Critical QNAP .NET Flaw Lets Attackers Bypass Security Protections

A significant security vulnerability has emerged affecting QNAP’s NetBak PC Agent software through a critical flaw in Microsoft ASP.NET Core.

The vulnerability, tracked as CVE-2025-55315, exploits HTTP Request Smuggling techniques to bypass essential security controls and could expose thousands of backup-dependent systems to unauthorized access and data manipulation.

The flaw resides in ASP.NET Core’s HTTP request handling mechanisms, allowing authenticated attackers to craft specially designed requests that confuse the web server’s security processing.

Once exploited, attackers gain the ability to access sensitive data stored on affected systems, modify critical server files, or trigger limited denial-of-service conditions that disrupt backup operations.

For organizations relying on NetBak PC Agent for data protection, this represents a direct threat to backup integrity and system security.

Overview of the Vulnerability

NetBak PC Agent depends on Microsoft ASP.NET Core during installation and runtime operation. Any Windows system running this backup solution likely contains the vulnerable ASP.NET Core components unless previously patched.

The HTTP Request Smuggling technique exploits inconsistencies in how different system components interpret HTTP messages, creating a gap that attackers can weaponize.

This type of vulnerability has historically been used in sophisticated attacks targeting enterprise infrastructure and sensitive data repositories.

The CVE-2025-55315 vulnerability requires authentication, meaning attackers must already have some level of system access or credentials.

However, insider threats and compromised accounts present realistic attack scenarios in many organizations. Once an authenticated attacker gains foothold, the vulnerability becomes a powerful tool for lateral movement and privilege escalation.

QNAP has issued urgent recommendations for all NetBak PC Agent users to update their ASP.NET Core runtime immediately.

The organisation emphasises that ensuring Windows systems contain the latest Microsoft ASP.NET Core updates is essential for protecting backup infrastructure from exploitation.

Users can address this vulnerability through two primary methods. The first approach involves reinstalling NetBak PC Agent completely.

Users should navigate to Settings, locate the application in installed apps, and uninstall it entirely.

After downloading the latest version from QNAP’s official utilities page, reinstalling the software automatically deploys the current ASP.NET Core runtime components with necessary security patches applied.

For users preferring not to reinstall, manual ASP.NET Core updates provide an alternative solution. This method requires downloading the latest ASP.NET Core Runtime Hosting Bundle from Microsoft’s official .NET 8.0 download page.

As of October 2025, the current version stands at 8.0.21. After installation, system administrators should restart their applications or systems to ensure the updated components are properly initialized.

Security professionals recommend testing updates in controlled environments before deploying them across entire organizations.

Organizations should also verify that all deployed instances of NetBak PC Agent receive the necessary updates to prevent inconsistent security postures across their infrastructure.

The discovery of CVE-2025-55315 underscores the importance of maintaining current patch levels across all software dependencies, particularly those handling critical backup operations.

Regular vulnerability scanning and automated patch management systems can help organizations identify similar risks before attackers do.

Users who have not yet updated should prioritize this patch immediately, given the vulnerability’s potential impact on backup systems and data security.

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