ASP.NET Core 5-RC1 HTTP Header Injection Vulnerability

ASP.NET Core is a open-source and cross-platform framework for building modern cloud based internet connected applications, such as web apps, IoT apps and mobile backends. ASP.NET Core apps can run on .NET Core or on the full .NET Framework. It was architected to provide an optimized development framework for apps that are deployed to the cloud or run on-premises. [1]

The ASP.NET 5-RC1 Web Application Template is vulnerable to a HTTP Header Injection. Control characters are not filtered in the URL. This allows to send a carriage return character (%0D and %0A) in the ReturnUrl parameter. The content after the carriage return character is injected into the HTTP Response Header.

As a proof of concept, we show how this HTTP header injection can be used to redirect the victim from the destination website to We use the string “/Account/\r\nLocation:” as “ReturnUrl” parameter:

Connection: close
Content-Type: application/x-www-form-urlencoded
Content-Length: 240
Username=[VALID USERNAME]&Password=[VALID PASSWORD]&__RequestVerificationToken=[CUT

The server generates the following response, with the parameter from the GET requested inserted into the HTTP response headers. In this case a manipulated redirection location (“Location:”):

HTTP/1.1 302 Found
Cache-Control: no-cache
Pragma: no-cache
Content-Length: 0
Expires: -1
Server: Microsoft-IIS/8.0
Set-Cookie: .AspNet.Microsoft.AspNet.Identity.Application=[CUT BY COMPASS]; path=/;
secure; httponly
X-Powered-By: ASP.NET
Date: Tue, 12 Apr 2016 13:50:23 GMT
Connection: close

With the same attack, also other HTTP Header values can be manipulated, e.g. it’s possible to set new cookies, or to overwrite existing cookies.

Microsoft patched the software since the version 1.0.0-RC2 of ASP.NET Core. Their bugfix consists of refusing parameters with control characters (ASCII values smaller than 0x20).

Github source code change:

New character check in the code:

+        public static void ValidateHeaderCharacters(string headerCharacters)
+        {
+            if (headerCharacters != null)
+            {
+                foreach (var ch in headerCharacters)
+                {
+                    if (ch < 0x20)
+                    {
+                        throw new InvalidOperationException(string.Format("Invalid control character in header: 0x{0:X2}", (byte)ch));
+                    }
+                }
+            }
+        }

The Compass advisory is available via here

Microsoft paid 5000$ based on their bug bounty program for Microsoft .NET Core: