CODEWITHSUNDEEP
c#asp.net-corecookies
GrayCat
GrayCat

Reputation: 1797

SameSite Cookie attribute ommited by ASP.NET Core

I am trying to explicitly set SameCookie attribute of the cookie with ASP.NET Core to None.

The way I tried to do this was to set property value of CookieOptions like this:

var options = new CookieOptions
{
    SameSite = SameSiteMode.None
};

(other attributes omitted for brevity)

However when I examine server response headers (where server is supposed to set the cookie with SameSite=None) I can see SameSite is omitted. On the contrary I can see Value, Expires, Path even Secure stated explicitly.

If I set SameSite in C# code to Lax or Strict I can see it explicitly included in Set-Cookie header. If I set it to None - I cannot.

I did check on two browsers - Firefox and Chrome 77 (I am aware of changes that this version introduces to SameSite).

There is a hack to include SameSite=None. You just need to add following line to Path property of CookieOptions:

options.Path += "; samesite=None";

Then it can be found in Set-Cookie header of the response.

Is there a way to configure Kestrel (no IIS used for hosting, bare Kestrel) to include SameSite=None in headers without hacking it like this?

Upvotes: 9

Views: 25854

Answers (5)

Arquentis
Arquentis

Reputation: 11

Using Microsoft.Net.Http.Headers 2.2.8 fixed the problem for me. Currently using target framework: .Net Core 2.2 for the project.

Upvotes: 1

Phil Dennis
Phil Dennis

Reputation: 381

The approach outlined by Charles Chen - using a handler to make a copy of each cookie with SameSite=None and Secure set - has the advantage of being unobtrusive to implement, combined with a simple approach to compatibility with browsers which do not support SameSite=None correctly. For my situation - supporting an older .NET version - the approach is a life-saver, however when attempting to use Charles' code, I ran into a few issues which prevented it from working for me "as is".

Here is updated code, which addresses the issues I ran into:

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Web;

namespace SameSiteHttpModule
{
    public class SameSiteModule : IHttpModule
    {
        // Suffix includes a randomly generated code to minimize possibility of cookie copies colliding with original names
        private const string SuffixForCookieCopy = "-same-site-j4J6bSt0";
        private Regex _cookieNameRegex;
        private Regex _cookieSameSiteAttributeRegex;
        private Regex _cookieSecureAttributeRegex;

        /// <inheritdoc />
        /// <summary>
        ///     Set up the event handlers.
        /// </summary>
        public void Init(HttpApplication context)
        {
            // Initialize regular expressions used for making a cookie copy
            InitializeMatchExpressions();

            // This one is the OUTBOUND side; we add the extra cookies
            context.PreSendRequestHeaders += OnPreSendRequestHeaders;

            // This one is the INBOUND side; we coalesce the cookies
            context.BeginRequest += OnBeginRequest;
        }

        /// <summary>
        ///     The OUTBOUND LEG; we add the extra cookie
        /// </summary>
        private void OnPreSendRequestHeaders(object sender, EventArgs e)
        {
            var application = (HttpApplication) sender;
            var response = application.Context.Response;
            var cookieCopies = CreateCookieCopiesToSave(response);
            SaveCookieCopies(response, cookieCopies);
        }

        /// <summary>
        ///     The INBOUND LEG; we coalesce the cookies
        /// </summary>
        private void OnBeginRequest(object sender, EventArgs e)
        {
            var application = (HttpApplication) sender;
            var request = application.Context.Request;
            var cookiesToRestore = CreateCookiesToRestore(request);
            RestoreCookies(request, cookiesToRestore);
        }

        #region Supporting code for saving cookies

        private IEnumerable<string> CreateCookieCopiesToSave(HttpResponse response)
        {
            var cookieStrings = response.Headers.GetValues("set-cookie") ?? new string[0];
            var cookieCopies = new List<string>();

            foreach (var cookieString in cookieStrings)
            {
                bool createdCopy;
                var cookieStringCopy = TryMakeSameSiteCookieCopy(cookieString, out createdCopy);
                if (!createdCopy) continue;
                cookieCopies.Add(cookieStringCopy);
            }

            return cookieCopies;
        }

        private static void SaveCookieCopies(HttpResponse response, IEnumerable<string> cookieCopies)
        {
            foreach (var cookieCopy in cookieCopies)
            {
                response.Headers.Add("set-cookie", cookieCopy);
            }
        }

        private void InitializeMatchExpressions()
        {
            _cookieNameRegex = new Regex(@"
                (?'prefix'          # Group 1: Everything prior to cookie name
                    ^\s*                # Start of value followed by optional whitespace
                )
                (?'cookie_name'     # Group 2: Cookie name
                    [^\s=]+             # One or more characters that are not whitespace or equals
                )            
                (?'suffix'          # Group 3: Everything after the cookie name
                    .*$                 # Arbitrary characters followed by end of value
                )",
                RegexOptions.IgnoreCase | RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);

            _cookieSameSiteAttributeRegex = new Regex(@"
                (?'prefix'          # Group 1: Everything prior to SameSite attribute value
                    ^.*                 # Start of value followed by 0 or more arbitrary characters
                    ;\s*                # Semicolon followed by optional whitespace
                    SameSite            # SameSite attribute name
                    \s*=\s*             # Equals sign (with optional whitespace around it)
                )
                (?'attribute_value' # Group 2: SameSite attribute value
                    [^\s;]+             # One or more characters that are not whitespace or semicolon
                )
                (?'suffix'          # Group 3: Everything after the SameSite attribute value
                    .*$                 # Arbitrary characters followed by end of value
                )",
                RegexOptions.IgnoreCase | RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);

            _cookieSecureAttributeRegex = new Regex(@"
                ;\s*                # Semicolon followed by optional whitespace
                Secure              # Secure attribute value
                \s*                 # Optional whitespace
                (?:;|$)             # Semicolon or end of value",
                RegexOptions.IgnoreCase | RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);
        }

        private string TryMakeSameSiteCookieCopy(string cookie, out bool success)
        {
            if (!AddNameSuffix(ref cookie))
            {
                // could not add the name suffix so unable to copy cookie (generally should not happen)
                success = false;
                return null;
            }

            var addedSameSiteNone = AddSameSiteNone(ref cookie);
            var addedSecure = AddSecure(ref cookie);

            if (!addedSameSiteNone && !addedSecure)
            {
                // cookie already has SameSite and Secure attributes so don't make copy
                success = false;
                return null;
            }

            success = true;
            return cookie;
        }

        private bool AddNameSuffix(ref string cookie)
        {
            var match = _cookieNameRegex.Match(cookie);
            if (!match.Success)
            {
                // Could not find the cookie name in order to modify it
                return false;
            }

            var groups = match.Groups;
            var nameForCopy = groups["cookie_name"] + SuffixForCookieCopy;
            cookie = string.Concat(groups["prefix"].Value, nameForCopy, groups["suffix"].Value);
            return true;
        }

        private bool AddSameSiteNone(ref string cookie)
        {
            var match = _cookieSameSiteAttributeRegex.Match(cookie);
            if (!match.Success)
            {
                cookie += "; SameSite=None";
                return true;
            }

            var groups = match.Groups;

            if (groups["attribute_value"].Value.Equals("None", StringComparison.OrdinalIgnoreCase))
            {
                // SameSite=None is already present, so we will not add it
                return false;
            }

            // Replace existing SameSite value with "None"
            cookie = string.Concat(groups["prefix"].Value, "None", groups["suffix"].Value);
            return true;
        }

        private bool AddSecure(ref string cookie)
        {
            if (_cookieSecureAttributeRegex.IsMatch(cookie))
            {
                // Secure is already present so we will not add it
                return false;
            }

            cookie += "; Secure";
            return true;
        }

        #endregion

        #region Supporting code for restoring cookies

        private static IEnumerable<HttpCookie> CreateCookiesToRestore(HttpRequest request)
        {
            var cookiesToRestore = new List<HttpCookie>();

            for (var i = 0; i < request.Cookies.Count; i++)
            {
                var inboundCookie = request.Cookies[i];
                if (inboundCookie == null) continue;

                var cookieName = inboundCookie.Name;

                if (!cookieName.EndsWith(SuffixForCookieCopy, StringComparison.OrdinalIgnoreCase))
                {
                    continue; // Not interested in this cookie since it is not a copied cookie.
                }

                var originalName = cookieName.Substring(0, cookieName.Length - SuffixForCookieCopy.Length);

                if (request.Cookies[originalName] != null)
                {
                    continue; // We have the original cookie, so we are OK; just continue.
                }

                cookiesToRestore.Add(new HttpCookie(originalName, inboundCookie.Value));
            }

            return cookiesToRestore;
        }

        private static void RestoreCookies(HttpRequest request, IEnumerable<HttpCookie> cookiesToRestore)
        {
            // We need to inject cookies as if they were the original.
            foreach (var cookie in cookiesToRestore)
            {
                // Add to the cookie header for non-managed modules
                // https://support.microsoft.com/en-us/help/2666571/cookies-added-by-a-managed-httpmodule-are-not-available-to-native-ihtt
                if (request.Headers["cookie"] == null)
                {
                    request.Headers.Add("cookie", $"{cookie.Name}={cookie.Value}");
                }
                else
                {
                    request.Headers["cookie"] += $"; {cookie.Name}={cookie.Value}";
                }

                // Also add to the request cookies collection for managed modules.
                request.Cookies.Add(cookie);
            }
        }

        #endregion

        public void Dispose()
        {
        }
    }
}

Some concerns handed by this code:

  • Copied cookies preserve attributes, such as Path and Expires which can be necessary for correct functioning of sites.
  • When restoring cookies, in addition to adding to the Cookie header, they are added to the .NET HttpRequest.Cookies collection, which is necessary, for example to avoid losing the ASP.NET session.
  • When restoring cookies, avoids the possibility of creating a duplicate Cookie header, which would be contrary to RFC 6265 and can cause problems with applications.

Some options for deployment:

  • Add code for handler to an existing application
  • Compile to a DLL for deployment to an application's bin folder
  • Compile to a DLL and add to the GAC

Configuration (e.g. for web.config):

<system.webServer>
  ...
  <modules>
    <add name="SameSiteModule" type="SameSiteHttpModule.SameSiteModule, CustomSameSiteModule" />

p.s. Charles, I'm a fan of var, sorry :)

Upvotes: 3

Charles Chen
Charles Chen

Reputation: 1565

For anyone that may need a side-loaded option, I've written, tested, and released a simple solution which plugs into the IIS HTTP request pipeline as an IHttpModule. The solution basically adds the cookie twice: one with SameSite, once without. This provides 100% browser compatibility as the browsers that understand SameSite=None; Secure use that one while the browsers that do not understand it will use the normal cookie. This is a solution originally proposed by Google themselves and implemented by Auth0 for their product (in a different form).

The gist of the code is below:

using System;
using System.Linq;
using System.Web;

namespace SameSiteHttpModule
{
    public class SameSiteDoomsdayModule : IHttpModule
    {
        /// <summary>
        ///     Set up the event handlers.
        /// </summary>
        public void Init(HttpApplication context)
        {
            // This one is the OUTBOUND side; we add the extra cookie
            context.PreSendRequestHeaders += OnEndRequest;

            // This one is the INBOUND side; we coalesce the cookies.
            context.BeginRequest += OnBeginRequest;
        }

        /// <summary>
        ///     The OUTBOUND LEG; we add the extra cookie.
        /// </summary>
        private void OnEndRequest(object sender, EventArgs e)
        {
            HttpApplication application = (HttpApplication)sender;

            HttpContext context = application.Context;

            // IF NEEDED: Add URL filter here

            for (int i = 0; i < context.Response.Cookies.Count; i++)
            {
                HttpCookie responseCookie = context.Response.Cookies[i];

                context.Response.Headers.Add("Set-Cookie", $"{responseCookie.Name}-same-site={responseCookie.Value};SameSite=None; Secure");
            }
        }

        /// <summary>
        ///     The INBOUND LEG; we coalesce the cookies.
        /// </summary>
        private void OnBeginRequest(object sender, EventArgs e)
        {
            HttpApplication application = (HttpApplication)sender;

            HttpContext context = application.Context;

            // IF NEEDED: Add URL filter here

            string[] keys = context.Request.Cookies.AllKeys;

            for (int i = 0; i < context.Request.Cookies.Count; i++)
            {
                HttpCookie inboundCookie = context.Request.Cookies[i];

                if (!inboundCookie.Name.Contains("-same-site"))
                {
                    continue; // Not interested in this cookie.
                }

                // Check to see if we have a root cookie without the -same-site
                string actualName = inboundCookie.Name.Replace("-same-site", string.Empty);

                if (keys.Contains(actualName))
                {
                    continue; // We have the actual key, so we are OK; just continue.
                }

                // We don't have the actual name, so we need to inject it as if it were the original
                // https://support.microsoft.com/en-us/help/2666571/cookies-added-by-a-managed-httpmodule-are-not-available-to-native-ihtt
                // HttpCookie expectedCookie = new HttpCookie(actualName, inboundCookie.Value);
                context.Request.Headers.Add("Cookie", $"{actualName}={inboundCookie.Value}");
            }
        }

        public void Dispose()
        {

        }
    }
}

This gets installed like any other HTTP module:

<?xml version="1.0" encoding="utf-8"?>
<configuration>    
    <system.webServer>
        <modules>
            <add type="SameSiteHttpModule.SameSiteDoomsdayModule, SameSiteHttpModule" name="SameSiteDoomsdayModule"/>
        </modules>
        <handlers>        
            <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
        </handlers>
        <aspNetCore processPath=".\IC.He.IdentityServices.exe" arguments="" forwardWindowsAuthToken="false" requestTimeout="00:10:00" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" />
    </system.webServer>
</configuration>

You can find more info here: https://charliedigital.com/2020/01/22/adventures-in-single-sign-on-samesite-doomsday/

It will provide the fix for ANY .NET version, ANY .NET Core version, ANY scenario whether you own the original source code or not.

Upvotes: 2

Xavierh
Xavierh

Reputation: 476

The issue is now fixed with latest release of .NET Framework and .NET Core.

As I already posted in this other post https://stackoverflow.com/a/58998232/906046, the cookie options SameSiteMode.None is now working as intended.

Upvotes: 3

rowan_m
rowan_m

Reputation: 3050

It looks like the issue is that while the SameSite Enum has a None value that's interpreted as the default value of simply not providing a SameSite attribute. You can see this in the code for SetCookieHeaderValue which only has token values for Strict and Lax.

To set a SameSite=None; Secure cookie you should send the Set-Cookie header yourself.

(Side note: I'll try to sort out a pull request for the core to add proper None support)

Upvotes: 3

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