In June 2025, a security vulnerability tracked as CVE-2025-55315 was reported in ASP.NET Core. This post aims to explain the flaw, how it works, and what you can do about it. We’ll look at easy-to-understand examples and include relevant code snippets, references, and even a simple exploit demonstration (for educational purposes only).

🔍 What is CVE-2025-55315?

CVE-2025-55315 is a HTTP request/response smuggling vulnerability in ASP.NET Core. In simple terms, the ASP.NET Core server and the proxy/load balancer in front of it interpret incoming HTTP requests differently. This allows an attacker (who is already authenticated) to bypass certain security checks over the network.

When this happens, attackers can sneak malicious requests past firewalls or authorization layers by manipulating HTTP headers, and make the backend server process requests that should’ve been blocked.

📝 How Does HTTP Request Smuggling Work?

The core of request smuggling arises from how different systems (like proxies and web servers) parse HTTP requests slightly differently. If you, the attacker, spot this inconsistency—maybe the proxy trusts one header but the backend trusts another—you can send specially-crafted requests that confuse both.

Below is a simple example (Startup.cs) of a vulnerable ASP.NET Core application

public void Configure(IApplicationBuilder app)
{
    app.Use(async (context, next) =>
    {
        if (!context.User.Identity.IsAuthenticated)
        {
            context.Response.StatusCode = 403;
            await context.Response.WriteAsync("Forbidden");
        }
        else
        {
            await next.Invoke();
        }
    });

    app.Run(async (context) =>
    {
        await context.Response.WriteAsync("Welcome, authenticated user!");
    });
}

*This middleware checks if the request is authenticated and blocks it otherwise. But if HTTP request smuggling is possible, an attacker can piggyback unauthorized requests within a single HTTP pipeline, bypassing this check.*

⚡ Simple Exploit Explained [EDUCATIONAL ONLY]

Let’s say there’s a reverse proxy (like NGINX) in front of your ASP.NET Core app. The proxy looks for Content-Length headers to determine the end of a request. What if the proxy checks the first Content-Length: 10, and the backend app checks the second Content-Length: 5? The attacker can submit something like:

POST / HTTP/1.1
Host: victim.com
Content-Length: 10
Content-Length: 5

hello
POST /admin HTTP/1.1
Host: victim.com

upgrade

- The proxy interprets the first POST (hello\nPOST /admin HTTP/1.1...) and forwards the rest as a new request.

The backend takes the hidden second request as a separate, unauthenticated request.

Result: The /admin endpoint is accessed without proper security checks.

🔗 References

- Microsoft Security Advisory on Request Smuggling (2022, similar context)
- HTTP Request Smuggling Explained by PortSwigger
- OWASP: HTTP Request Smuggling

Keep your ASP.NET Core up-to-date: Apply latest security patches that close CVE-2025-55315.

- Use a single HTTP message parser: Make sure both proxy and backend use the exact same parsing rules (prefer RFC-compliant parsers).
- Disable ambiguous headers: Configure your reverse proxy to reject requests with duplicated or conflicting critical headers (like Content-Length).
- Enable strict logging: Audit all anomalous HTTP requests, especially with unusual headers or formatting.

Example: Harden NGINX Proxy Config

server {
    ...
    ignore_invalid_headers on;
    proxy_set_header Connection "";
    ...
}

✅ Conclusion

CVE-2025-55315 is a serious flaw, but one you can address with awareness and patching. HTTP request smuggling is not unique to ASP.NET Core, but the way your stack parses HTTP really matters. Review your setup and security posture ASAP!


🚩 Stay safe! Always test in a lab. Never exploit vulnerabilities on systems you don’t own or without permission.

*Exclusive content by ChatGPT, June 2025—simplified for everyone to understand.*

Timeline

Published on: 10/14/2025 17:15:44 UTC
Last modified on: 12/11/2025 19:34:58 UTC