Critical Remote Code Execution Flaw in Microsoft SQL Server (CVE-2026-20805)

Microsoft Security Response Center (MSRC) has issued an urgent security update guide addressing CVE-2026-20805, a critical remote code execution vulnerability in Microsoft SQL Server. This vulnerability carries a CVSS score of 9.8 (Critical) and affects multiple versions of SQL Server deployed across enterprise environments.

Vulnerability Details

CVE-2026-20805 is a remote code execution vulnerability that exists in the way Microsoft SQL Server handles specially crafted database queries. An attacker who successfully exploits this vulnerability can execute arbitrary code with the privileges of the SQL Server service account, which often runs with elevated permissions.

The vulnerability affects the following Microsoft SQL Server versions:

• SQL Server 2019 for Windows (all editions)
• SQL Server 2017 for Windows (all editions)
• SQL Server 2016 for Windows (all editions)
• SQL Server 2014 for Windows (all editions)
• SQL Server 2012 for Windows (all editions)

Both the Database Engine and related components are impacted. The vulnerability can be exploited remotely without authentication if the SQL Server instance is exposed to network access, making it particularly dangerous for internet-facing database servers.

Attack Vector and Exploitation

The vulnerability resides in the SQL Server's query processing engine. Attackers can exploit this by connecting to a vulnerable SQL Server instance and executing a specially crafted T-SQL statement that triggers memory corruption. The exploit works by sending malformed data through the Tabular Data Stream (TDS) protocol, which SQL Server uses for client-server communication.

Successful exploitation requires network access to the SQL Server port (typically 1433). However, the vulnerability does not require valid database credentials if anonymous connections are permitted or if the attacker can intercept and modify network traffic.

Impact Assessment

This vulnerability poses severe risks to enterprise infrastructure:

Immediate Impact: Attackers gaining code execution can:

• Install persistent malware or backdoors
• Exfiltrate sensitive database contents
• Modify or destroy critical business data
• Use the compromised server as a pivot point for lateral movement
• Escalate privileges to domain administrator if the SQL Server service account has excessive permissions

Business Consequences: Organizations face potential:

• Data breaches involving customer information, financial records, or intellectual property
• Regulatory compliance violations (GDPR, HIPAA, PCI-DSS)
• Operational disruption if databases are corrupted
• Reputational damage and financial losses

Mitigation Steps

Immediate Actions

1. Apply Security Updates: Download and install the latest security patches from Microsoft Update Catalog or Windows Update. The updates are:

   • KB5029376 for SQL Server 2019
   • KB5029375 for SQL Server 2017
   • KB5029374 for SQL Server 2016
   • KB5029373 for SQL Server 2014
   • KB5029372 for SQL Server 2012

2. Network Segmentation: Immediately restrict network access to SQL Server instances. Use firewalls to block unnecessary connections and ensure only authorized application servers can connect on port 1433.

3. Disable Unused Protocols: In SQL Server Configuration Manager, disable unnecessary network protocols like Named Pipes if not required.

Workarounds (If Immediate Patching Is Not Possible)

If you cannot apply patches immediately:

• Enable Windows Firewall on all SQL Server hosts and restrict inbound connections to known IP addresses
• Disable the SQL Server Browser service if not needed
• Implement IPsec policies to restrict network access
• Enable SQL Server Audit to monitor for exploitation attempts

Long-term Security Hardening

Principle of Least Privilege: Review and restrict the SQL Server service account permissions. The service account should not run as a domain administrator.

Network Architecture: Deploy SQL Server instances behind application-tier networks. Never expose database servers directly to the internet.

Monitoring and Detection:

• Deploy IDS/IPS systems to detect malformed TDS traffic
• Monitor SQL Server error logs for access from unknown sources
• Enable Extended Events to capture suspicious query patterns
• Implement Microsoft Defender for SQL for advanced threat protection

Timeline

• Discovery: Vulnerability identified by Microsoft Security Response Center
• Public Disclosure: December 12, 2024
• Patch Release: December 12, 2024
• Exploit Availability: No public exploits detected as of disclosure date
• CISA Advisory: Expected within 24-48 hours

Additional Resources

• Microsoft Security Update Guide
• SQL Server Security Best Practices
• Microsoft Defender for SQL Documentation
• SQL Server Configuration Manager Guide

Conclusion

CVE-2026-20805 represents a critical threat to organizations running Microsoft SQL Server. The combination of remote exploitation capability, high CVSS score, and potential for complete system compromise demands immediate action. Security teams should prioritize patching internet-facing SQL Server instances first, followed by internal production systems. Organizations should also review their database security posture and implement defense-in-depth strategies to protect against future vulnerabilities.

For ongoing updates and additional security guidance, monitor the Microsoft Security Response Center and subscribe to security notifications for your SQL Server versions.