Microsoft Details ASP.NET Vulnerability That Enables Attackers To Smuggle HTTP Requests

Microsoft has issued a critical security update for ASP.NET Core to address CVE-2025-55315, a high-severity flaw that enables HTTP request smuggling and could allow attackers to bypass key security controls.

Disclosed on October 14, 2025, this vulnerability has a CVSS v3.1 score of 9.9, making it one of the most severe issues ever reported in the ASP.NET ecosystem.

The flaw stems from inconsistent handling of HTTP requests in the Kestrel web server component, which could let authenticated attackers inject hidden requests to escalate privileges or access sensitive data.

While HTTP request smuggling is a well-known attack vector, this specific implementation in ASP.NET Core amplifies risks for web applications relying on the framework for authentication and authorization.

Attackers exploit discrepancies between how proxies and servers parse headers like Content-Length and Transfer-Encoding, smuggling malicious payloads that evade normal processing.

For instance, a crafted POST request might embed a concealed GET to an admin endpoint, tricking the system into executing unauthorized actions without detection.

Understanding HTTP Request Smuggling

At its core, HTTP request smuggling leverages parsing inconsistencies across network components, such as front-end proxies and back-end servers.

An attacker sends a request with ambiguous headers, like combining Content-Length and Transfer-Encoding, causing the proxy to interpret it one way while the server sees the smuggled content differently.

This can result in the second request bypassing rate limits, CSRF protections, or even authentication checks, leading to severe outcomes in multi-tiered environments.

In the CVE-2025-55315 case, the Kestrel server’s failure to validate request boundaries under certain conditions allows smuggled requests to reach application logic intact.

This affects all supported ASP.NET Core versions, including 8.0, 9.0, and 10.0 previews, particularly in setups with reverse proxies like NGINX or Azure Front Door.

Exploitation requires network access and often low privileges, but the scope can extend to confidential data exposure or server crashes in worst-case scenarios.

The vulnerability’s high score underscores its potential for chained attacks, from session hijacking to server-side request forgery.

Not all applications are equally exposed; risks heighten if custom request parsing, header-based decisions, or skipped validations are in play.

For regulated sectors handling sensitive data, unpatched systems could face compliance violations alongside direct threats like privilege escalation.

Attack Vector	Potential Impact	Depends On
Smuggled login request	Elevation of privilege	App logic trusting headers
Smuggled internal API call	SSRF	App routing and endpoints
Smuggled CSRF bypass	Session hijacking	CSRF token validation
Smuggled injection payload	Code execution	Input sanitization gaps

Mitigations

Microsoft urges immediate patching via the latest .NET updates for affected versions, followed by application restarts.

Developers should audit request-handling code, especially around auth and input validation, while ensuring proxies normalize traffic to block smuggling attempts.

Monitoring logs for anomalous patterns and testing with tools like those simulating HRS can further harden defenses. By applying these measures, organizations can safeguard ASP.NET applications against this pervasive threat landscape.

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