Critical Remote Code Execution Vulnerability in Microsoft SQL Server (CVE-2026-20811)

Microsoft Security Response Center (MSRC) has published guidance for CVE-2026-20811, a critical remote code execution vulnerability affecting multiple versions of Microsoft SQL Server. This vulnerability carries a CVSS base score of 9.8 (Critical), representing significant risk to enterprise infrastructure.

Affected Products and Versions

The vulnerability impacts the following Microsoft SQL Server editions:

• SQL Server 2019 (all builds prior to CU 18)
• SQL Server 2017 (all builds prior to CU 31)
• SQL Server 2016 (all builds prior to SP2 CU 18)
• SQL Server 2014 (all builds prior to SP3 CU 4)
• SQL Server 2012 (all builds prior to SP4 CU 10)

Additionally, Azure SQL Database and Azure SQL Managed Instance are protected automatically; no customer action is required for these cloud services.

Vulnerability Details

CVE-2026-20811 stems from improper input validation in the SQL Server Database Engine's query processing component. When the engine processes certain malformed T-SQL statements containing nested subqueries with specific aggregate functions, it fails to properly validate memory allocation boundaries.

An authenticated attacker with db_datareader or higher privileges can craft a malicious SQL query that, when executed, triggers a buffer overflow condition. The overflow occurs in the sqlservr.exe process memory space, allowing the attacker to overwrite critical structures and achieve arbitrary code execution at the service account privilege level.

Attack Vector: Network-based Privileges Required: Low (authenticated database user) User Interaction: None Scope: Unchanged

Exploitation Requirements

For successful exploitation, the attacker must:

1. Have valid credentials to connect to the SQL Server instance
2. Possess permissions to execute queries against at least one database
3. Be able to deploy a specially crafted T-SQL statement

The vulnerability does not require sysadmin privileges, significantly lowering the barrier for lateral movement in compromised networks.

Impact Assessment

Successful exploitation grants the attacker code execution capabilities with the same privileges as the SQL Server service account. In typical configurations, this runs under a domain service account with elevated permissions on the database server and potentially other network resources.

Real-world impact scenarios include:

• Data exfiltration: Attackers can access and extract sensitive database contents
• Ransomware deployment: SQL Server service account privileges enable file encryption across accessible shares
• Lateral movement: Service account credentials can be harvested or used to pivot to other systems
• Persistence: Attackers can create malicious stored procedures or modify existing ones

Mitigation Steps

Immediate Actions

1. Apply Security Updates

   • Download and install the latest cumulative updates from the Microsoft Update Catalog
   • For SQL Server 2019: Update to CU 18 or later
   • For SQL Server 2017: Update to CU 31 or later
   • For SQL Server 2016: Update to SP2 CU 18 or later
   • For SQL Server 2014: Update to SP3 CU 4 or later
   • For SQL Server 2012: Update to SP4 CU 10 or later

2. Restart SQL Server Services

   • The update requires a service restart to take effect
   • Plan for maintenance window if running production workloads

Network-Level Protections

If immediate patching is not feasible:

• Implement IP restrictions: Limit SQL Server port 1433 access to trusted IP ranges only
• Enable firewall rules: Block inbound connections from untrusted networks
• Use VPN for remote access: Ensure all database connections traverse encrypted tunnels

Detection and Monitoring

Deploy detection rules to identify potential exploitation attempts:

• Monitor SQL Server error logs for unusual query patterns or memory-related errors
• Enable SQL Server Audit to log all SELECT statements against sensitive databases
• Watch for unexpected sqlservr.exe child processes (indicates successful exploitation)
• Monitor network traffic for anomalous query volumes from single sources

Timeline and Disclosure

• Discovery: Vulnerability identified by Microsoft Security Response Center internal research
• Reported to Microsoft: January 15, 2026
• Patch Development: January 16 - February 28, 2026
• Coordinated Release: March 11, 2026
• Public Disclosure: March 11, 2026 at 10:00 AM PST

Additional Resources

• Microsoft Security Update Guide - CVE-2026-20811
• SQL Server Cumulative Update Downloads
• Microsoft SQL Server Security Best Practices
• MSRC Blog Post on CVE-2026-20811

Recommended Priority

CRITICAL - This vulnerability should be treated as emergency priority for all affected SQL Server instances exposed to untrusted networks or used by multi-tenant applications. Organizations with compliance requirements (PCI-DSS, HIPAA, SOX) should treat this as a compliance-critical patch.

For environments where immediate patching is not possible, implement the network-level protections and enhanced monitoring as temporary measures while planning maintenance windows for patch deployment.