Critical ASP.NET Vulnerability CVE-2025-55315 Reported, Upgrade Now

Microsoft has recently disclosed a critical vulnerability, CVE-2025-55315, impacting all versions of ASP.NET Core. This vulnerability, with an alarming CVSS score of 9.9, highlights the importance of staying current with your software updates.

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The Technical Details

The vulnerability, which involves inconsistent interpretation of HTTP requests in ASP.NET Core, enables a sophisticated attack known as HTTP Request Smuggling. While HTTP Request Smuggling in ASP.NET Core is a moderate threat, Microsoft's 9.9 CVSS score reflects a deeper concern, which is the potential impact on applications built on top of ASP.NET Core. This is because HTTP Request Smuggling allows an attacker to embed a hidden HTTP request within another. The potential danger lies in what this smuggled request can then achieve targeting applications built on ASP.NET Core.

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Depending on your ASP.NET Core application, HTTP Request Smuggling could potentially lead to:

• Elevation of Privilege (EOP): An attacker logging in as a different user.
• Server-Side Request Forgery (SSRF): Making internal requests within your network.
• Bypassing CSRF Checks: Circumventing cross-site request forgery protections.
• Injection Attacks: Exploiting vulnerabilities through malicious data injection.

Microsoft scores CVEs with the worst-case scenario in mind, where a security feature bypass could fundamentally change the scope of an attack. Hence the 9.9 CVSS score, ASP.NET’s highest CVSS score to date.

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Why a 9.9 Rating is a Red Alert

A CVSS score of 9.9 is just shy of the absolute maximum, 10, signifying a severe threat. This rating indicates that the vulnerability is depending on your ASP.NET application:

• Trivial to Exploit: The vulnerability requires minimal effort to exploit, often without authentication, and with remote access possible.
• Complete Compromise: It's highly likely to result in a complete compromise of confidentiality, integrity, and/or availability of your systems.
• Active Exploitation Risk: Such vulnerabilities often become targets for active exploitation or pose an immediate and severe threat if left unpatched.

When you see a 9.9 rating, it's a clear signal that patching or mitigating this issue should be your absolute top priority.

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Upgrade .NET 8, 9, or 10

The good news is that Microsoft has swiftly addressed this vulnerability, and it is patched in .NET 8 and 9, and in release candidate 2 of .NET 10.

For all .NET developers running ASP.NET applications on these versions, update immediately. Upgrading to the latest supported versions ensures you benefit from the most recent security enhancements and protections.

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For .NET 6

If your environment still runs older .NET versions like .NET 6, act now. HeroDevs has the solution and will keep you secure. While migrating to a newer major release is the long-term fix, we know immediate upgrades aren’t always possible. When you can’t migrate right away, contact HeroDevs for .NET NES (Never-Ending Support). Our NES program for .NET delivers essential security patches and ongoing maintenance for legacy .NET versions, protecting your applications and systems today while you plan and execute a migration to a newer .NET release.

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Undetected Threats

A word of caution. Because Microsoft does not report CVEs on EOL software, such as .NET 6, security scanners and other endpoint management tools may not detect this critical vulnerability on your systems. However, it is crucial to understand that the absence of a CVE detection does not mean the vulnerability is not present. The threat remains, and systems running on unsupported versions of .NET are still exposed to the risks outlined by CVE-2025-55315, making proactive mitigation or upgrade essential.

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Proactive Open Source Security

Staying proactive with your security updates is the best defense against evolving threats. By upgrading to .NET 8, 9, or 10, or by securing extended support for older versions of .NET, you're safeguarding your applications and your users.

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