Microsoft has released a critical security update addressing a severe vulnerability in ASP.NET Core that could enable attackers to execute HTTP request smuggling attacks.
On October 14, 2025, the company issued patches for CVE-2025-55315, a security feature bypass flaw affecting the Kestrel web server component with an alarming CVSS score of 9.9, placing it in the critical severity category.
| CVE ID | Affected Product | Vulnerability Type | CVSS 3.1 Score | Exploitation Prerequisites |
| CVE-2025-55315 | ASP.NET Core (Kestrel web server) | Security Feature Bypass / HTTP Request Smuggling | 9.9 (Critical) | ASP.NET Core application using Kestrel server with specific config |
Understanding the Critical Threat
The vulnerability stems from improper request parsing within ASP.NET Core’s Kestrel web server, which serves as the foundation for many enterprise applications.
Under certain conditions, Kestrel fails to properly validate request boundaries, creating an opportunity for attackers to inject hidden malicious requests within legitimate traffic.
This security feature bypass can undermine authentication and authorization mechanisms that applications rely on to protect sensitive resources.
HTTP request smuggling exploits inconsistencies between different components in the request processing chain, such as proxies and backend servers.
Attackers manipulate HTTP headers like Content-Length and Transfer-Encoding to disguise a second request within a seemingly normal one.
When the proxy and backend server interpret these headers differently, the hidden request can bypass security controls and reach protected application code undetected.
The 9.9 CVSS score reflects the severity of potential impacts, which include elevation of privilege, server-side request forgery, session hijacking, and in some scenarios, potential code execution.
While not every ASP.NET Core application is vulnerable, the high score accounts for worst-case scenarios to encourage immediate remediation, particularly for applications handling sensitive or highly regulated data.
Consider a scenario where an attacker crafts a request that exploits parsing differences between a proxy server and the Kestrel backend.
By manipulating request headers, the attacker smuggles a hidden request that bypasses normal routing and security checks. This smuggled request might target administrative endpoints, internal APIs, or authentication mechanisms that would typically be protected.
For example, a smuggled login request could lead to privilege escalation if application logic trusts certain headers without proper validation.
Similarly, smuggled calls to internal APIs could enable server-side request forgery, allowing attackers to access resources that should remain isolated.
In cases where CSRF token validation is weak, smuggled requests could facilitate session hijacking. When combined with input sanitization gaps, injection payloads smuggled through this vulnerability could potentially lead to code execution.
Organizations running ASP.NET Core applications should prioritize applying the October 14, 2025 security update immediately.
The vulnerability requires no user interaction and can be exploited remotely over the network with low attack complexity, making it an attractive target for malicious actors.
System administrators should review their ASP.NET Core deployments, especially those using Kestrel in production environments, and implement the patches without delay to maintain robust security postures.
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