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mod_cluster

mod_cluster is an intelligent native Apache httpd-based and pure-Java Undertow-based load-balancer

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X-aspnet-version 4.0.3 Vulnerabilities [work] -

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The X-AspNet-Version: 4.0.30319 header is a historical artifact that has outlived its usefulness. While it was designed to help debug compatibility, in modern security postures, it serves only as a beacon for attackers. The vulnerabilities associated with ASP.NET 4.0.3—from padding oracles to ViewState deserialization RCE—are well-documented, weaponized in penetration testing frameworks, and actively exploited in the wild. x-aspnet-version 4.0.3 vulnerabilities

The X-AspNet-Version HTTP header explicitly identifies the version of ASP.NET running on a web server. When it reveals version (often shortened to 4.0.3), it provides attackers with a roadmap for targeted exploitation. While the header itself is a configuration choice rather than a bug, broadcasting this specific version exposes the application to several known risks associated with the .NET Framework 4.0 lifecycle. The Risk of Information Disclosure : The X-AspNet-Version: 4

According to Wappalyzer and BuiltWith data, over 18% of detected ASP.NET sites still emit the X-AspNet-Version header, with a significant portion running actual 4.0.x runtime libraries. The Risk of Information Disclosure According to Wappalyzer

One of the most notorious risks involves . In older versions of ASP.NET, if the enableViewStateMac property is improperly configured (or vulnerable due to the framework version), attackers can craft malicious ViewState data. When the server deserializes this data, it can lead to arbitrary code execution on the server.

Versions prior to ASP.NET 4.5.2 used HMACSHA1 for ViewState integrity. By 2025, SHA1 is considered cryptographically broken. If an application running "4.0.3" has not manually upgraded its machine key configuration, it remains vulnerable to hash collision attacks.

While this header was originally intended for debugging and compatibility purposes, in the hands of a malicious actor, it serves as a reconnaissance tool. It tells the attacker exactly which weapon to select from their arsenal.