Critical Remote Code Execution Vulnerability in Microsoft SQL Server - CVE-2026-20818

Microsoft Security Response Center has released guidance for CVE-2026-20818, a critical remote code execution vulnerability that poses significant risk to enterprise environments running Microsoft SQL Server. This vulnerability enables unauthenticated attackers to achieve code execution at the service account privilege level.

Vulnerability Details

CVE-2026-20818 affects multiple versions of Microsoft SQL Server, including SQL Server 2012 SP4, 2014 SP3, 2016 SP2, 2017, 2019, and 2022. The vulnerability resides in the SQL Server service's handling of specially crafted network packets. Attackers can exploit this flaw remotely without authentication by sending malicious requests to TCP port 1433 (default SQL Server port).

The vulnerability carries a CVSS v3.1 base score of 9.8 (Critical), reflecting its network attack vector, low attack complexity, no privileges required, and no user interaction needed. The impact metrics show high values for confidentiality, integrity, and availability impact.

Attack Vector and Risk Assessment

This vulnerability allows remote code execution through the SQL Server service. The attacker does not need to authenticate to the instance. A successful exploit grants the attacker the same privileges as the SQL Server service account, which typically runs with elevated permissions in enterprise environments.

In most deployments, the SQL Server service runs under a domain account or local system privileges. This means a successful exploit could lead to domain compromise if the service account has domain admin rights, or complete system takeover if running as SYSTEM.

The vulnerability is exploitable over the network. The attack requires no user interaction. An attacker only needs network connectivity to the SQL Server instance. This makes internet-facing SQL Server installations particularly vulnerable.

Affected Products and Versions

The following Microsoft SQL Server versions are affected:

• SQL Server 2012 SP4 (all editions)
• SQL Server 2014 SP3 (all editions)
• SQL Server 2016 SP2 (all editions)
• SQL Server 2017 (all versions prior to CU31)
• SQL Server 2019 (all versions prior to CU23)
• SQL Server 2022 (all versions prior to CU14)

SQL Server 2008 R2 and earlier versions are not affected as they are out of support. Azure SQL Database and Azure SQL Managed Instance are not affected.

Exploitation Status

Microsoft has not observed active exploitation of CVE-2026-20818 in the wild at the time of disclosure. However, the vulnerability details are now public, increasing the likelihood of exploit development and use.

Security researchers have demonstrated proof-of-concept exploitation in controlled environments. The vulnerability is relatively straightforward to exploit once the attack vector is understood.

Mitigation Steps

Immediate Actions

1. Apply Security Updates

   • Download and install the latest security updates from Microsoft Security Update Guide
   • Prioritize internet-facing SQL Server instances
   • Test updates in non-production environments first

2. Network Segmentation

   • Restrict network access to SQL Server ports (1433, 1434) to trusted hosts only
   • Implement firewall rules to block unnecessary access
   • Use VPN or private networks for database connectivity

3. Disable SQL Server Browser Service

   • If not needed, disable the SQL Server Browser service
   • This reduces the attack surface

Workarounds (if immediate patching is not possible)

1. Enable SQL Server Protocol Encryption

   • Force encrypted connections only
   • This may limit some attack vectors

2. Implement Network Access Control

   • Use IPsec policies to restrict access
   • Configure Windows Firewall rules

Detection and Monitoring

Monitor for exploitation attempts by reviewing SQL Server error logs for unusual connection attempts or malformed packets. Enable SQL Server audit logging to track authentication attempts and query patterns.

Network monitoring tools can detect anomalous traffic patterns to SQL Server ports. Security Information and Event Management (SIEM) systems should alert on multiple failed connection attempts followed by successful connections.

Long-term Security Posture

This vulnerability highlights the importance of defense-in-depth for database systems:

• Keep SQL Server updated with the latest cumulative updates
• Run SQL Server services with least-privilege accounts
• Implement network segmentation for database servers
• Use Azure Defender for SQL for cloud-based threat detection
• Regularly review and audit SQL Server configurations

References

• Microsoft Security Update Guide
• Microsoft SQL Server Security Best Practices
• SQL Server Network Configuration
• CISA Known Exploited Vulnerabilities Catalog