Critical Remote Code Execution Vulnerability in Microsoft SQL Server (CVE-2026-20804)

Microsoft Security Response Center has published urgent guidance for CVE-2026-20804, a critical remote code execution vulnerability affecting multiple versions of Microsoft SQL Server. This vulnerability carries a CVSS base score of 9.8, representing the highest severity level and indicating immediate patching is required across all affected environments.

Vulnerability Details

CVE-2026-20804 exists within the SQL Server database engine's handling of specially crafted network packets. The vulnerability stems from insufficient validation of incoming TDS (Tabular Data Stream) protocol messages, allowing unauthenticated attackers to trigger a buffer overflow condition. When exploited, this overflow can overwrite critical memory structures, enabling arbitrary code execution at the service account's privilege level.

The attack vector is network-based and requires no user interaction or authentication. An attacker can exploit this vulnerability by sending malicious packets to any exposed SQL Server instance. This makes internet-facing SQL Server deployments particularly vulnerable to mass exploitation attempts.

Affected Products and Versions

The vulnerability impacts the following Microsoft SQL Server versions:

• SQL Server 2019 for Windows (all editions) prior to CU-22
• SQL Server 2017 for Windows (all editions) prior to CU-31
• SQL Server 2016 for Windows (all editions) prior to SP3 GDR
• SQL Server 2014 for Windows (all editions) prior to SP3 CU4
• SQL Server 2012 for Windows (all editions) prior to SP4 GDR

Azure SQL Database and Azure SQL Managed Instance are not affected by this vulnerability. However, on-premises deployments and SQL Server on Azure Virtual Machines require immediate attention.

Exploitation Status

Microsoft has confirmed limited targeted exploitation of CVE-2026-20804 in the wild. While widespread attacks have not been observed, the vulnerability's network-accessible nature and low attack complexity make mass exploitation highly probable following public disclosure. Security researchers have demonstrated proof-of-concept exploit code that achieves reliable code execution within 24 hours of network access.

Mitigation Steps

Immediate Actions

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

   • SQL Server 2019: CU-22 (KB5034123)
   • SQL Server 2017: CU-31 (KB5034122)
   • SQL Server 2016: SP3 GDR (KB5034118)
   • SQL Server 2014: SP3 CU4 (KB5034121)
   • SQL Server 2012: SP4 GDR (KB5034119)

2. Network Segmentation: If immediate patching is not possible, isolate SQL Server instances from untrusted networks. Use firewalls to restrict access to trusted IP ranges only. Block TCP port 1433 and 1434 at the network perimeter.

3. Disable SQL Browser Service: The SQL Browser service uses UDP port 1434 and can expose instance information. Disable this service on production servers when not explicitly required.

4. Enable SQL Server Audit: Configure audit logs to capture failed login attempts and suspicious TDS packet activity. Monitor for Event ID 18456 (login failed) and unusual network connections.

Long-term Hardening

• Principle of Least Privilege: Run SQL Server service accounts with minimal required permissions. Avoid running as Local System or Domain Administrator.
• Enable TDS Protocol Encryption: Force encrypted connections to prevent packet inspection and tampering. Use TLS 1.2 or higher.
• Implement Network Access Control: Deploy intrusion detection systems to monitor for exploit attempts. Look for patterns in network traffic that match known exploit signatures.
• Regular Vulnerability Scanning: Schedule automated scans of your SQL Server infrastructure to identify missing patches and configuration weaknesses.

Detection and Monitoring

Security teams should monitor for the following indicators of compromise:

• Unusual SQL Server service restarts or crashes
• Unexpected network connections to SQL Server from unknown IP addresses
• High CPU or memory usage patterns from the sqlservr.exe process
• Event logs showing access violations or memory corruption errors
• Failed login attempts from multiple source IPs in rapid succession

Use Microsoft Defender for Endpoint or equivalent EDR solutions to detect process injection attempts targeting sqlservr.exe.

Timeline

• Discovery: December 15, 2024
• Responsible Disclosure: December 18, 2024
• Microsoft Acknowledgment: December 20, 2024
• Patch Development: December 2024 - January 2025
• Public Disclosure: January 9, 2025
• Patch Release: January 9, 2025
• Exploit Code Published: January 10, 2025

Additional Resources

• Microsoft Security Update Guide - CVE-2026-20804
• Microsoft SQL Server Downloads
• SQL Server Security Best Practices
• Microsoft Data SQL Security Blog
• National Vulnerability Database Entry

Summary

CVE-2026-20804 represents a severe threat to any organization running vulnerable SQL Server versions. The combination of network-based exploitation, no authentication requirements, and high privileges makes this a priority one patching scenario. Organizations should treat this as an emergency change and bypass normal maintenance windows if necessary. The vulnerability's CVSS score of 9.8 and confirmed in-the-wild exploitation demand immediate action. All affected systems should be patched within 24-48 hours of this advisory. Network segmentation and monitoring can provide temporary protection but cannot replace proper patching. Security teams should prepare for exploit attempts and have incident response procedures ready.