diff --git a/src/Bifrost.Web/Features/ConceptVotes/VoterKeyResolver.cs b/src/Bifrost.Web/Features/ConceptVotes/VoterKeyResolver.cs
index ce9fbec..2948700 100644
--- a/src/Bifrost.Web/Features/ConceptVotes/VoterKeyResolver.cs
+++ b/src/Bifrost.Web/Features/ConceptVotes/VoterKeyResolver.cs
@@ -1,4 +1,5 @@
 using System.Security.Cryptography;
+using Bifrost.Web.Features.Shared;
 using Microsoft.AspNetCore.DataProtection;
 
 namespace Bifrost.Web.Features.ConceptVotes;
@@ -7,8 +8,8 @@ namespace Bifrost.Web.Features.ConceptVotes;
 // decides the voter key and class. The cookie holds a protected random id, never a count, class or account.
 public sealed class VoterKeyResolver(IDataProtectionProvider dataProtection)
 {
-    public const string CookieName = "bifrost_voter";
-    public const string CookiePath = "/mill";
+    public const string CookieName = BifrostCookies.Voter;
+    public const string CookiePath = BifrostCookies.Path;
     public static readonly TimeSpan CookieLifetime = TimeSpan.FromDays(365);
 
     private const string Purpose = "Bifrost.ConceptVotes.DeviceToken.v1";
diff --git a/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs b/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
index 3e0b3a1..0d39c2f 100644
--- a/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
+++ b/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
@@ -446,13 +446,13 @@ public sealed record HeimdallAccount(
 
 /// <summary>
 /// The attempt cookies: a ring of <see cref="SlotNames"/> fixed names, so the Cookie header a browser sends to
-/// /auth/heimdall is bounded by construction (a worst-case attempt is about 2.4 KB; nginx's line limit is 8 KB).
+/// every path is bounded by construction (the __Host- prefix forces Path=/; a worst-case attempt is about 2.4 KB, nginx's line limit is 8 KB).
 /// A start takes a free slot or overwrites the one expiring first; an overwritten attempt finds no cookie and
 /// is refused. The attempt's nonce lives inside the protected value, never in the cookie name.
 /// </summary>
 public static class HeimdallAttemptCookie
 {
-    public const string Prefix = "bifrost_heimdall_";
+    public const string Prefix = BifrostCookies.HeimdallAttemptPrefix;
     public const int SlotCount = 2;
 
     public static readonly TimeSpan Lifetime = TimeSpan.FromMinutes(10);
@@ -465,13 +465,14 @@ public static class HeimdallAttemptCookie
         HttpOnly = true,
         Secure = true,
         SameSite = SameSiteMode.Lax,
-        Path = "/auth/heimdall",
+        Path = BifrostCookies.Path,
         MaxAge = Lifetime
     };
 
     /// <summary>
-    /// The options a deletion carries: only the attempt cookie's Path, which a cookie is matched on. It carries no
-    /// Max-Age, which would take precedence over the expiry Delete writes and leave the emptied cookie alive.
+    /// The options a deletion carries: the attempt cookie's Path, which a cookie is matched on, and Secure, without
+    /// which a browser ignores a Set-Cookie for a __Host- name. It carries no Max-Age, which would take precedence
+    /// over the expiry Delete writes and leave the emptied cookie alive.
     /// </summary>
-    public static CookieOptions DeleteOptions() => new() { Path = "/auth/heimdall" };
+    public static CookieOptions DeleteOptions() => new() { Path = BifrostCookies.Path, Secure = true };
 }
diff --git a/src/Bifrost.Web/Features/Shared/BifrostCookies.cs b/src/Bifrost.Web/Features/Shared/BifrostCookies.cs
new file mode 100644
index 0000000..1c099e8
--- /dev/null
+++ b/src/Bifrost.Web/Features/Shared/BifrostCookies.cs
@@ -0,0 +1,17 @@
+namespace Bifrost.Web.Features.Shared;
+
+// Every cookie Bifrost sets carries the __Host- prefix: the browser then accepts it only with Secure, Path=/ and no
+// Domain, so no sibling subdomain of gamecult.org (the preview host is same-site) can plant or shadow one by setting
+// a Domain=gamecult.org cookie of the same name. The prefix forbids a narrower Path, so these all live at Path=/.
+public static class BifrostCookies
+{
+    public const string HostPrefix = "__Host-";
+
+    public const string Session = HostPrefix + "bifrost_session";
+    public const string OAuthCorrelation = HostPrefix + "bifrost_oauth_correlation";
+    public const string Antiforgery = HostPrefix + "bifrost_antiforgery";
+    public const string Voter = HostPrefix + "bifrost_voter";
+    public const string HeimdallAttemptPrefix = HostPrefix + "bifrost_heimdall_";
+
+    public const string Path = "/";
+}
diff --git a/src/Bifrost.Web/Program.cs b/src/Bifrost.Web/Program.cs
index d0656d3..c8a524a 100644
--- a/src/Bifrost.Web/Program.cs
+++ b/src/Bifrost.Web/Program.cs
@@ -76,6 +76,13 @@ builder.Services
     .AddOptions<ConceptVoteOptions>()
     .BindConfiguration(ConceptVoteOptions.SectionName);
 
+builder.Services.AddAntiforgery(options =>
+{
+    options.Cookie.Name = BifrostCookies.Antiforgery;
+    options.Cookie.SecurePolicy = CookieSecurePolicy.Always;
+    options.Cookie.Path = BifrostCookies.Path;
+});
+
 builder.Services.AddRazorPages(options =>
 {
     options.Conventions.AuthorizePage("/Membership/Status");
@@ -167,6 +174,10 @@ var authenticationBuilder = builder.Services
     .AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
     .AddCookie(options =>
     {
+        options.Cookie.Name = BifrostCookies.Session;
+        options.Cookie.SecurePolicy = CookieSecurePolicy.Always;
+        options.Cookie.Path = BifrostCookies.Path;
+        options.Cookie.Domain = null;
         options.LoginPath = "/auth/sign-in";
         options.AccessDeniedPath = "/Membership/Status";
         options.SlidingExpiration = true;
@@ -183,6 +194,9 @@ if (configuredGitHubOAuthOptions.IsConfigured)
         options.TokenEndpoint = "https://github.com/login/oauth/access_token";
         options.UserInformationEndpoint = "https://api.github.com/user";
         options.SaveTokens = true;
+        options.CorrelationCookie.Name = BifrostCookies.OAuthCorrelation;
+        options.CorrelationCookie.SecurePolicy = CookieSecurePolicy.Always;
+        options.CorrelationCookie.Path = BifrostCookies.Path;
         options.Scope.Add("read:user");
         options.Scope.Add("user:email");
         options.ClaimActions.MapJsonKey(ClaimTypes.NameIdentifier, "id");
diff --git a/tests/Bifrost.Web.Tests/ConceptVotePageTests.cs b/tests/Bifrost.Web.Tests/ConceptVotePageTests.cs
index 5434e8d..d0675d7 100644
--- a/tests/Bifrost.Web.Tests/ConceptVotePageTests.cs
+++ b/tests/Bifrost.Web.Tests/ConceptVotePageTests.cs
@@ -1,3 +1,4 @@
+using Bifrost.Web.Features.Shared;
 using System.Net;
 using System.Text.RegularExpressions;
 using Bifrost.Web.Data;
@@ -14,7 +15,7 @@ namespace Bifrost.Web.Tests;
 
 public sealed partial class ConceptVotePageTests : IClassFixture<TestWebApplicationFactory>
 {
-    private const string CookieName = "bifrost_voter";
+    private const string CookieName = BifrostCookies.Voter;
     private const string Purpose = "Bifrost.ConceptVotes.DeviceToken.v1";
 
     private readonly TestWebApplicationFactory _factory;
@@ -40,7 +41,7 @@ public sealed partial class ConceptVotePageTests : IClassFixture<TestWebApplicat
         Assert.Contains("httponly", attributes);
         Assert.Contains("secure", attributes);
         Assert.Contains("samesite=lax", attributes);
-        Assert.Contains("path=/mill", attributes);
+        Assert.Contains("path=/", attributes);
         Assert.Contains("max-age=31536000", attributes);
 
         using (var page = await visitor.Get(collection, "alpha"))
diff --git a/tests/Bifrost.Web.Tests/ConceptVoteRateLimitTests.cs b/tests/Bifrost.Web.Tests/ConceptVoteRateLimitTests.cs
index 04cdff4..991f009 100644
--- a/tests/Bifrost.Web.Tests/ConceptVoteRateLimitTests.cs
+++ b/tests/Bifrost.Web.Tests/ConceptVoteRateLimitTests.cs
@@ -1,3 +1,4 @@
+using Bifrost.Web.Features.Shared;
 using System.Net;
 using Bifrost.Web.Data;
 using Bifrost.Web.Domain;
@@ -19,7 +20,7 @@ namespace Bifrost.Web.Tests;
 public sealed class ConceptVoteRateLimitTests : IClassFixture<ConceptVoteRateLimitTests.LimitedFactory>
 {
     private const string Site = "https://localhost";
-    private const string CookieName = "bifrost_voter";
+    private const string CookieName = BifrostCookies.Voter;
 
     private readonly LimitedFactory _factory;
 
diff --git a/tests/Bifrost.Web.Tests/CookieHardeningTests.cs b/tests/Bifrost.Web.Tests/CookieHardeningTests.cs
new file mode 100644
index 0000000..287e8ac
--- /dev/null
+++ b/tests/Bifrost.Web.Tests/CookieHardeningTests.cs
@@ -0,0 +1,100 @@
+using Bifrost.Web.Features.ConceptVotes;
+using Bifrost.Web.Features.Heimdall;
+using Bifrost.Web.Features.Shared;
+using Bifrost.Web.Tests.Support;
+using Microsoft.AspNetCore.Antiforgery;
+using Microsoft.AspNetCore.Authentication.Cookies;
+using Microsoft.AspNetCore.Authentication.OAuth;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
+
+namespace Bifrost.Web.Tests;
+
+// Every cookie is host-only: __Host- name, Secure, Path=/, no Domain. These pin the literals on purpose, so removing
+// the prefix, Secure or Path=/ from any of them fails here and not only in a test that reads the same constant.
+public sealed class CookieHardeningTests(CookieHardeningTests.OAuthConfiguredFactory factory)
+    : IClassFixture<CookieHardeningTests.OAuthConfiguredFactory>
+{
+    // Program reads GitHubOAuth at startup, before the base factory's in-memory settings land, so the GitHub scheme
+    // is only registered when the settings arrive as host settings.
+    public sealed class OAuthConfiguredFactory : TestWebApplicationFactory
+    {
+        protected override void ConfigureWebHost(Microsoft.AspNetCore.Hosting.IWebHostBuilder builder)
+        {
+            builder.UseSetting("GitHubOAuth:ClientId", "configured-for-tests");
+            builder.UseSetting("GitHubOAuth:ClientSecret", "configured-for-tests");
+            base.ConfigureWebHost(builder);
+        }
+    }
+
+    [Fact]
+    public void Cookie_names_carry_the_host_prefix_and_live_at_the_root()
+    {
+        Assert.Equal("__Host-bifrost_session", BifrostCookies.Session);
+        Assert.Equal("__Host-bifrost_oauth_correlation", BifrostCookies.OAuthCorrelation);
+        Assert.Equal("__Host-bifrost_antiforgery", BifrostCookies.Antiforgery);
+        Assert.Equal("__Host-bifrost_voter", VoterKeyResolver.CookieName);
+        Assert.Equal("__Host-bifrost_heimdall_", HeimdallAttemptCookie.Prefix);
+        Assert.All(HeimdallAttemptCookie.SlotNames, x => Assert.StartsWith("__Host-", x, StringComparison.Ordinal));
+        Assert.Equal("/", BifrostCookies.Path);
+        Assert.Equal("/", VoterKeyResolver.CookiePath);
+    }
+
+    [Fact]
+    public void Session_cookie_is_host_only()
+    {
+        var cookie = factory.Services.GetRequiredService<IOptionsMonitor<CookieAuthenticationOptions>>()
+            .Get(CookieAuthenticationDefaults.AuthenticationScheme).Cookie;
+        AssertHostOnly(cookie.Name, cookie.SecurePolicy, cookie.Path, cookie.Domain);
+        Assert.True(cookie.HttpOnly);
+    }
+
+    [Fact]
+    public void Oauth_correlation_cookie_is_host_only()
+    {
+        var cookie = factory.Services.GetRequiredService<IOptionsMonitor<OAuthOptions>>().Get("GitHub").CorrelationCookie;
+        AssertHostOnly(cookie.Name, cookie.SecurePolicy, cookie.Path, cookie.Domain);
+        Assert.True(cookie.HttpOnly);
+    }
+
+    [Fact]
+    public void Antiforgery_cookie_is_host_only()
+    {
+        var cookie = factory.Services.GetRequiredService<IOptions<AntiforgeryOptions>>().Value.Cookie;
+        AssertHostOnly(cookie.Name, cookie.SecurePolicy, cookie.Path, cookie.Domain);
+    }
+
+    [Fact]
+    public void Voter_and_attempt_cookies_set_secure_root_path_and_no_domain()
+    {
+        var context = new DefaultHttpContext();
+        new VoterKeyResolver(new Microsoft.AspNetCore.DataProtection.EphemeralDataProtectionProvider()).Mint(context);
+        context.Response.Cookies.Append("__Host-bifrost_heimdall_0", "v", HeimdallAttemptCookie.Options());
+        var deletion = new DefaultHttpContext();
+        deletion.Response.Cookies.Delete("__Host-bifrost_heimdall_0", HeimdallAttemptCookie.DeleteOptions());
+
+        var headers = context.Response.Headers.SetCookie.Concat(deletion.Response.Headers.SetCookie).ToArray();
+        Assert.Equal(3, headers.Length);
+        foreach (var header in headers)
+        {
+            var attributes = header!.Split(';').Select(x => x.Trim().ToLowerInvariant()).ToArray();
+            Assert.StartsWith("__host-", attributes[0], StringComparison.Ordinal);
+            Assert.Contains("path=/", attributes);
+            Assert.DoesNotContain(attributes, x => x.StartsWith("domain", StringComparison.Ordinal));
+        }
+
+        // A browser ignores a __Host- Set-Cookie that lacks Secure, a deletion included.
+        Assert.All(headers, header => Assert.Contains("secure", header!.Split(';').Select(x => x.Trim().ToLowerInvariant())));
+        Assert.All(headers.Take(2), header =>
+            Assert.Contains("httponly", header!.Split(';').Select(x => x.Trim().ToLowerInvariant())));
+    }
+
+    private static void AssertHostOnly(string? name, CookieSecurePolicy secure, string? path, string? domain)
+    {
+        Assert.StartsWith("__Host-", name, StringComparison.Ordinal);
+        Assert.Equal(CookieSecurePolicy.Always, secure);
+        Assert.Equal("/", path);
+        Assert.Null(domain);
+    }
+}
diff --git a/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs b/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
index 3439b43..532f669 100644
--- a/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
+++ b/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
@@ -49,6 +49,29 @@ public sealed class HeimdallCompletionTests
     private static void AssertTokenRefused(HeimdallHost host, string rule) =>
         AssertLogged(host, $"Heimdall token refused: {rule}.");
 
+    /// <summary>A __Host- cookie is only deleted by a Set-Cookie that is itself Secure at Path=/; a browser ignores any other.</summary>
+    private static void AssertAttemptCookieCleared(string? header)
+    {
+        Assert.NotNull(header);
+        var attributes = header!.Split(';').Select(x => x.Trim().ToLowerInvariant()).ToArray();
+        Assert.Contains(attributes, x => x.StartsWith("expires=", StringComparison.Ordinal) && x.Contains("1970"));
+        Assert.Contains("secure", attributes);
+        Assert.Contains("path=/", attributes);
+        Assert.DoesNotContain(attributes, x => x.StartsWith("domain", StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public async Task A_refused_completion_clears_the_attempt_cookie_with_a_set_cookie_the_browser_honours()
+    {
+        using var host = new HeimdallHost();
+        var attempt = await host.StartAsync();
+
+        var refused = await host.CompleteAsync(attempt.Nonce, "unknown", attempt.Cookie);
+
+        AssertRefusedRedirect(refused);
+        AssertAttemptCookieCleared(HeimdallHost.SetCookieHeader(refused.Response, attempt.CookieName!));
+    }
+
     private static void AssertRefusedRedirect(CompletionResult result)
     {
         Assert.Equal(HttpStatusCode.Redirect, result.Response.StatusCode);
@@ -81,8 +104,7 @@ public sealed class HeimdallCompletionTests
         Assert.Equal(MembershipStatus.Authenticated, account.Membership!.Status);
         Assert.Empty(account.Membership.RoleAssignments);
         var cleared = HeimdallHost.SetCookieHeader(done.Response, attempt.CookieName!);
-        Assert.NotNull(cleared);
-        Assert.Contains("1970", cleared);
+        AssertAttemptCookieCleared(cleared);
 
         var again = await host.CompleteAsync(attempt, code);
 
@@ -526,7 +548,7 @@ public sealed class HeimdallCompletionTests
         Assert.Contains("httponly", attributes);
         Assert.Contains("secure", attributes);
         Assert.Contains("samesite=lax", attributes);
-        Assert.Contains("path=/auth/heimdall", attributes);
+        Assert.Contains("path=/", attributes);
         Assert.Contains("max-age=600", attributes);
         Assert.NotEqual(first.Cookie, second.Cookie);
         Assert.DoesNotContain(first.Nonce, first.Cookie!);
@@ -564,7 +586,7 @@ public sealed class HeimdallCompletionTests
         Assert.Contains("<noscript>", page);
         Assert.Contains("/?auth=heimdall-error", page);
         Assert.DoesNotContain("CANARY", page);
-        Assert.Null(HeimdallHost.CookieValue(response, ".AspNetCore.Cookies"));
+        Assert.Null(HeimdallHost.CookieValue(response, BifrostCookies.Session));
     }
 
     [Theory]
@@ -696,7 +718,7 @@ public sealed class HeimdallCompletionTests
                 "Heimdall completion has no attempt for this browser."),
             ("no attempt field", await host.CompleteAsync(null, host.CodeFor(attempt, accountId), attempt.Cookie),
                 "Heimdall completion has no attempt for this browser."),
-            ("malformed attempt", await host.CompleteAsync("short", "code", attempt.Cookie, "bifrost_heimdall_short"),
+            ("malformed attempt", await host.CompleteAsync("short", "code", attempt.Cookie, BifrostCookies.HeimdallAttemptPrefix + "short"),
                 "Heimdall completion has no attempt for this browser."),
             ("no code", await host.CompleteAsync(attempt.Nonce, null, attempt.Cookie), "Heimdall completion carried no code."),
             ("blank code", await host.CompleteAsync(attempt.Nonce, "  ", attempt.Cookie), "Heimdall completion carried no code."),
@@ -730,7 +752,7 @@ public sealed class HeimdallCompletionTests
         {
             AssertRefusedRedirect(result);
             Assert.Equal(string.Empty, await result.Response.Content.ReadAsStringAsync());
-            Assert.Null(HeimdallHost.CookieValue(result.Response, ".AspNetCore.Cookies"));
+            Assert.Null(HeimdallHost.CookieValue(result.Response, BifrostCookies.Session));
             AssertLogged(host, reason);
             Assert.True(result.Response.Headers.Location!.OriginalString == ErrorRedirect, name);
         }
@@ -1109,7 +1131,7 @@ public sealed class HeimdallCompletionTests
         responses.Add((await host.CompleteAsync(issued, host.CodeFor(issued, accountCanary, wrongSubject))).Response);
         responses.Add((await host.CompleteAsync(issued, host.CodeFor(issued, accountCanary, returnTo: returnToCanary))).Response);
         responses.Add((await host.CompleteAsync(issued.Nonce, codeCanary, cookieCanary)).Response);
-        responses.Add((await host.CompleteAsync(attemptCanary, codeCanary, cookieCanary, "bifrost_heimdall_" + attemptCanary)).Response);
+        responses.Add((await host.CompleteAsync(attemptCanary, codeCanary, cookieCanary, BifrostCookies.HeimdallAttemptPrefix + attemptCanary)).Response);
         responses.Add((await host.CompleteAsync(issued.Nonce, codeCanary, issued.Cookie)).Response);
         using (var client = host.NewClient())
         {
@@ -1188,7 +1210,7 @@ public sealed class HeimdallCompletionTests
         Assert.NotNull(header);
         Assert.StartsWith(name + "=;", header);
         var attributes = header!.Split(';').Select(x => x.Trim().ToLowerInvariant()).ToArray();
-        Assert.Contains("path=/auth/heimdall", attributes);
+        Assert.Contains("path=/", attributes);
         Assert.Contains(attributes, x => x.StartsWith("expires=", StringComparison.Ordinal) && x.Contains("1970"));
         Assert.DoesNotContain(attributes, x => x.StartsWith("max-age", StringComparison.Ordinal));
     }
@@ -1277,7 +1299,7 @@ public sealed class HeimdallCompletionTests
         foreach (var header in deletions)
         {
             var attributes = header.Split(';').Skip(1).Select(x => x.Trim().ToLowerInvariant()).ToArray();
-            Assert.Contains("path=/auth/heimdall", attributes);
+            Assert.Contains("path=/", attributes);
             Assert.DoesNotContain(attributes, x => x.StartsWith("max-age", StringComparison.Ordinal));
         }
 
@@ -1346,7 +1368,7 @@ public sealed class HeimdallCompletionTests
         {
             host.Clock.Advance(TimeSpan.FromSeconds(1));
             var attempt = await host.StartAsync(
-                returnUrl: "/App", cookieHeader: (jar.Count == 0 ? "" : CookieHeader(jar) + "; ") + ".AspNetCore.Cookies=session; x=1");
+                returnUrl: "/App", cookieHeader: (jar.Count == 0 ? "" : CookieHeader(jar) + "; ") + BifrostCookies.Session + "=session; x=1");
             ApplySetCookies(jar, attempt.Response);
             started.Add(attempt);
         }
@@ -1393,7 +1415,7 @@ public sealed class HeimdallCompletionTests
         using var post = new HttpRequestMessage(HttpMethod.Post, action) { Content = new FormUrlEncodedContent(fields) };
         post.Headers.Add("Cookie", $"{attempt.CookieName}={attempt.Cookie}");
         using var response = await client.SendAsync(post);
-        Assert.False(string.IsNullOrEmpty(HeimdallHost.CookieValue(response, ".AspNetCore.Cookies")));
+        Assert.False(string.IsNullOrEmpty(HeimdallHost.CookieValue(response, BifrostCookies.Session)));
     }
 
     [Fact]
diff --git a/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs b/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs
index 4d86991..bd5f917 100644
--- a/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs
+++ b/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs
@@ -1,3 +1,4 @@
+using Bifrost.Web.Features.Shared;
 using System.Net;
 using System.Security.Cryptography;
 using System.Text;
@@ -482,7 +483,7 @@ public sealed class HeimdallHost : TestWebApplicationFactory
         }
 
         var response = await client.SendAsync(request);
-        var session = CookieValue(response, ".AspNetCore.Cookies");
+        var session = CookieValue(response, BifrostCookies.Session);
         return new CompletionResult(response, !string.IsNullOrEmpty(session));
     }