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 :)
