CVE-2026-21929: MySQL Server Parser DoS Vulnerability

CVE-2026-21929 Overview

CVE-2026-21929 is a denial of service vulnerability affecting the Parser component of Oracle MySQL Server. This vulnerability allows a low-privileged attacker with network access to cause a complete denial of service condition, resulting in a hang or frequently repeatable crash of the MySQL Server instance. While the attack complexity is high, successful exploitation can severely impact database availability for organizations relying on affected MySQL Server deployments.

Critical Impact

Successful exploitation enables attackers to cause complete denial of service of MySQL Server, disrupting database operations and potentially affecting dependent applications and services.

Affected Products

• MySQL Server versions 9.0.0 through 9.5.0
• Oracle MySQL Server (Parser component)
• Systems accessible via network protocols supporting MySQL connections

Discovery Timeline

• January 20, 2026 - CVE-2026-21929 published to NVD
• January 20, 2026 - Last updated in NVD database

Technical Details for CVE-2026-21929

Vulnerability Analysis

This vulnerability resides in the Server Parser component of MySQL Server, which is responsible for interpreting and processing SQL queries submitted by clients. The Parser component is a critical element of the database engine, handling syntax validation, query structure analysis, and preparation of execution plans.

The flaw allows an authenticated attacker with low privileges to craft specific inputs that trigger an abnormal condition in the parser's processing logic. When exploited, this results in the database server entering a hung state or experiencing a crash that can be repeatedly triggered, effectively denying service to legitimate users and applications.

The network-based attack vector means exploitation can occur remotely across multiple MySQL-supported protocols, though the high attack complexity indicates that specific conditions or carefully crafted payloads are required for successful exploitation. Importantly, this vulnerability only impacts availability—there is no data confidentiality or integrity breach.

Root Cause

The vulnerability stems from improper handling within the MySQL Server's Parser component when processing certain malformed or edge-case SQL constructs. The parser fails to properly validate or handle specific input patterns, leading to resource exhaustion, infinite loops, or null pointer dereference conditions that cause the server to hang or crash.

This type of parser vulnerability typically occurs when boundary conditions or unexpected input combinations are not adequately handled during query parsing phases, allowing specially crafted queries to trigger undefined behavior in the parsing routines.

Attack Vector

The attack is executed over the network using standard MySQL communication protocols. An attacker must possess valid credentials with at least low-level privileges to authenticate to the MySQL Server. Once authenticated, the attacker can submit specially crafted SQL statements designed to trigger the vulnerability in the Parser component.

The high attack complexity suggests that exploitation requires precise timing, specific configuration states, or carefully constructed payloads rather than simple arbitrary input. The attack does not require user interaction and operates within the scope of the MySQL Server process itself.

Detection Methods for CVE-2026-21929

Indicators of Compromise

• Unexpected MySQL Server crashes or hangs coinciding with database connection activity
• Repeated server restarts without clear operational cause
• Anomalous SQL query patterns in MySQL general or slow query logs from low-privileged accounts
• Increased connection timeouts reported by applications connecting to MySQL

Detection Strategies

• Monitor MySQL error logs for crash dump signatures or parser-related error messages
• Implement query analysis to detect unusual or malformed SQL statement patterns
• Deploy database activity monitoring (DAM) solutions to flag suspicious query structures
• Configure alerting for MySQL service availability drops or unexpected restarts

Monitoring Recommendations

• Enable MySQL general query logging temporarily during incident investigation to capture suspicious queries
• Implement connection monitoring to track authentication attempts and session durations
• Establish baseline metrics for MySQL Server responsiveness to detect degradation indicative of DoS attempts
• Configure automated health checks to detect and alert on MySQL service interruptions

How to Mitigate CVE-2026-21929

Immediate Actions Required

• Review and restrict network access to MySQL Server instances to trusted hosts and networks only
• Audit user privileges and remove unnecessary low-privileged accounts that have network access
• Implement connection rate limiting to reduce the impact of repeated exploitation attempts
• Ensure MySQL Server instances are monitored for availability with automatic alerting

Patch Information

Oracle has addressed this vulnerability in the January 2026 Critical Patch Update (CPU). Administrators should apply the latest security patches from Oracle to remediate this vulnerability. Detailed patch information and download links are available in the Oracle Security Alert - CPU January 2026.

Organizations should prioritize patching MySQL Server instances running versions 9.0.0 through 9.5.0 and follow Oracle's standard patch deployment procedures for database systems.

Workarounds

• Restrict MySQL Server network exposure using firewall rules to limit connections to trusted IP addresses
• Implement additional authentication layers such as VPN or SSH tunneling for database access
• Consider deploying a database proxy or firewall that can filter or block potentially malicious SQL patterns
• Temporarily disable or restrict accounts with network access until patches can be applied

# Example: Restrict MySQL connections to trusted networks using iptables
iptables -A INPUT -p tcp --dport 3306 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 3306 -j DROP

# Example: Review MySQL user privileges
mysql -u admin -p -e "SELECT user, host FROM mysql.user WHERE host NOT IN ('localhost', '127.0.0.1');"