Restrict access to a specific controller by IP address in ASP.NET MVC Beta

I know this is an old question, but I needed to have this functionality today so I implemented it and thought about posting it here.

Using the IPList class from here (http://www.codeproject.com/KB/IP/ipnumbers.aspx)

The filter attribute FilterIPAttribute.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http;
using System.Security.Principal;
using System.Configuration;

namespace Miscellaneous.Attributes.Controller
{

    /// <summary>
    /// Filter by IP address
    /// </summary>
    public class FilterIPAttribute : AuthorizeAttribute
    {

        #region Allowed
        /// <summary>
        /// Comma seperated string of allowable IPs. Example "10.2.5.41,192.168.0.22"
        /// </summary>
        /// <value></value>
        public string AllowedSingleIPs { get; set; }

        /// <summary>
        /// Comma seperated string of allowable IPs with masks. Example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0"
        /// </summary>
        /// <value>The masked I ps.</value>
        public string AllowedMaskedIPs { get; set; }

        /// <summary>
        /// Gets or sets the configuration key for allowed single IPs
        /// </summary>
        /// <value>The configuration key single I ps.</value>
        public string ConfigurationKeyAllowedSingleIPs { get; set; }

        /// <summary>
        /// Gets or sets the configuration key allowed mmasked IPs
        /// </summary>
        /// <value>The configuration key masked I ps.</value>
        public string ConfigurationKeyAllowedMaskedIPs { get; set; }

        /// <summary>
        /// List of allowed IPs
        /// </summary>
        IPList allowedIPListToCheck = new IPList();
        #endregion

        #region Denied
        /// <summary>
        /// Comma seperated string of denied IPs. Example "10.2.5.41,192.168.0.22"
        /// </summary>
        /// <value></value>
        public string DeniedSingleIPs { get; set; }

        /// <summary>
        /// Comma seperated string of denied IPs with masks. Example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0"
        /// </summary>
        /// <value>The masked I ps.</value>
        public string DeniedMaskedIPs { get; set; }


        /// <summary>
        /// Gets or sets the configuration key for denied single IPs
        /// </summary>
        /// <value>The configuration key single I ps.</value>
        public string ConfigurationKeyDeniedSingleIPs { get; set; }

        /// <summary>
        /// Gets or sets the configuration key for denied masked IPs
        /// </summary>
        /// <value>The configuration key masked I ps.</value>
        public string ConfigurationKeyDeniedMaskedIPs { get; set; }

        /// <summary>
        /// List of denied IPs
        /// </summary>
        IPList deniedIPListToCheck = new IPList();
        #endregion


        /// <summary>
        /// Determines whether access to the core framework is authorized.
        /// </summary>
        /// <param name="actionContext">The HTTP context, which encapsulates all HTTP-specific information about an individual HTTP request.</param>
        /// <returns>
        /// true if access is authorized; otherwise, false.
        /// </returns>
        /// <exception cref="T:System.ArgumentNullException">The <paramref name="httpContext"/> parameter is null.</exception>
        protected override bool IsAuthorized(HttpActionContext actionContext)
        {
            if (actionContext == null)
                throw new ArgumentNullException("actionContext");

            string userIpAddress = ((HttpContextWrapper)actionContext.Request.Properties["MS_HttpContext"]).Request.UserHostName;

            try
            {
                // Check that the IP is allowed to access
                bool ipAllowed = CheckAllowedIPs(userIpAddress);

                // Check that the IP is not denied to access
                bool ipDenied = CheckDeniedIPs(userIpAddress);    

                // Only allowed if allowed and not denied
                bool finallyAllowed = ipAllowed && !ipDenied;

                return finallyAllowed;
            }
            catch (Exception e)
            {
                // Log the exception, probably something wrong with the configuration
            }

            return true; // if there was an exception, then we return true
        }

        /// <summary>
        /// Checks the allowed IPs.
        /// </summary>
        /// <param name="userIpAddress">The user ip address.</param>
        /// <returns></returns>
        private bool CheckAllowedIPs(string userIpAddress)
        {
            // Populate the IPList with the Single IPs
            if (!string.IsNullOrEmpty(AllowedSingleIPs))
            {
                SplitAndAddSingleIPs(AllowedSingleIPs, allowedIPListToCheck);
            }

            // Populate the IPList with the Masked IPs
            if (!string.IsNullOrEmpty(AllowedMaskedIPs))
            {
                SplitAndAddMaskedIPs(AllowedMaskedIPs, allowedIPListToCheck);
            }

            // Check if there are more settings from the configuration (Web.config)
            if (!string.IsNullOrEmpty(ConfigurationKeyAllowedSingleIPs))
            {
                string configurationAllowedAdminSingleIPs = ConfigurationManager.AppSettings[ConfigurationKeyAllowedSingleIPs];
                if (!string.IsNullOrEmpty(configurationAllowedAdminSingleIPs))
                {
                    SplitAndAddSingleIPs(configurationAllowedAdminSingleIPs, allowedIPListToCheck);
                }
            }

            if (!string.IsNullOrEmpty(ConfigurationKeyAllowedMaskedIPs))
            {
                string configurationAllowedAdminMaskedIPs = ConfigurationManager.AppSettings[ConfigurationKeyAllowedMaskedIPs];
                if (!string.IsNullOrEmpty(configurationAllowedAdminMaskedIPs))
                {
                    SplitAndAddMaskedIPs(configurationAllowedAdminMaskedIPs, allowedIPListToCheck);
                }
            }

            return allowedIPListToCheck.CheckNumber(userIpAddress);
        }

        /// <summary>
        /// Checks the denied IPs.
        /// </summary>
        /// <param name="userIpAddress">The user ip address.</param>
        /// <returns></returns>
        private bool CheckDeniedIPs(string userIpAddress)
        {
            // Populate the IPList with the Single IPs
            if (!string.IsNullOrEmpty(DeniedSingleIPs))
            {
                SplitAndAddSingleIPs(DeniedSingleIPs, deniedIPListToCheck);
            }

            // Populate the IPList with the Masked IPs
            if (!string.IsNullOrEmpty(DeniedMaskedIPs))
            {
                SplitAndAddMaskedIPs(DeniedMaskedIPs, deniedIPListToCheck);
            }

            // Check if there are more settings from the configuration (Web.config)
            if (!string.IsNullOrEmpty(ConfigurationKeyDeniedSingleIPs))
            {
                string configurationDeniedAdminSingleIPs = ConfigurationManager.AppSettings[ConfigurationKeyDeniedSingleIPs];
                if (!string.IsNullOrEmpty(configurationDeniedAdminSingleIPs))
                {
                    SplitAndAddSingleIPs(configurationDeniedAdminSingleIPs, deniedIPListToCheck);
                }
            }

            if (!string.IsNullOrEmpty(ConfigurationKeyDeniedMaskedIPs))
            {
                string configurationDeniedAdminMaskedIPs = ConfigurationManager.AppSettings[ConfigurationKeyDeniedMaskedIPs];
                if (!string.IsNullOrEmpty(configurationDeniedAdminMaskedIPs))
                {
                    SplitAndAddMaskedIPs(configurationDeniedAdminMaskedIPs, deniedIPListToCheck);
                }
            }

            return deniedIPListToCheck.CheckNumber(userIpAddress);
        }

        /// <summary>
        /// Splits the incoming ip string of the format "IP,IP" example "10.2.0.0,10.3.0.0" and adds the result to the IPList
        /// </summary>
        /// <param name="ips">The ips.</param>
        /// <param name="list">The list.</param>
        private void SplitAndAddSingleIPs(string ips,IPList list)
        {
            var splitSingleIPs = ips.Split(',');
            foreach (string ip in splitSingleIPs)
                list.Add(ip);
        }

        /// <summary>
        /// Splits the incoming ip string of the format "IP;MASK,IP;MASK" example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0" and adds the result to the IPList
        /// </summary>
        /// <param name="ips">The ips.</param>
        /// <param name="list">The list.</param>
        private void SplitAndAddMaskedIPs(string ips, IPList list)
        {
            var splitMaskedIPs = ips.Split(',');
            foreach (string maskedIp in splitMaskedIPs)
            {
                var ipAndMask = maskedIp.Split(';');
                list.Add(ipAndMask[0], ipAndMask[1]); // IP;MASK
            }
        }

        public override void OnAuthorization(AuthorizationContext filterContext)
        {
            base.OnAuthorization(filterContext);
        }
    }
}

Example usage:

1. Directly specifying the IPs in the code

    [FilterIP(
         AllowedSingleIPs="10.2.5.55,192.168.2.2",
         AllowedMaskedIPs="10.2.0.0;255.255.0.0,192.168.2.0;255.255.255.0"
    )]
    public class HomeController {
      // Some code here
    }

2. Or, Loading the configuration from the Web.config

    [FilterIP(
         ConfigurationKeyAllowedSingleIPs="AllowedAdminSingleIPs",
         ConfigurationKeyAllowedMaskedIPs="AllowedAdminMaskedIPs",
         ConfigurationKeyDeniedSingleIPs="DeniedAdminSingleIPs",
         ConfigurationKeyDeniedMaskedIPs="DeniedAdminMaskedIPs"
    )]
    public class HomeController {
      // Some code here
    }


<configuration>
<appSettings>
    <add key="AllowedAdminSingleIPs" value="localhost,127.0.0.1"/> <!-- Example "10.2.80.21,192.168.2.2" -->
    <add key="AllowedAdminMaskedIPs" value="10.2.0.0;255.255.0.0"/> <!-- Example "10.2.0.0;255.255.0.0,192.168.2.0;255.255.255.0" -->
    <add key="DeniedAdminSingleIPs" value=""/>    <!-- Example "10.2.80.21,192.168.2.2" -->
    <add key="DeniedAdminMaskedIPs" value=""/>    <!-- Example "10.2.0.0;255.255.0.0,192.168.2.0;255.255.255.0" -->
</appSettings>
</configuration>

You should have access to the UserHostAddress in the Request object in your controller to do the restriction on. I'd suggest that you may want to extend the AuthorizeAttribute and add your IP address restrictions on it so that you can simply decorate any methods or controllers that need this protection.

I needed a solution for this problem in MVC4 that could handle IPv6 and IP ranges. Also, I needed to use white and black-lists for authorization but also use a regular authorization process when the IP was neither.

This is the solution I came to after taking a lot from @sabbour and @Richard Szalay(How to check a input IP fall in a specific IP range) awesome posts so I post it back here for whomever it may help.

public class MagniAuthorizeAttribute : FilterAttribute, IAuthorizationFilter
{

    #region Allowed

    public bool IsPublic { get; set; }
    /// <summary>
    /// Comma seperated string of allowable IPs. Example "10.2.5.41,192.168.0.22"
    /// </summary>
    /// <value></value>        
    public string AllowedSingleIPs { get; set; }

    /// <summary>
    /// Comma seperated string of allowable IPs with masks. Example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0"
    /// </summary>
    /// <value>The masked I ps.</value>
    public string AllowedIPRanges { get; set; }

    /// <summary>
    /// Gets or sets the configuration key for allowed single IPs
    /// </summary>
    /// <value>The configuration key single I ps.</value>
    public string ConfigurationKeyAllowedSingleIPs { get; set; }

    /// <summary>
    /// Gets or sets the configuration key allowed mmasked IPs
    /// </summary>
    /// <value>The configuration key masked I ps.</value>
    public string ConfigurationKeyAllowedMaskedIPs { get; set; }

    #endregion

    #region Denied
    /// <summary>
    /// Comma seperated string of denied IPs. Example "10.2.5.41,192.168.0.22"
    /// </summary>
    /// <value></value>
    public string DeniedSingleIPs { get; set; }

    /// <summary>
    /// Comma seperated string of denied IPs with masks. Example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0"
    /// </summary>
    /// <value>The masked I ps.</value>
    public string DeniedIPRanges { get; set; }


    /// <summary>
    /// Gets or sets the configuration key for denied single IPs
    /// </summary>
    /// <value>The configuration key single I ps.</value>
    public string ConfigurationKeyDeniedSingleIPs { get; set; }

    /// <summary>
    /// Gets or sets the configuration key for denied masked IPs
    /// </summary>
    /// <value>The configuration key masked I ps.</value>
    public string ConfigurationKeyDeniedMaskedIPs { get; set; }

    #endregion


    /// <summary>
    /// Checks the allowed IPs.
    /// </summary>
    /// <param name="userIpAddress">The user ip address.</param>
    /// <returns></returns>
    private bool CheckAllowedIPs(IPAddress userIpAddress)
    {
        List<IPAddress> allowedIPsToCheck = new List<IPAddress>();
        List<IPAddressRange> allowedIPRangesToCheck = new List<IPAddressRange>();

        // Populate the IPList with the Single IPs
        if (!string.IsNullOrEmpty(AllowedSingleIPs))
        {
            SplitAndAddSingleIPs(AllowedSingleIPs, allowedIPsToCheck);
        }

        // Populate the IPList with the Masked IPs
        if (!string.IsNullOrEmpty(AllowedIPRanges))
        {
            SplitAndAddIPRanges(AllowedIPRanges, allowedIPRangesToCheck);
        }

        // Check if there are more settings from the configuration (Web.config)
        if (!string.IsNullOrEmpty(ConfigurationKeyAllowedSingleIPs))
        {
            string configurationAllowedAdminSingleIPs = ConfigurationManager.AppSettings[ConfigurationKeyAllowedSingleIPs];
            if (!string.IsNullOrEmpty(configurationAllowedAdminSingleIPs))
            {
                SplitAndAddSingleIPs(configurationAllowedAdminSingleIPs, allowedIPsToCheck);
            }
        }

        if (!string.IsNullOrEmpty(ConfigurationKeyAllowedMaskedIPs))
        {
            string configurationAllowedAdminMaskedIPs = ConfigurationManager.AppSettings[ConfigurationKeyAllowedMaskedIPs];
            if (!string.IsNullOrEmpty(configurationAllowedAdminMaskedIPs))
            {
                SplitAndAddIPRanges(configurationAllowedAdminMaskedIPs, allowedIPRangesToCheck);
            }
        }

        return allowedIPsToCheck.Any(a => a.Equals(userIpAddress)) || allowedIPRangesToCheck.Any(a => a.IsInRange(userIpAddress));
    }

    /// <summary>
    /// Checks the denied IPs.
    /// </summary>
    /// <param name="userIpAddress">The user ip address.</param>
    /// <returns></returns>
    private bool CheckDeniedIPs(IPAddress userIpAddress)
    {
        List<IPAddress> deniedIPsToCheck = new List<IPAddress>();
        List<IPAddressRange> deniedIPRangesToCheck = new List<IPAddressRange>();

        // Populate the IPList with the Single IPs
        if (!string.IsNullOrEmpty(DeniedSingleIPs))
        {
            SplitAndAddSingleIPs(DeniedSingleIPs, deniedIPsToCheck);
        }

        // Populate the IPList with the Masked IPs
        if (!string.IsNullOrEmpty(DeniedIPRanges))
        {
            SplitAndAddIPRanges(DeniedIPRanges, deniedIPRangesToCheck);
        }

        // Check if there are more settings from the configuration (Web.config)
        if (!string.IsNullOrEmpty(ConfigurationKeyDeniedSingleIPs))
        {
            string configurationDeniedAdminSingleIPs = ConfigurationManager.AppSettings[ConfigurationKeyDeniedSingleIPs];
            if (!string.IsNullOrEmpty(configurationDeniedAdminSingleIPs))
            {
                SplitAndAddSingleIPs(configurationDeniedAdminSingleIPs, deniedIPsToCheck);
            }
        }

        if (!string.IsNullOrEmpty(ConfigurationKeyDeniedMaskedIPs))
        {
            string configurationDeniedAdminMaskedIPs = ConfigurationManager.AppSettings[ConfigurationKeyDeniedMaskedIPs];
            if (!string.IsNullOrEmpty(configurationDeniedAdminMaskedIPs))
            {
                SplitAndAddIPRanges(configurationDeniedAdminMaskedIPs, deniedIPRangesToCheck);
            }
        }

        return deniedIPsToCheck.Any(a => a.Equals(userIpAddress)) || deniedIPRangesToCheck.Any(a => a.IsInRange(userIpAddress));
    }

    /// <summary>
    /// Splits the incoming ip string of the format "IP,IP" example "10.2.0.0,10.3.0.0" and adds the result to the IPAddress list
    /// </summary>
    /// <param name="ips">The ips.</param>
    /// <param name="list">The list.</param>
    private void SplitAndAddSingleIPs(string ips, List<IPAddress> list)
    {
        var splitSingleIPs = ips.Split(',');
        IPAddress ip;

        foreach (string ipString in splitSingleIPs)
        {
            if(IPAddress.TryParse(ipString, out ip))
                list.Add(ip);
        }
    }

    /// <summary>
    /// Splits the incoming ip ranges string of the format "IP-IP,IP-IP" example "10.2.0.0-10.2.255.255,10.3.0.0-10.3.255.255" and adds the result to the IPAddressRange list
    /// </summary>
    /// <param name="ips">The ips.</param>
    /// <param name="list">The list.</param>
    private void SplitAndAddIPRanges(string ips, List<IPAddressRange> list)
    {
        var splitMaskedIPs = ips.Split(',');
        IPAddress lowerIp;
        IPAddress upperIp;
        foreach (string maskedIp in splitMaskedIPs)
        {
            var ipRange = maskedIp.Split('-');
            if (IPAddress.TryParse(ipRange[0], out lowerIp) && IPAddress.TryParse(ipRange[1], out upperIp))
                list.Add(new IPAddressRange(lowerIp, upperIp));
        }
    }

    protected void HandleUnauthorizedRequest(AuthorizationContext context)
    {
        context.Result = new RedirectToRouteResult(new RouteValueDictionary { { "Controller", "Home" },
                                                                                    { "Action", "Login" },
                                                                                    { "OriginalURL", context.HttpContext.Request.Url.AbsoluteUri } });
    }

    protected bool AuthorizeCore(AuthorizationContext context)
    {
        try
        {
            string userIPString = context.HttpContext.Request.UserHostName;
            IPAddress userIPAddress = IPAddress.Parse(userIPString);

            // Check that the IP is allowed to access
            bool? ipAllowed = CheckAllowedIPs(userIPAddress) ? true : (bool?)null;

            // Check that the IP is not denied to access
            ipAllowed = CheckDeniedIPs(userIPAddress) ? false : ipAllowed;

            if (ipAllowed.HasValue)
            {
                return ipAllowed.Value;
            }

            var serverSession = context.HttpContext.Session;

            UserSession session = null;

            //usersession in server session
            if (serverSession[Settings.HttpContextUserSession] != null)
            {
                session = (UserSession)serverSession[Settings.HttpContextUserSession];
                Trace.TraceInformation($"[{MethodBase.GetCurrentMethod().Name}] UserId:" + session.UserId + ". ClientId: " + session.ClientId);
                return true;
            }

            //usersession in database from cookie
            session = UserSession.GetSession(context.HttpContext.Request.Cookies.Get("sessionId").Value);
            if (session != null)
            {
                Trace.TraceInformation($"[{MethodBase.GetCurrentMethod().Name}] Session found for cookie {context.HttpContext.Request.Cookies.Get("sessionId").Value}");
                serverSession[Settings.HttpContextUserSession] = session;
                Trace.TraceInformation($"[{MethodBase.GetCurrentMethod().Name}] UserId:" + session.UserId + ". ClientId: " + session.ClientId);

                return true;
            }
            else
            {
                Trace.TraceInformation($"[{MethodBase.GetCurrentMethod().Name}] No session found for cookie {serverSession["cookie"]}");
                return false;
            }

        }
        catch (Exception ex)
        {
            Trace.TraceError($"[{MethodBase.GetCurrentMethod().Name}] exception: {ex.Message} - trace {ex.StackTrace}");
            return false;
        }
    }

    public void OnAuthorization(AuthorizationContext actionContext)
    {
        if (IsPublic == false && AuthorizeCore(actionContext) == false)
        {
            HandleUnauthorizedRequest(actionContext);
        }
    }
}

I found sabbour's solution excellent but needed to make two changes to be more useful for my purposes:

1. Allow access if the allowed list is empty. This way you can allow any IP by simply changing the config (e.g. for test deployments) or allow all IPs except explicitly denied ones. To do this I extended IPList to include a Count property and check it as part of CheckAllowedIPs:

   return _allowedIpListToCheck.Count == 0 || _allowedIpListToCheck.CheckNumber(userIpAddress);
   

2. Override HandleUnauthorizedRequest to always return 403. The AuthorizeAtrribute returns 401 by default:

   public override void OnAuthorization(AuthorizationContext actionContext)
   {
       if (AuthorizeCore((HttpContextBase)actionContext.HttpContext))
           return;
       HandleUnauthorizedRequest(actionContext);
   }
   
   protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
   {
           filterContext.Result = new HttpStatusCodeResult(403, "IP Access Denied");
   }
   

Here is the full variation of the FilterIpAttribute class:

public class FilterIpAttribute:AuthorizeAttribute
{

    #region Allowed
    /// <summary>
    /// Comma seperated string of allowable IPs. Example "10.2.5.41,192.168.0.22"
    /// </summary>
    /// <value></value>
    public string AllowedSingleIPs { get; set; }

    /// <summary>
    /// Comma seperated string of allowable IPs with masks. Example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0"
    /// </summary>
    /// <value>The masked I ps.</value>
    public string AllowedMaskedIPs { get; set; }

    /// <summary>
    /// Gets or sets the configuration key for allowed single IPs
    /// </summary>
    /// <value>The configuration key single I ps.</value>
    public string ConfigurationKeyAllowedSingleIPs { get; set; }

    /// <summary>
    /// Gets or sets the configuration key allowed mmasked IPs
    /// </summary>
    /// <value>The configuration key masked I ps.</value>
    public string ConfigurationKeyAllowedMaskedIPs { get; set; }

    /// <summary>
    /// List of allowed IPs
    /// </summary>
    readonly IpList _allowedIpListToCheck = new IpList();
    #endregion

    #region Denied
    /// <summary>
    /// Comma seperated string of denied IPs. Example "10.2.5.41,192.168.0.22"
    /// </summary>
    /// <value></value>
    public string DeniedSingleIPs { get; set; }

    /// <summary>
    /// Comma seperated string of denied IPs with masks. Example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0"
    /// </summary>
    /// <value>The masked I ps.</value>
    public string DeniedMaskedIPs { get; set; }


    /// <summary>
    /// Gets or sets the configuration key for denied single IPs
    /// </summary>
    /// <value>The configuration key single I ps.</value>
    public string ConfigurationKeyDeniedSingleIPs { get; set; }

    /// <summary>
    /// Gets or sets the configuration key for denied masked IPs
    /// </summary>
    /// <value>The configuration key masked I ps.</value>
    public string ConfigurationKeyDeniedMaskedIPs { get; set; }

    /// <summary>
    /// List of denied IPs
    /// </summary>
    readonly IpList _deniedIpListToCheck = new IpList();
    #endregion

    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        if (httpContext == null)
            throw new ArgumentNullException("httpContext");

        string userIpAddress = httpContext.Request.UserIp();

        try
        {
            // Check that the IP is allowed to access
            bool ipAllowed = CheckAllowedIPs(userIpAddress);

            // Check that the IP is not denied to access
            bool ipDenied = CheckDeniedIPs(userIpAddress);

            //Only allowed if allowed and not denied

            bool finallyAllowed = ipAllowed && !ipDenied;


            return finallyAllowed;
        }
        catch (Exception e)
        {
            // Log the exception, probably something wrong with the configuration
        }

        return true; // if there was an exception, then we return true
    }

    /// <summary>
    /// Checks the allowed IPs.
    /// </summary>
    /// <param name="userIpAddress">The user ip address.</param>
    /// <returns></returns>
    private bool CheckAllowedIPs(string userIpAddress)
    {
        // Populate the IPList with the Single IPs
        if (!string.IsNullOrEmpty(AllowedSingleIPs))
        {
            SplitAndAddSingleIPs(AllowedSingleIPs, _allowedIpListToCheck);
        }

        // Populate the IPList with the Masked IPs
        if (!string.IsNullOrEmpty(AllowedMaskedIPs))
        {
            SplitAndAddMaskedIPs(AllowedMaskedIPs, _allowedIpListToCheck);
        }

        // Check if there are more settings from the configuration (Web.config)
        if (!string.IsNullOrEmpty(ConfigurationKeyAllowedSingleIPs))
        {
            string configurationAllowedAdminSingleIPs = ConfigurationManager.AppSettings[ConfigurationKeyAllowedSingleIPs];
            if (!string.IsNullOrEmpty(configurationAllowedAdminSingleIPs))
            {
                SplitAndAddSingleIPs(configurationAllowedAdminSingleIPs, _allowedIpListToCheck);
            }
        }

        if (!string.IsNullOrEmpty(ConfigurationKeyAllowedMaskedIPs))
        {
            string configurationAllowedAdminMaskedIPs = ConfigurationManager.AppSettings[ConfigurationKeyAllowedMaskedIPs];
            if (!string.IsNullOrEmpty(configurationAllowedAdminMaskedIPs))
            {
                SplitAndAddMaskedIPs(configurationAllowedAdminMaskedIPs, _allowedIpListToCheck);
            }
        }

        return _allowedIpListToCheck.Count == 0 || _allowedIpListToCheck.CheckNumber(userIpAddress);
    }

    /// <summary>
    /// Checks the denied IPs.
    /// </summary>
    /// <param name="userIpAddress">The user ip address.</param>
    /// <returns></returns>
    private bool CheckDeniedIPs(string userIpAddress)
    {
        // Populate the IPList with the Single IPs
        if (!string.IsNullOrEmpty(DeniedSingleIPs))
        {
            SplitAndAddSingleIPs(DeniedSingleIPs, _deniedIpListToCheck);
        }

        // Populate the IPList with the Masked IPs
        if (!string.IsNullOrEmpty(DeniedMaskedIPs))
        {
            SplitAndAddMaskedIPs(DeniedMaskedIPs, _deniedIpListToCheck);
        }

        // Check if there are more settings from the configuration (Web.config)
        if (!string.IsNullOrEmpty(ConfigurationKeyDeniedSingleIPs))
        {
            string configurationDeniedAdminSingleIPs = ConfigurationManager.AppSettings[ConfigurationKeyDeniedSingleIPs];
            if (!string.IsNullOrEmpty(configurationDeniedAdminSingleIPs))
            {
                SplitAndAddSingleIPs(configurationDeniedAdminSingleIPs, _deniedIpListToCheck);
            }
        }

        if (!string.IsNullOrEmpty(ConfigurationKeyDeniedMaskedIPs))
        {
            string configurationDeniedAdminMaskedIPs = ConfigurationManager.AppSettings[ConfigurationKeyDeniedMaskedIPs];
            if (!string.IsNullOrEmpty(configurationDeniedAdminMaskedIPs))
            {
                SplitAndAddMaskedIPs(configurationDeniedAdminMaskedIPs, _deniedIpListToCheck);
            }
        }

        return _deniedIpListToCheck.CheckNumber(userIpAddress);
    }

    /// <summary>
    /// Splits the incoming ip string of the format "IP,IP" example "10.2.0.0,10.3.0.0" and adds the result to the IPList
    /// </summary>
    /// <param name="ips">The ips.</param>
    /// <param name="list">The list.</param>
    private void SplitAndAddSingleIPs(string ips, IpList list)
    {
        var splitSingleIPs = ips.Split(',');
        foreach (string ip in splitSingleIPs)
            list.Add(ip);
    }

    /// <summary>
    /// Splits the incoming ip string of the format "IP;MASK,IP;MASK" example "10.2.0.0;255.255.0.0,10.3.0.0;255.255.0.0" and adds the result to the IPList
    /// </summary>
    /// <param name="ips">The ips.</param>
    /// <param name="list">The list.</param>
    private void SplitAndAddMaskedIPs(string ips, IpList list)
    {
        var splitMaskedIPs = ips.Split(',');
        foreach (string maskedIp in splitMaskedIPs)
        {
            var ipAndMask = maskedIp.Split(';');
            list.Add(ipAndMask[0], ipAndMask[1]); // IP;MASK
        }
    }

    public override void OnAuthorization(AuthorizationContext actionContext)
    {
        if (AuthorizeCore((HttpContextBase)actionContext.HttpContext))
            return;
        HandleUnauthorizedRequest(actionContext);
    }

    protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
    {
            filterContext.Result = new HttpStatusCodeResult(403, "IP Access Denied");
    }
}

An extension method to get the User IP as suggested by osa on github

public static class HttpUtils {

    public static string UserIp(this HttpRequestBase request)
    {
        var ip = request["HTTP_X_FORWARDED_FOR"];

        if (!string.IsNullOrWhiteSpace(ip))
        {
            ip = ip.Split(',').Last().Trim();
        }

        if (string.IsNullOrWhiteSpace(ip))
        {
            ip = request.UserHostAddress;
        }

        return ip;
    }
}

And finally the IPList modification (full source is here) :

internal class IpArrayList
{
    //[...]
    public int Count
    {
        get { return _ipNumList.Count; }
    }

 }

public class IpList
{
      //[...]
     public int Count
     {
         get { return _usedList.Count; }
     }
}