CVE-2026-94520 Overview
CVE-2026-94520 is a format string vulnerability in Microsoft SQL Server that allows an authenticated attacker to trigger a denial-of-service condition over the network. The flaw is categorized under [CWE-134] Use of Externally-Controlled Format String. An authorized attacker can send crafted input that is interpreted as a format specifier, causing the SQL Server process to crash or become unresponsive.
Critical Impact
Successful exploitation disrupts database availability for all dependent applications and services, requiring high privileges but no user interaction.
Affected Products
- Microsoft SQL Server (see Microsoft CVE-2026-94520 Update Guide for specific versions)
Discovery Timeline
- 2026-10-13 - CVE CVE-2026-94520 published to NVD
- 2026-10-13 - Last updated in NVD database
Technical Details for CVE-2026-94520
Vulnerability Analysis
The vulnerability resides in a SQL Server code path that passes attacker-controlled data directly into a format string function without sanitization. When the server processes the crafted input, format specifiers inside the attacker's payload are interpreted at runtime. This can cause invalid memory access and terminate the SQL Server service.
The attack is network-reachable and requires authenticated access with elevated privileges. There is no confidentiality or integrity impact, but availability is fully disrupted. The current EPSS probability is 0.606%.
Root Cause
The root cause is the unsafe use of a format string function where user-supplied input is passed as the format argument rather than as a data argument. This pattern, tracked as [CWE-134], permits specifiers such as %s, %n, or %x to be processed against unintended memory regions. In this case, the result is a process-level fault rather than code execution.
Attack Vector
An attacker with authenticated, high-privilege access to SQL Server submits a crafted query or request containing format specifiers. The server processes the payload through the vulnerable function and crashes. Repeated requests sustain the denial-of-service condition, breaking availability for downstream applications. No code examples are published; refer to the Microsoft CVE-2026-94520 Update Guide for vendor details.
Detection Methods for CVE-2026-94520
Indicators of Compromise
- Unexpected SQL Server process termination or repeated service restarts recorded in Windows Event Logs.
- SQL Server error log entries referencing access violations or dump file generation (SQLDump*.mdmp).
- Spikes in failed query execution events originating from a single authenticated principal.
Detection Strategies
- Audit T-SQL queries containing suspicious format specifiers such as %s, %n, or long sequences of %x from non-administrative accounts.
- Correlate SQL Server crash events with recent authentication activity to identify the originating session.
- Monitor for unusual query patterns from service accounts that typically execute predictable workloads.
Monitoring Recommendations
- Forward SQL Server error logs and Windows Application logs to a central SIEM for correlation and alerting.
- Enable SQL Server Audit on login events and failed statement execution to track attacker reconnaissance.
- Baseline normal query volume per account and alert on sudden deviations that precede service failures.
How to Mitigate CVE-2026-94520
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft CVE-2026-94520 Update Guide as soon as it is available for your SQL Server version.
- Review and reduce the number of accounts holding elevated SQL Server privileges to shrink the attack surface.
- Rotate credentials for any high-privilege SQL Server accounts suspected of exposure.
Patch Information
Microsoft has issued guidance through its Security Response Center. Administrators should consult the Microsoft CVE-2026-94520 Update Guide for the specific patched builds mapped to each supported SQL Server version and cumulative update track.
Workarounds
- Restrict network access to SQL Server listeners using firewall rules so only trusted application tiers can connect.
- Enforce least-privilege role assignments and remove unnecessary sysadmin or db_owner memberships.
- Enable login auditing and automated alerting on repeated SQL Server service restarts to shorten response time.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
