CVE-2026-67631: SQL Server RCE Vulnerability

CVE-2026-67631 Overview

CVE-2026-67631 is a heap-based buffer overflow vulnerability in Microsoft SQL Server. An authorized attacker can trigger the flaw over a network to execute arbitrary code on the database server. The vulnerability is classified under [CWE-122] and carries a CVSS 3.1 score of 8.8.

Microsoft published details in the Microsoft Security Update Guide CVE-2026-67631. Exploitation requires low-privilege authenticated access and no user interaction, making the flaw relevant for any environment where SQL Server accepts remote database connections from untrusted or compromised accounts.

Critical Impact
Successful exploitation grants code execution in the context of the SQL Server process, leading to full compromise of database confidentiality, integrity, and availability.

Affected Products

Microsoft SQL Server (specific supported versions listed in the Microsoft Security Update Guide)

Discovery Timeline

2026-09-08 - CVE-2026-67631 published to NVD
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-67631

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow within SQL Server. An attacker with valid database credentials sends crafted input over the network protocol used by SQL Server. The server writes attacker-controlled data past the bounds of an allocated heap buffer.

The overflow corrupts adjacent heap metadata or object pointers within the SQL Server process. When the corrupted structures are later dereferenced, the attacker can redirect execution flow. This yields arbitrary code execution in the security context of the SQL Server service account.

Root Cause

The underlying weakness is [CWE-122] Heap-based Buffer Overflow. A code path in SQL Server fails to correctly validate the size of a user-supplied element before copying it into a fixed or under-sized heap allocation. Microsoft has not published a granular technical decomposition of the affected component in public advisories.

Attack Vector

Exploitation occurs over the network against an accessible SQL Server instance. The attacker must first authenticate, but only low privileges are required. No user interaction is needed, and the scope is unchanged. Refer to the Microsoft Security Update Guide CVE-2026-67631 for exploitability assessments per product build.

No public proof-of-concept exploit is currently available. The EPSS probability is 0.628%.

Detection Methods for CVE-2026-67631

Indicators of Compromise

Unexpected crashes, access violations, or restarts of the sqlservr.exe process, particularly correlated with inbound TDS traffic from a specific principal.
Creation of new child processes under sqlservr.exe such as cmd.exe, powershell.exe, or rundll32.exe.
Unusual outbound network connections initiated by the SQL Server service account.
New or modified xp_cmdshell activity, linked server changes, or unfamiliar CLR assemblies loaded into the instance.

Detection Strategies

Monitor SQL Server error logs and Windows Application event logs for stack dumps, EXCEPTION_ACCESS_VIOLATION, or minidumps associated with heap corruption.
Alert on anomalous authenticated sessions issuing malformed queries or large parameter payloads to database endpoints.
Correlate low-privilege database logins with subsequent process spawning or file writes from the SQL Server host.

Monitoring Recommendations

Enable SQL Server audit for login events, failed logins, and privileged action execution across all production instances.
Forward sqlservr.exe process telemetry, module loads, and child-process events to a centralized detection platform.
Track outbound connections from database servers and baseline expected destinations to identify deviations.

How to Mitigate CVE-2026-67631

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide CVE-2026-67631 to all affected SQL Server builds.
Inventory SQL Server instances exposed to untrusted networks and prioritize patching internet-reachable or multi-tenant systems.
Rotate credentials for database accounts that may have been exposed, especially low-privilege application accounts.

Patch Information

Microsoft has released fixed builds through the Security Update Guide. Administrators should identify the correct cumulative update or GDR package for each SQL Server version and apply it via standard patch management workflows. Validate patched build numbers after installation.

Workarounds

Restrict network access to SQL Server ports (default TCP 1433) using host and network firewalls, allowing only trusted application subnets.
Enforce least privilege for database logins and remove unused accounts to reduce the pool of principals capable of triggering the flaw.
Require authentication through a jump host or private link, and disable public endpoint access on managed SQL deployments where feasible.

bash

# Example: restrict inbound access to SQL Server TCP 1433 to a trusted subnet
New-NetFirewallRule -DisplayName "SQL Server 1433 - Allow AppTier Only" `
  -Direction Inbound -Protocol TCP -LocalPort 1433 `
  -RemoteAddress 10.20.30.0/24 -Action Allow

New-NetFirewallRule -DisplayName "SQL Server 1433 - Block All Others" `
  -Direction Inbound -Protocol TCP -LocalPort 1433 `
  -Action Block