Share via

Registering an Application to a URI Scheme

The About Asynchronous Pluggable Protocols article describes how to develop handlers for Uniform Resource Identifier (URI) schemes. In some cases, it may be desirable to invoke another application to handle a custom URI scheme. To do so, register the existing application as a URI pluggable protocol handler and associate it with the custom URI scheme. Once the application has successfully launched, it can use command-line parameters to retrieve the URI that launched it. These settings apply to pluggable protocol handlers launched from within Windows Internet Explorer and from Windows Explorer using the Run... command (Windows logo key+R).

Security Warning: Applications that handle URI schemes must consider how to respond to malicious data. Because handler applications can receive data from untrusted sources, the URI and other parameter values passed to the application may contain malicious data that attempts to exploit the handling application.

This topic contains the following sections:

  • Registering the Application Handling the Custom URI Scheme
  • Launching the Handler
  • Security Issues
  • Example Pluggable Protocol Handler
  • Related topics

Registering the Application Handling the Custom URI Scheme

To register an application to handle a particular URI scheme, add a new key, along with the appropriate subkeys and values, to HKEY_CLASSES_ROOT. The root key must match the URI scheme that is being added. For instance, to add an "alert:" scheme, add an alert key to HKEY_CLASSES_ROOT, as follows:

      URL Protocol = ""

Under this new key, the URL Protocol string value indicates that this key declares a custom pluggable protocol handler. Without this key, the handler application will not launch. The value should be an empty string.

Keys should also be added for DefaultIcon and shell. The Default string value of the DefaultIcon key must be the file name to use as an icon for this new URI scheme. The string takes the form "path, iconindex" with a maximum length of MAX_PATH. The name of the first key under the shell key should be an action verb, such as open. Under this key, a command key or a DDEEXEC key indicate how the handler should be invoked. The values under the command and DDEEXEC keys describe how to launch the application handling the new protocol.

Finally, the Default string value should contain the display name of the new URI scheme. The following example shows how to register an application, alert.exe in this case, to handle the alert scheme.

      (Default) = "URL:Alert Protocol"
      URL Protocol = ""
         (Default) = "alert.exe,1"
               (Default) = "C:\Program Files\Alert\alert.exe" "%1"

When a user clicks a link containing your custom URI scheme, Windows Internet Explorer launches the pluggable protocol handler registered for that URI scheme. If the specified open command specified in the registry contains a %1 parameter, Internet Explorer passes the URI to the registered pluggable protocol handler application.

Launching the Handler

By adding the above settings to the registry, navigating to URIs such as alert:Hello%20World would cause an attempt to launch alert.exe with the complete URI on the command line. Internet Explorer percent-decodes the URI, but the Windows Run... command does not. If a URI contains percent-encoded spaces, it may be split across more than one argument on the command line.

For example, if the link above is followed through Internet Explorer, the command line would be:

"C:\Program Files\Alert\alert.exe" "alert:Hello World"

If this link is followed through Windows Explorer, the Windows Run command, or some other application, the command line would be:

"C:\Program Files\Alert\alert.exe" "alert:Hello%20World"

Because Internet Explorer will decode all percent-encoded octets in the URI before passing the resulting string to ShellExecute, URIs such as alert:%3F? will be given to the alert application pluggable protocol handler as alert:??. The handler won't know that the first question mark was percent-encoded. To avoid this issue, pluggable protocol handlers and their associated URI scheme must not rely on encoding. If encoding is necessary, protocol handlers should use another type of encoding that is compatible with URI syntax, such as Base64 encoding. Double percent-encoding is not a good solution either; if the application protocol URI isn't processed by Internet Explorer, it will not be decoded.

When ShellExecute executes the pluggable protocol handler with a stringon the command line, any non-encoded spaces, quotes, and backslashes in the URI will be interpreted as part of the command line. This means that if you use C/C++'s argc and argv to determine the arguments passed to your application, the string may be broken across multiple parameters. To mitigate this issue:

  • Avoid spaces, quotes, or backslashes in your URI
  • Quote the %1 in the registration ("%1" as written in the 'alert' example registration)

However, avoidance doesn't completely solve the problem of quotes in the URI or a backslash at the end of the URI.

Security Issues

As noted above, the string that is passed to a pluggable protocol handler might be broken across multiple parameters. Malicious parties could use additional quote or backslash characters to pass additional command line parameters. For this reason, pluggable protocol handlers should assume that any parameters on the command line could come from malicious parties, and carefully validate them. Applications that could initiate dangerous actions based on external data must first confirm those actions with the user. In addition, handling applications should be tested with URIs that are overly long or contain unexpected (or undesirable) character sequences.

For more information, please see Writing Secure Code.

Example Pluggable Protocol Handler

The following sample code contains a simple C# console application demonstrating one way to implement a pluggable protocol handler for the alert URI scheme.

using System;
using System.Collections.Generic;
using System.Text;

namespace Alert
  class Program
    static string ProcessInput(string s)
       // TODO Verify and validate the input 
       // string as appropriate for your application.
       return s;

    static void Main(string[] args)
      Console.WriteLine("Alert.exe invoked with the following parameters.\r\n");
      Console.WriteLine("Raw command-line: \n\t" + Environment.CommandLine);

      foreach (string s in args)
        Console.WriteLine("\t" + ProcessInput(s));
      Console.WriteLine("\nPress any key to continue...");

When invoked with the URI alert:"Hello%20World" (note extra quotes) from Internet Explorer, the program responds with:

Alert.exe invoked with the following parameters.

Raw command-line:
        "C:\Program Files\Alert\alert.exe" "alert:"Hello World""



Press any key to continue...


About Asynchronous Pluggable Protocols

Debugging Tips