CVE-2026-77481: SQL Server Heap Buffer Overflow RCE Flaw

CVE-2026-77481 Overview

CVE-2026-77481 is a heap-based buffer overflow vulnerability in Microsoft SQL Server. An authorized attacker can exploit the flaw over a network to execute arbitrary code within the SQL Server process context. The weakness is categorized under [CWE-122], heap-based buffer overflow, and requires only low-privilege authenticated access to trigger.

Successful exploitation compromises confidentiality, integrity, and availability of the database instance. Attackers can read or modify database contents, deploy persistence, and pivot into connected systems. Microsoft published the vulnerability advisory through the Microsoft Security Response Center (MSRC).

Critical Impact

A low-privileged authenticated attacker can trigger a heap corruption over the network to execute code within the SQL Server process, leading to full database compromise.

Affected Products

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

Discovery Timeline

• 2026-09-08 - CVE-2026-77481 published to NVD
• 2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-77481

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow [CWE-122] in Microsoft SQL Server. The flaw occurs when SQL Server processes attacker-controlled input that exceeds the bounds of a heap-allocated buffer. The overflow corrupts adjacent heap metadata or object pointers, giving the attacker the ability to hijack control flow.

Exploitation requires network access to the SQL Server instance and valid low-privilege credentials. No user interaction is needed. Because SQL Server typically runs as a service account with database-wide permissions, code execution translates directly into full instance compromise.

Attackers who chain this vulnerability with credential theft or lateral movement techniques can escalate from a single database foothold to Active Directory or connected application tiers. The Exploit Prediction Scoring System (EPSS) currently rates the probability of exploitation at 0.617%.

Root Cause

The root cause is missing or incorrect bounds validation on data written into a heap buffer during SQL Server request handling. When crafted input is submitted, the write operation exceeds the allocated size and corrupts heap structures. Microsoft has not publicly disclosed the specific component or code path.

Attack Vector

The attack vector is network-based. An authenticated attacker sends a specially crafted query, stored procedure call, or protocol message to the SQL Server listener. The malformed request triggers the overflow during parsing or processing, allowing attacker-controlled data to overwrite heap memory and redirect execution.

Refer to the Microsoft Security Update Guide for technical details on affected components and the vulnerable request path.

Detection Methods for CVE-2026-77481

Indicators of Compromise

• Unexpected SQL Server service crashes, restarts, or Dr. Watson dumps referencing sqlservr.exe
• Anomalous child processes spawned by sqlservr.exe, such as cmd.exe, powershell.exe, or rundll32.exe
• Outbound network connections initiated by the SQL Server service account to unknown destinations
• New or modified SQL logins, linked servers, or extended stored procedures created shortly after suspicious sessions

Detection Strategies

• Monitor SQL Server error logs and Windows Application event logs for access violations, stack dumps, and heap corruption messages
• Baseline normal query and RPC patterns per application account, then alert on oversized payloads and malformed TDS traffic
• Deploy endpoint telemetry on database hosts to detect process injection, memory tampering, and unusual module loads within sqlservr.exe

Monitoring Recommendations

• Enable SQL Server Audit for failed logins, permission changes, and privileged command execution
• Forward database host telemetry, TDS network metadata, and Windows security events into a centralized analytics platform for correlation
• Alert on any interactive shell or scripting host launched by the SQL Server service account

How to Mitigate CVE-2026-77481

Immediate Actions Required

• Apply the Microsoft security update referenced in the Microsoft Security Update Guide as soon as testing permits
• Inventory all SQL Server instances, including developer, test, and embedded deployments, to confirm patch coverage
• Rotate credentials for SQL logins and service accounts on any instance suspected of exposure

Patch Information

Microsoft has issued security updates addressing CVE-2026-77481. Administrators should consult the Microsoft Security Update Guide for the specific KB articles, cumulative updates, and supported product versions applicable to their SQL Server deployments.

Workarounds

• Restrict network access to SQL Server ports (default TCP 1433) using host firewalls and network ACLs, permitting only trusted application hosts
• Enforce least privilege on database logins and remove unnecessary db_owner, sysadmin, or CONNECT grants
• Require TLS encryption for all SQL Server client connections to reduce exposure to unauthenticated reconnaissance
• Isolate SQL Server hosts from direct internet exposure and route access through hardened application tiers or bastions

# Configuration example: restrict inbound SQL Server access on Windows
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 Any" `
  -Direction Inbound -Protocol TCP -LocalPort 1433 `
  -Action Block