CVE-2026-83945: SQL Server Heap Buffer Overflow RCE Flaw

CVE-2026-83945 Overview

CVE-2026-83945 is a heap-based buffer overflow vulnerability in Microsoft SQL Server. An authorized attacker can exploit the flaw to execute arbitrary code over a network. The weakness is classified under [CWE-122], Heap-based Buffer Overflow.

Successful exploitation requires high privileges and high attack complexity, which limits broad opportunistic abuse. However, the flaw still delivers full confidentiality, integrity, and availability impact when conditions are met. No public proof-of-concept exploit or in-the-wild activity has been reported at the time of publication.

Critical Impact

An authenticated attacker with elevated database privileges can trigger memory corruption to run attacker-controlled code in the context of the SQL Server process.

Affected Products

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

Discovery Timeline

• 2026-10-13 - CVE CVE-2026-83945 published to NVD
• 2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-83945

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow inside SQL Server's handling of network-reachable database operations. An authorized attacker sends crafted input that is processed without proper bounds enforcement on a heap-allocated buffer. The write past the allocation boundary corrupts adjacent heap metadata or object pointers.

Controlled corruption of heap structures can be leveraged to redirect execution flow inside the sqlservr.exe process. Because the SQL Server service typically runs with elevated service privileges, successful code execution yields high impact across confidentiality, integrity, and availability within the database host.

Exploitation requires valid authenticated access with high privileges to the SQL Server instance. The attack complexity is high, meaning the attacker must satisfy additional conditions such as timing, heap layout, or specific data states to reliably trigger the overflow. The exploit prediction scoring suggests limited near-term weaponization likelihood.

Root Cause

The root cause is improper validation of input size relative to a heap-allocated destination buffer during request processing within SQL Server. The affected routine copies attacker-influenced data into a buffer whose capacity was not correctly computed or enforced, producing an out-of-bounds write on the heap, consistent with [CWE-122].

Attack Vector

The attack vector is network-based. An attacker authenticated to the SQL Server instance with sufficient privileges issues crafted queries or protocol messages against the Tabular Data Stream (TDS) endpoint. The malformed payload reaches the vulnerable parsing or data-handling code path and overflows the heap buffer, enabling arbitrary code execution in the SQL Server process context.

See the Microsoft Security Update Guide for technical details and affected build information.

Detection Methods for CVE-2026-83945

Indicators of Compromise

• Unexpected crashes or access-violation events in the sqlservr.exe process, often recorded in the Windows Application event log and SQL Server error log.
• Creation of child processes under sqlservr.exe such as cmd.exe, powershell.exe, or rundll32.exe, which is anomalous for a database engine.
• Outbound network connections initiated by the SQL Server service to untrusted destinations.

Detection Strategies

• Hunt for authenticated SQL sessions originating from unusual source hosts that precede SQL Server service crashes or restarts.
• Alert on SQL Server spawning interactive shells, scripting hosts, or writing executable files to disk.
• Correlate SQL audit logs with endpoint telemetry to identify high-privileged logins issuing abnormal query patterns against the engine.

Monitoring Recommendations

• Enable SQL Server login auditing and track successful logins for sysadmin and other high-privilege roles.
• Forward SQL Server error logs, Windows event logs, and process telemetry to a centralized analytics platform for correlation.
• Monitor for repeated malformed TDS traffic patterns and unexpected memory-related exceptions in the database host.

How to Mitigate CVE-2026-83945

Immediate Actions Required

• Apply the Microsoft security update referenced in the Microsoft Security Update Guide for the affected SQL Server build.
• Inventory all SQL Server instances, including those embedded in third-party applications, to confirm patch coverage.
• Review and reduce the number of accounts holding high-privilege roles such as sysadmin or db_owner.

Patch Information

Microsoft has published fix information through the Microsoft Security Update Guide entry for CVE-2026-83945. Administrators should apply the cumulative update or security-only patch for their SQL Server version and restart the service to complete remediation.

Workarounds

• Restrict network exposure of SQL Server by binding listeners to internal interfaces and enforcing firewall rules that limit access to known application hosts.
• Enforce least privilege for database logins and remove unnecessary elevated role memberships.
• Require strong authentication and, where feasible, enforce TLS for client-to-SQL Server connections to protect the authenticated channel.

# Configuration example: restrict SQL Server TCP access to trusted application subnet
New-NetFirewallRule -DisplayName "Allow SQL from AppTier" `
  -Direction Inbound -Protocol TCP -LocalPort 1433 `
  -RemoteAddress 10.20.30.0/24 -Action Allow

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