CVE-2026-67373: SQL Server Heap Buffer Overflow RCE Vulnerability

CVE-2026-67373 Overview

CVE-2026-67373 is a heap-based buffer overflow vulnerability in Microsoft SQL Server. The flaw allows an authorized attacker to execute arbitrary code over a network. The vulnerability is classified under [CWE-122] (Heap-based Buffer Overflow) and affects the memory management of SQL Server's network-facing components.

An attacker with low-level authenticated access can send crafted input that overflows a heap buffer. Successful exploitation leads to code execution in the SQL Server process context, compromising confidentiality, integrity, and availability of the database instance.

Critical Impact

Authenticated network attackers can achieve arbitrary code execution on affected SQL Server instances, enabling full database compromise and potential lateral movement into the underlying host.

Affected Products

• Microsoft SQL Server (versions specified in the Microsoft Security Update CVE-2026-67373)

Discovery Timeline

• 2026-09-08 - CVE-2026-67373 published to NVD
• 2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-67373

Vulnerability Analysis

The vulnerability resides in SQL Server's handling of network-supplied input that is copied into a heap-allocated buffer. When the input size exceeds the allocated buffer capacity, adjacent heap memory is overwritten. This corruption can be leveraged to overwrite function pointers, vtable entries, or heap metadata used by the SQL Server process.

Exploitation requires the attacker to hold valid credentials on the target SQL Server instance. Once authenticated, the attacker sends crafted queries or protocol messages that trigger the vulnerable code path. The attack does not require user interaction and can be executed remotely across the network.

Code execution occurs in the security context of the SQL Server service account. On typical deployments, this account has broad access to database files and can interact with the underlying operating system through features such as xp_cmdshell or CLR integration.

Root Cause

The root cause is improper bounds validation before writing attacker-controlled data into a heap buffer. The affected routine trusts a length value or fails to enforce the destination buffer size, resulting in an out-of-bounds heap write consistent with [CWE-122].

Attack Vector

The attack vector is network-based and requires low-privilege authentication. An attacker with any authenticated SQL Server session can reach the vulnerable code path. Attack complexity is low and no user interaction is required. Refer to the Microsoft Security Update CVE-2026-67373 advisory for protocol-level details.

No public proof-of-concept exploit is available at the time of publication. The vulnerability mechanism is described in prose because verified exploit code has not been released.

Detection Methods for CVE-2026-67373

Indicators of Compromise

• Unexpected crashes or restarts of the sqlservr.exe process, particularly with access violation exceptions in Windows Event Logs.
• Anomalous outbound network connections originating from the SQL Server service account.
• New or modified stored procedures, jobs, or CLR assemblies created shortly after suspicious authenticated sessions.

Detection Strategies

• Monitor SQL Server error logs for exception dumps referencing heap corruption or access violations in network protocol handlers.
• Enable and review SQL Server Audit for authenticated sessions that issue unusually large payloads or malformed protocol messages.
• Correlate authentication events with process behavior on the SQL Server host to identify post-authentication code execution attempts.

Monitoring Recommendations

• Baseline normal query patterns and alert on statistical outliers in payload size from authenticated principals.
• Alert on child processes spawned by sqlservr.exe, which is uncommon in hardened deployments.
• Track privilege changes and new logins on SQL Server hosts following suspicious database activity.

How to Mitigate CVE-2026-67373

Immediate Actions Required

• Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-67373 advisory to all affected SQL Server instances.
• Inventory SQL Server deployments, including containerized and cloud-hosted instances, to confirm patch coverage.
• Rotate credentials for any SQL Server login that may have been exposed prior to patching.

Patch Information

Microsoft has published a security update for this vulnerability. Consult the Microsoft Security Update CVE-2026-67373 advisory for the applicable cumulative update or security-only package for each supported SQL Server version.

Workarounds

• Restrict network access to SQL Server TCP port 1433 and dynamic instance ports using host firewalls and network segmentation.
• Enforce the principle of least privilege for SQL Server logins and disable unused authentication methods.
• Require strong authentication and disable legacy or shared accounts that could be abused to reach the vulnerable code path.

# Configuration example: restrict SQL Server inbound access with Windows Firewall
New-NetFirewallRule -DisplayName "SQL Server - Restrict to Application Tier" `
  -Direction Inbound -Protocol TCP -LocalPort 1433 `
  -RemoteAddress 10.20.30.0/24 -Action Allow

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