Work with SameSite cookies in ASP.NET Core

By Rick Anderson

SameSite is an IETF draft designed to provide some protection against cross-site request forgery (CSRF) attacks. The SameSite 2019 draft:

  • Treats cookies as SameSite=Lax by default.
  • States cookies that explicitly assert SameSite=None in order to enable cross-site delivery should be marked as Secure.

Lax works for most app cookies. Some forms of authentication like OpenID Connect (OIDC) and WS-Federation default to POST based redirects. The POST based redirects trigger the SameSite browser protections, so SameSite is disabled for these components. Most OAuth logins are not affected due to differences in how the request flows.

The None parameter causes compatibility problems with clients that implemented the prior 2016 draft standard (for example, iOS 12). See Supporting older browsers in this document.

Each ASP.NET Core component that emits cookies needs to decide if SameSite is appropriate.

API usage with SameSite

HttpContext.Response.Cookies.Append defaults to Unspecified, meaning no SameSite attribute added to the cookie and the client will use its default behavior (Lax for new browsers, None for old ones). The following code shows how to change the cookie SameSite value to SameSiteMode.Lax:

HttpContext.Response.Cookies.Append(
                     "name", "value",
                     new CookieOptions() { SameSite = SameSiteMode.Lax });

All ASP.NET Core components that emit cookies override the preceding defaults with settings appropriate for their scenarios. The overridden preceding default values haven't changed.

Component cookie Default
CookieBuilder SameSite Unspecified
Session SessionOptions.Cookie Lax
CookieTempDataProvider CookieTempDataProviderOptions.Cookie Lax
IAntiforgery AntiforgeryOptions.Cookie Strict
Cookie Authentication CookieAuthenticationOptions.Cookie Lax
AddTwitter TwitterOptions.StateCookie Lax
RemoteAuthenticationHandler<TOptions> RemoteAuthenticationOptions.CorrelationCookie None
AddOpenIdConnect OpenIdConnectOptions.NonceCookie None
HttpContext.Response.Cookies.Append CookieOptions Unspecified

ASP.NET Core 3.1 and later provides the following SameSite support:

  • Redefines the behavior of SameSiteMode.None to emit SameSite=None
  • Adds a new value SameSiteMode.Unspecified to omit the SameSite attribute.
  • All cookies APIs default to Unspecified. Some components that use cookies set values more specific to their scenarios. See the table above for examples.

In ASP.NET Core 3.0 and later the SameSite defaults were changed to avoid conflicting with inconsistent client defaults. The following APIs have changed the default from SameSiteMode.Lax to -1 to avoid emitting a SameSite attribute for these cookies:

History and changes

SameSite support was first implemented in ASP.NET Core in 2.0 using the 2016 draft standard. The 2016 standard was opt-in. ASP.NET Core opted-in by setting several cookies to Lax by default. After encountering several issues with authentication, most SameSite usage was disabled.

Patches were issued in November 2019 to update from the 2016 standard to the 2019 standard. The 2019 draft of the SameSite specification:

  • Is not backwards compatible with the 2016 draft. For more information, see Supporting older browsers in this document.
  • Specifies cookies are treated as SameSite=Lax by default.
  • Specifies cookies that explicitly assert SameSite=None in order to enable cross-site delivery should be marked as Secure. None is a new entry to opt out.
  • Is supported by patches issued for ASP.NET Core 2.1, 2.2, and 3.0. ASP.NET Core 3.1 has additional SameSite support.
  • Is scheduled to be enabled by Chrome by default in Feb 2020. Browsers started moving to this standard in 2019.

APIs impacted by the change from the 2016 SameSite draft standard to the 2019 draft standard

Supporting older browsers

The 2016 SameSite standard mandated that unknown values must be treated as SameSite=Strict values. Apps accessed from older browsers which support the 2016 SameSite standard may break when they get a SameSite property with a value of None. Web apps must implement browser detection if they intend to support older browsers. ASP.NET Core doesn't implement browser detection because User-Agents values are highly volatile and change frequently. An extension point in Microsoft.AspNetCore.CookiePolicy allows plugging in User-Agent specific logic.

In Startup.Configure, add code that calls UseCookiePolicy before calling UseAuthentication or any method that writes cookies:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Error");
        app.UseHsts();
    }

    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseCookiePolicy();
    app.UseAuthentication();
    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapRazorPages();
    });
}

In Startup.ConfigureServices, add code similar to the following:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        options.MinimumSameSitePolicy = SameSiteMode.Unspecified;
        options.OnAppendCookie = cookieContext =>
            CheckSameSite(cookieContext.Context, cookieContext.CookieOptions);
        options.OnDeleteCookie = cookieContext =>
            CheckSameSite(cookieContext.Context, cookieContext.CookieOptions);
    });

    services.AddRazorPages();
}

private void CheckSameSite(HttpContext httpContext, CookieOptions options)
{
    if (options.SameSite == SameSiteMode.None)
    {
        var userAgent = httpContext.Request.Headers["User-Agent"].ToString();
        if (MyUserAgentDetectionLib.DisallowsSameSiteNone(userAgent))
        {
            options.SameSite = SameSiteMode.Unspecified;
        }
    }
}
public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        options.MinimumSameSitePolicy = (SameSiteMode)(-1);
        options.OnAppendCookie = cookieContext =>
            CheckSameSite(cookieContext.Context, cookieContext.CookieOptions);
        options.OnDeleteCookie = cookieContext =>
            CheckSameSite(cookieContext.Context, cookieContext.CookieOptions);
    });

    services.AddRazorPages();
}

private void CheckSameSite(HttpContext httpContext, CookieOptions options)
{
    if (options.SameSite == SameSiteMode.None)
    {
        var userAgent = httpContext.Request.Headers["User-Agent"].ToString();
        if (MyUserAgentDetectionLib.DisallowsSameSiteNone(userAgent))
        {
            options.SameSite = (SameSiteMode)(-1);
        }

    }
}

In the preceding sample, MyUserAgentDetectionLib.DisallowsSameSiteNone is a user supplied library that detects if the user agent doesn't support SameSite None:

if (MyUserAgentDetectionLib.DisallowsSameSiteNone(userAgent))
{
    options.SameSite = SameSiteMode.Unspecified;
}

The following code shows a sample DisallowsSameSiteNone method:

Warning

The following code is for demonstration only:

  • It should not be considered complete.
  • It is not maintained or supported.
public static bool DisallowsSameSiteNone(string userAgent)
{
    // Cover all iOS based browsers here. This includes:
    // - Safari on iOS 12 for iPhone, iPod Touch, iPad
    // - WkWebview on iOS 12 for iPhone, iPod Touch, iPad
    // - Chrome on iOS 12 for iPhone, iPod Touch, iPad
    // All of which are broken by SameSite=None, because they use the iOS networking
    // stack.
    if (userAgent.Contains("CPU iPhone OS 12") ||
        userAgent.Contains("iPad; CPU OS 12"))
    {
        return true;
    }

    // Cover Mac OS X based browsers that use the Mac OS networking stack. 
    // This includes:
    // - Safari on Mac OS X.
    // This does not include:
    // - Chrome on Mac OS X
    // Because they do not use the Mac OS networking stack.
    if (userAgent.Contains("Macintosh; Intel Mac OS X 10_14") &&
        userAgent.Contains("Version/") && userAgent.Contains("Safari"))
    {
        return true;
    }

    // Cover Chrome 50-69, because some versions are broken by SameSite=None, 
    // and none in this range require it.
    // Note: this covers some pre-Chromium Edge versions, 
    // but pre-Chromium Edge does not require SameSite=None.
    if (userAgent.Contains("Chrome/5") || userAgent.Contains("Chrome/6"))
    {
        return true;
    }

    return false;
}

Test apps for SameSite problems

Apps that interact with remote sites such as through third-party login need to:

Test web apps using a client version that can opt-in to the new SameSite behavior. Chrome, Firefox, and Chromium Edge all have new opt-in feature flags that can be used for testing. After your app applies the SameSite patches, test it with older client versions, especially Safari. For more information, see Supporting older browsers in this document.

Test with Chrome

Chrome 78+ gives misleading results because it has a temporary mitigation in place. The Chrome 78+ temporary mitigation allows cookies less than two minutes old. Chrome 76 or 77 with the appropriate test flags enabled provides more accurate results. To test the new SameSite behavior toggle chrome://flags/#same-site-by-default-cookies to Enabled. Older versions of Chrome (75 and below) are reported to fail with the new None setting. See Supporting older browsers in this document.

Google does not make older chrome versions available. Follow the instructions at Download Chromium to test older versions of Chrome. Do not download Chrome from links provided by searching for older versions of chrome.

Test with Safari

Safari 12 strictly implemented the prior draft and fails when the new None value is in a cookie. None is avoided via the browser detection code Supporting older browsers in this document. Test Safari 12, Safari 13, and WebKit based OS style logins using MSAL, ADAL or whatever library you are using. The problem is dependent on the underlying OS version. OSX Mojave (10.14) and iOS 12 are known to have compatibility problems with the new SameSite behavior. Upgrading the OS to OSX Catalina (10.15) or iOS 13 fixes the problem. Safari does not currently have an opt-in flag for testing the new spec behavior.

Test with Firefox

Firefox support for the new standard can be tested on version 68+ by opting in on the about:config page with the feature flag network.cookie.sameSite.laxByDefault. There haven't been reports of compatibility issues with older versions of Firefox.

Test with Edge browser

Edge supports the old SameSite standard. Edge version 44 doesn't have any known compatibility problems with the new standard.

Test with Edge (Chromium)

SameSite flags are set on the edge://flags/#same-site-by-default-cookies page. No compatibility issues were discovered with Edge Chromium.

Test with Electron

Versions of Electron include older versions of Chromium. For example, the version of Electron used by Teams is Chromium 66, which exhibits the older behavior. You must perform your own compatibility testing with the version of Electron your product uses. See Supporting older browsers in the following section.

Additional resources