X-aspnet-version 4.0.3 Vulnerabilities Jun 2026

The "x-aspnet-version 4.0.3" header indicating the use of ASP.NET 4.0.3 should serve as a prompt for developers and system administrators to assess the security posture of their applications and servers. By understanding known vulnerabilities, applying patches, following best practices, and maintaining a proactive stance on security, risks associated with outdated versions can be significantly mitigated.

When an ASP.NET application handles a request (e.g., .aspx , .ashx , or MVC routes), the runtime automatically appends a response header similar to: x-aspnet-version 4.0.3 vulnerabilities

X-AspNet-Version: 4.0.3 is a for attackers targeting end-of-life ASP.NET applications. Removing the header via enableVersionHeader="false" is a simple but mandatory first step. However, due to the unsupported nature of .NET 4.0.3, organizations must prioritize migration to a supported .NET runtime. Relying solely on header suppression offers no protection against known remote code execution or padding oracle vulnerabilities. The "x-aspnet-version 4

| CVE ID | Description | Impact | |--------|-------------|--------| | | Padding Oracle in ASP.NET (MS14-026) | Plaintext recovery, session hijacking | | CVE-2014-4149 | XSS in ASP.NET web forms | Remote code execution via unsanitized input | | CVE-2013-3131 | ViewState MAC validation bypass | Privilege escalation, arbitrary postback | | CVE-2012-1882 | Request validation bypass (MVC 3/4) | XSS, injection attacks | | CVE-2011-3416 | ASP.NET Forms Authentication ticket weakness | Authentication bypass | | CVE ID | Description | Impact |

For custom applications, follow secure coding practices. This includes validating inputs to prevent SQL injection and XSS attacks.

By default, ASP.NET includes this header to help development tools like Visual Studio identify the environment. However, in a production environment, this is pure reconnaissance data for an attacker. Shhh… don't let your response headers talk too loudly