Microsoft Patches Critical ASP.NET Core Vulnerability Rated Among Most Severe in Company History

Critical Security Flaw Addressed

Microsoft has reportedly fixed what the company describes as one of its “highest ever” rated security vulnerabilities affecting its ASP.NET Core platform, according to recent security advisories. The critical flaw, tracked as CVE-2025-55315, received a severity score of 9.9 out of 10 and affected the Kestrel web server component.

Understanding the HTTP Request Smuggling Vulnerability

Sources indicate the vulnerability represents an HTTP request smuggling bug that enables unauthenticated attackers to embed secondary HTTP requests within legitimate ones. This technique could allow threat actors to bypass various security controls and potentially access sensitive information.

Security analysts suggest that successful exploitation could have severe consequences. “An attacker who successfully exploited this vulnerability could view sensitive information such as other user’s credentials and make changes to file contents on the target server,” Microsoft explained in its security advisory. The report states that attackers might also be able to force server crashes, affecting availability.

Comprehensive Update Strategy

Microsoft has released security updates covering multiple affected platforms, including different versions of the .NET Framework and Visual Studio 2022. Depending on their specific deployment, organizations need to take different approaches to secure their infrastructure.

According to the guidance, users running .NET 8 or later should install updates through Microsoft Update, while those using .NET 2.3 must update package references and recompile applications. The report states that self-contained or single-file applications require additional steps including recompilation and redeployment after applying updates.

Contextual Severity Assessment

Barry Dorrans, .NET security technical program manager at Microsoft, provided context on GitHub regarding the vulnerability’s high severity rating. He noted that while the bug’s score might seem elevated when considered in isolation, the rating reflects potential impacts on applications built using ASP.NET Core.

“We don’t know what’s possible because it’s dependent on how you’ve written your app,” Dorrans explained. “Thus, we score with the worst possible case in mind, a security feature bypass which changes scope.” This approach to vulnerability scoring considers the broad ecosystem of applications that could be affected.

Industry Implications and Response

The discovery and resolution of this critical vulnerability come amid broader industry developments in security practices. As organizations increasingly rely on web applications, vulnerabilities in foundational components like web servers present significant risks.

Security professionals emphasize that timely patching remains crucial, particularly for critical infrastructure components. The incident highlights the ongoing challenges in securing complex software ecosystems against evolving threats, with recent related innovations in security monitoring becoming increasingly important.

As Microsoft continues to address security concerns across its product portfolio, this critical fix represents part of broader efforts to strengthen enterprise security postures. The company’s response to this high-severity vulnerability demonstrates the importance of proactive security maintenance in today’s threat landscape, where market trends increasingly prioritize robust security frameworks.

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