Critical Remote Code Execution Vulnerability in Microsoft SQL Server (CVE-2026-20958)

Executive Summary

Microsoft Security Response Center (MSRC) has released security guidance for CVE-2026-20958, a critical remote code execution vulnerability in Microsoft SQL Server. This vulnerability carries a CVSS base score of 8.8 (High) and affects multiple versions of SQL Server deployed across enterprise environments worldwide.

The vulnerability exists in the SQL Server service's handling of specially crafted network requests. An attacker who successfully exploits this flaw can execute arbitrary code with the privileges of the SQL Server service account, potentially leading to full system compromise, data exfiltration, or lateral movement within corporate networks.

Affected Products and Versions

Microsoft SQL Server 2012 (x64) Service Pack 4 (KB5014242) Microsoft SQL Server 2014 Service Pack 3 (KB5014242) Microsoft SQL Server 2016 Service Pack 2 (KB5014242) Microsoft SQL Server 2017 (KB5014242) Microsoft SQL Server 2019 (KB5014242) Microsoft SQL Server 2022 (KB5014242)

The vulnerability affects both the Database Engine and related SQL Server components. Standalone SQL Server installations, as well as those bundled with other Microsoft products like SharePoint, Dynamics, and System Center, are all vulnerable.

Technical Details

The vulnerability resides in the SQL Server's Tabular Data Stream (TDS) protocol parser. TDS is the proprietary protocol used for client-server communication between SQL Server and its clients. When processing incoming TDS packets, the SQL Server service fails to properly validate the length fields and memory boundaries.

An attacker can craft a malicious TDS packet containing overflow data that overwrites critical memory structures within the SQL Server process space. By carefully controlling the overflow contents, the attacker can achieve arbitrary code execution in the context of the SQL Server service account.

Key technical characteristics:

• Attack Vector: Network-based (requires connectivity to SQL Server port 1433)
• Attack Complexity: Low (exploitation does not require user interaction)
• Privileges Required: None (authentication not required for initial exploitation)
• User Interaction: None
• Scope: Unchanged

The vulnerability can be triggered without authentication in default configurations, making it particularly dangerous for internet-facing SQL Server instances or those within compromised internal networks.

Severity Assessment

CVSS v3.1 Base Score: 8.8 (High)

• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): None (N)
• Scope (S): Unchanged (U)
• Confidentiality (C): High (H)
• Integrity (I): High (H)
• Availability (A): High (H)

This rating reflects the potential for complete system compromise, data theft, and service disruption. The lack of authentication requirements and low attack complexity make this vulnerability attractive to both opportunistic and targeted attackers.

Exploitation Status

Microsoft is aware of limited, targeted exploitation of this vulnerability in the wild. While widespread attacks have not been observed, the technical details required for exploitation are relatively straightforward, suggesting that broader exploitation attempts may follow.

Organizations should treat this as an active threat and prioritize patching accordingly.

Mitigation Strategies

Immediate Actions

1. Apply Security Updates: Install the cumulative updates released by Microsoft. The patches address the memory handling issue in the TDS protocol parser.

2. Network Segmentation: Restrict access to SQL Server ports (default 1433, 1434) to authorized clients only. Implement firewall rules that block unnecessary external access.

3. Disable SQL Server Browser Service: If not required, disable the SQL Server Browser service to reduce attack surface.

4. Enable SQL Server Authentication Logging: Configure enhanced logging to capture authentication attempts and TDS protocol errors.

Long-term Security Hardening

• Implement Least Privilege: Run SQL Server service accounts with minimal required privileges. Avoid running as Local System or Domain Administrator.

• Enable Windows Firewall: Configure Windows Firewall to restrict SQL Server traffic to specific IP ranges.

• Use Network Level Authentication: Where possible, enforce network-level authentication before establishing SQL Server connections.

• Regular Security Audits: Conduct periodic vulnerability assessments and penetration testing of SQL Server environments.

Patch Deployment Timeline

Release Date: June 11, 2024 Notification Period: Immediate Required Action: Deploy patches within 7 days for internet-facing systems, 30 days for internal systems

Microsoft recommends prioritizing patching based on exposure:

• Tier 1: Internet-facing SQL Server instances (patch immediately)
• Tier 2: Internal systems with direct client access (within 7 days)
• Tier 3: Isolated backend systems (within 30 days)

Detection and Monitoring

Indicators of Compromise

Monitor for the following suspicious activities:

• Unexpected SQL Server service restarts
• High CPU usage from sqlservr.exe process
• Unusual network connections to SQL Server from unknown IPs
• Failed authentication attempts in SQL Server error logs
• Windows Event Log entries indicating memory access violations

Recommended Monitoring Tools

• SQL Server Extended Events: Capture TDS protocol errors and suspicious query patterns
• Windows Event Forwarding: Centralize security event collection from SQL Server hosts
• Microsoft Defender for Endpoint: Detect post-exploitation activities on SQL Server systems
• Network IDS/IPS: Monitor for anomalous TDS traffic patterns

Additional Resources

• Microsoft Security Update Guide - CVE-2026-20958
• SQL Server Security Updates
• Microsoft SQL Server Best Practices
• MSRC Customer Guidance

Conclusion

CVE-2026-20958 represents a significant threat to organizations running Microsoft SQL Server. The combination of network-based exploitation, lack of authentication requirements, and high impact makes this vulnerability a priority for immediate remediation.

Security teams should verify SQL Server versions across their environment, deploy the necessary patches according to the exposure-based timeline, and implement additional monitoring to detect exploitation attempts. Organizations with legacy SQL Server installations should consider this vulnerability a catalyst for accelerating upgrade plans to supported versions.

For questions or additional support, contact Microsoft Security Response Center at [email protected] or visit the Microsoft Security Update Guide.