b) FROM test_table t2 WHERE t2.c = t1.c)
FROM test_table t1
JOIN (SELECT * FROM test_table WHERE d = 'trigger_bug') t3 ON t1.id = t3.id;

In vulnerable versions, variations of deeply nested subqueries or complex optimizer scenarios like this have triggered engine bugs, leading to hangs or crashes.

Proof of Concept (PoC): Testing for Vulnerability

If you’re a database administrator and need to check your system, always test in a safe development or staging environment first.

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If your server freezes or crashes on such queries, you may be vulnerable to CVE-2025-21536.

Exploit Details

- Exploitability: This bug is not "remote" in the sense that anyone on the web can abuse it. The attacker needs a valid login with high privileges.
- Payloads: Different payloads can trigger the bug, but most involve complex nested queries that interfere with the optimizer’s analysis and execution path.
- Impact: MySQL process becomes unusable or stops, requiring a manual restart. If the payload is run repeatedly (i.e., by a script or automated job), continuous DoS is possible.

Upgrade to 9..2 or higher

- Restrict Privileges: Do not grant unnecessary high privileges to user accounts, especially to apps or users that don’t need them.

- Monitoring: Watch for unexplained MySQL crashes or slowdowns, especially after complex queries from unfamiliar sources.

References

- Oracle Critical Patch Update Advisory - July 2024 *(official security bulletin)*
- NIST National Vulnerability Database listing for CVE-2025-21536

Conclusion

CVE-2025-21536 is a serious stability problem for anyone relying on MySQL in production, even though it needs an attacker with higher privileges. A crash-prone database can cause huge disruptions, especially if the vulnerability is abused in large organizations. The fix is to patch early, restrict privileges, and keep an eye on your server logs.

Stay safe, and make sure you’re not running an outdated version!


Disclaimer: The provided queries are for educational and detection purposes only. Never test on a production system.


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Timeline

Published on: 01/21/2025 21:15:19 UTC
Last modified on: 01/22/2025 19:15:12 UTC