This is the Trace Id: 98e96da3c43278037a75af90241cf633
Skip to main content
MSRC

Understanding CVE-2025-55315: What CISOs, security engineers, and sysadmins should know

MSRC
/ By MSRC /

On October 14, 2025, Microsoft released a security update addressing CVE-2025-55315, a vulnerability in ASP.NET Core that allows HTTP request smuggling. While request smuggling is a known technique, this security update addresses a scenario with a high CVSS score to help encourage mitigation actions.

The 9.9 score reflects the importance of promptly addressing this issue, which involves a security feature bypass that can influence how applications enforce authentication and authorization. Applying the patch ensures these protections remain strong.  The vulnerability could allow an attacker to craft a request that hides another request inside it. Depending on how your app processes requests, this could enable techniques like privilege escalation or request manipulation, if left unpatched. While these scenarios depend on specific configurations, applying the update helps maintain robust security for sensitive or regulated environments.

These are serious risks, especially for applications handling sensitive or highly regulated data. While not every app is vulnerable, the CVSS score accounts for the most severe potential impact to encourage timely updates. 

What is HTTP request smuggling?

HTTP request smuggling (HRS) exploits inconsistencies in how different components (like proxies and servers) parse HTTP requests. It typically involves manipulating headers like Content-Length and Transfer-Encoding to inject a hidden request inside another.

Example scenario

Let’s say a proxy and backend server disagree on how to interpret a request:

POST / HTTP/1.1
Host: vulnerable.app
Content-Length: 50
Transfer-Encoding: chunked

0

GET /admin HTTP/1.1
Host: vulnerable.app

This mismatch allows the attacker to “smuggle” a second request (GET /admin) that bypasses normal routing or security checks.

CVE-2025-55315 specifics

In ASP.NET Core, the vulnerability arises from how the Kestrel web server parses incoming requests. Under certain conditions, it fails to properly validate request boundaries, allowing a smuggled request to reach application code.

Impact table 

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

Is my app vulnerable?

It depends. The vulnerability exists in the framework, but exploitation depends on how your app handles requests.  Your app could be impacted if it:

  • Parses raw requests manually

  • Relies on headers for security decisions.

  • Skips validation based on request structure.

If any of these apply, review your configuration and apply the update to maintain strong protections.

What you should do

Microsoft strongly recommends updating affected systems as soon as possible. Here’s how to reduce risk:

  • Review your application’s request handling logic, especially around authentication, authorization, and input validation. 

  • Check your proxy configuration. If you use Azure Front Door, Azure Web Apps, nginx, or other reverse proxies, confirm they normalize requests and detect smuggling attempts.

  • Validate baseline security configurations. Ensure your app doesn’t skip critical checks based on request structure.

  • Monitor for unusual request patterns in logs and telemetry.

Learn more