diff --git a/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs b/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
index 7f32908..1672c96 100644
--- a/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
+++ b/src/Bifrost.Web/Features/Heimdall/HeimdallAuthService.cs
@@ -418,4 +418,16 @@ public static class HeimdallAttemptCookie
         Path = "/auth/heimdall",
         MaxAge = Lifetime
     };
+
+    /// <summary>
+    /// The options a deletion carries: the attempt cookie's Path, which a cookie is matched on, and no Max-Age,
+    /// which would take precedence over the expiry Delete writes and leave the emptied cookie alive.
+    /// </summary>
+    public static CookieOptions DeleteOptions() => new()
+    {
+        HttpOnly = true,
+        Secure = true,
+        SameSite = SameSiteMode.Lax,
+        Path = "/auth/heimdall"
+    };
 }
diff --git a/src/Bifrost.Web/Features/Shared/LocalReturnPath.cs b/src/Bifrost.Web/Features/Shared/LocalReturnPath.cs
index e9c97d1..1de135b 100644
--- a/src/Bifrost.Web/Features/Shared/LocalReturnPath.cs
+++ b/src/Bifrost.Web/Features/Shared/LocalReturnPath.cs
@@ -2,13 +2,16 @@ namespace Bifrost.Web.Features.Shared;
 
 /// <summary>
 /// A sign-in return target that stays on this site. The only way to make one is <see cref="Parse"/>, which
-/// accepts a path that starts with a single slash and holds no control character, and otherwise yields
-/// <see cref="Default"/>. Callers parse once at entry and carry the typed value from there.
+/// accepts a path of at most <see cref="MaxLength"/> characters that starts with a single slash and holds no
+/// control character, and otherwise yields <see cref="Default"/>. Callers parse once at entry and carry the typed value from there.
 /// </summary>
 public sealed record LocalReturnPath
 {
     private LocalReturnPath(string value) => Value = value;
 
+    /// <summary>Bounds the path so the attempt cookie that carries it stays far below a browser's 4096-byte limit.</summary>
+    public const int MaxLength = 256;
+
     public static LocalReturnPath Default { get; } = new("/App");
 
     public string Value { get; }
@@ -16,6 +19,7 @@ public sealed record LocalReturnPath
     public static LocalReturnPath Parse(string? candidate)
     {
         if (string.IsNullOrEmpty(candidate) ||
+            candidate.Length > MaxLength ||
             candidate[0] != '/' ||
             candidate.Any(char.IsControl) ||
             (candidate.Length > 1 && (candidate[1] == '/' || candidate[1] == '\\')))
diff --git a/src/Bifrost.Web/Program.cs b/src/Bifrost.Web/Program.cs
index 4bb73ac..1c96ea0 100644
--- a/src/Bifrost.Web/Program.cs
+++ b/src/Bifrost.Web/Program.cs
@@ -125,7 +125,8 @@ builder.Services.AddScoped<ApplicationBootstrapper>();
 builder.Services.AddSingleton<StartupConfigurationValidator>();
 builder.Services.AddScoped<IAuthorizationHandler, ActiveMemberAuthorizationHandler>();
 builder.Services.AddHttpClient<HeimdallAuthService>((serviceProvider, client) =>
-    HeimdallAuthService.ConfigureClient(client, serviceProvider.GetRequiredService<IOptions<HeimdallOptions>>().Value));
+    HeimdallAuthService.ConfigureClient(client, serviceProvider.GetRequiredService<IOptions<HeimdallOptions>>().Value))
+    .RedactLoggedHeaders(header => string.Equals(header, HeimdallAuthService.AppSecretHeader, StringComparison.OrdinalIgnoreCase));
 builder.Services.AddSingleton<HeimdallTokenVerifier>();
 builder.Services.AddHttpClient(HeimdallTokenVerifier.ClientName, (serviceProvider, client) =>
     HeimdallTokenVerifier.ConfigureClient(client, serviceProvider.GetRequiredService<IOptions<HeimdallOptions>>().Value));
@@ -537,7 +538,7 @@ app.MapPost("/auth/heimdall/complete", async (
             form?["code"].ToString(),
             cancellationToken);
         await httpContext.SignInAsync(CookieAuthenticationDefaults.AuthenticationScheme, signIn.Principal);
-        httpContext.Response.Cookies.Delete(cookieName!, HeimdallAttemptCookie.Options());
+        httpContext.Response.Cookies.Delete(cookieName!, HeimdallAttemptCookie.DeleteOptions());
         return Results.LocalRedirect(signIn.ReturnPath.Value);
     }
     catch (HeimdallAuthException error)
@@ -545,7 +546,7 @@ app.MapPost("/auth/heimdall/complete", async (
         logger.LogWarning("Heimdall completion refused: {Reason}", error.Message);
         if (cookieName is not null)
         {
-            httpContext.Response.Cookies.Delete(cookieName, HeimdallAttemptCookie.Options());
+            httpContext.Response.Cookies.Delete(cookieName, HeimdallAttemptCookie.DeleteOptions());
         }
 
         return Results.Redirect(HeimdallAuthService.ErrorRedirect);
diff --git a/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs b/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
index 1f2da00..0544438 100644
--- a/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
+++ b/tests/Bifrost.Web.Tests/HeimdallCompletionTests.cs
@@ -4,10 +4,14 @@ using System.Text.Json;
 using Bifrost.Web.Domain;
 using Bifrost.Web.Features.Heimdall;
 using Bifrost.Web.Features.Membership;
+using Bifrost.Web.Features.Shared;
 using Bifrost.Web.Tests.Support;
 using Microsoft.AspNetCore.Authentication.OAuth;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.Testing;
 using Microsoft.AspNetCore.WebUtilities;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 
 namespace Bifrost.Web.Tests;
@@ -1139,4 +1143,109 @@ public sealed class HeimdallCompletionTests
             Assert.DoesNotContain(canary, logs);
         }
     }
+
+    [Fact]
+    public async Task The_app_secret_reaches_Heimdall_but_is_never_logged_even_at_trace()
+    {
+        const string canary = "CANARY-app-secret-7c1e";
+        using var host = new HeimdallHost(appSecret: canary);
+        host.Heimdall.RequiredAppSecret = canary;
+        using var traced = host.WithWebHostBuilder(builder => builder.ConfigureLogging(logging =>
+        {
+            logging.SetMinimumLevel(LogLevel.Trace);
+            logging.AddFilter((string? _, LogLevel _) => true);
+            logging.AddFilter("System.Net.Http", LogLevel.Trace);
+            logging.AddFilter("Microsoft", LogLevel.Trace);
+        }));
+        using var client = traced.CreateClient(new WebApplicationFactoryClientOptions
+        {
+            AllowAutoRedirect = false,
+            HandleCookies = false,
+            BaseAddress = new Uri("https://localhost")
+        });
+
+        using var start = await client.GetAsync("/auth/heimdall/discord?access=member&returnUrl=/App");
+
+        Assert.Equal(HttpStatusCode.Redirect, start.StatusCode);
+        Assert.Equal(canary, Assert.Single(host.Heimdall.StartSecrets));
+        var logs = string.Join("
+", host.Logs.Lines);
+        Assert.Contains(HeimdallAuthService.AppSecretHeader, logs, StringComparison.OrdinalIgnoreCase);
+        Assert.DoesNotContain(canary, logs);
+    }
+
+    private static void AssertAttemptCookieExpired(HttpResponseMessage response, string nonce)
+    {
+        var name = HeimdallAttemptCookie.Prefix + nonce;
+        var header = HeimdallHost.SetCookieHeader(response, name);
+        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(attributes, x => x.StartsWith("expires=", StringComparison.Ordinal) && x.Contains("1970"));
+        Assert.DoesNotContain(attributes, x => x.StartsWith("max-age", StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public async Task The_attempt_cookie_is_expired_at_its_path_on_success_and_on_refusal()
+    {
+        using var host = new HeimdallHost();
+        var accountId = HeimdallShapes.NewAccountId();
+
+        var signedAttempt = await host.StartAsync();
+        var signedIn = await Genuine(host, signedAttempt, accountId);
+        Assert.True(signedIn.SignedIn);
+        AssertAttemptCookieExpired(signedIn.Response, signedAttempt.Nonce);
+
+        var unknownCode = await host.StartAsync();
+        var gone = await host.CompleteAsync(unknownCode, "unknown-code");
+        AssertRefusedRedirect(gone);
+        AssertAttemptCookieExpired(gone.Response, unknownCode.Nonce);
+
+        var tampered = await host.StartAsync();
+        var refused = await host.CompleteAsync(
+            tampered.Nonce, host.CodeFor(tampered, accountId), Corrupt(tampered.Cookie!));
+        AssertRefusedRedirect(refused);
+        AssertAttemptCookieExpired(refused.Response, tampered.Nonce);
+    }
+
+    [Fact]
+    public async Task Return_paths_are_bounded_so_the_attempt_cookie_stays_small()
+    {
+        using var host = new HeimdallHost();
+        var atBound = "/" + new string('a', LocalReturnPath.MaxLength - 1);
+        var overBound = atBound + "a";
+        var worstCase = "/" + new string('<', LocalReturnPath.MaxLength - 1);
+
+        Assert.Equal(atBound, LocalReturnPath.Parse(atBound).Value);
+        Assert.Equal(LocalReturnPath.Default, LocalReturnPath.Parse(overBound));
+
+        var kept = await host.StartAsync(returnUrl: atBound);
+        var keptSignIn = await Genuine(host, kept, HeimdallShapes.NewAccountId());
+        Assert.True(keptSignIn.SignedIn);
+        Assert.Equal(atBound, keptSignIn.Response.Headers.Location!.OriginalString);
+
+        var over = await host.StartAsync(returnUrl: overBound);
+        var overSignIn = await Genuine(host, over, HeimdallShapes.NewAccountId());
+        Assert.True(overSignIn.SignedIn);
+        Assert.Equal("/App", overSignIn.Response.Headers.Location!.OriginalString);
+
+        var heaviest = await host.StartAsync(returnUrl: worstCase);
+        var header = HeimdallHost.SetCookieHeader(heaviest.Response, HeimdallAttemptCookie.Prefix + heaviest.Nonce)!;
+        Assert.InRange(header.Split(';')[0].Length, 1, 3072);
+    }
+
+    [Fact]
+    public async Task A_blank_app_slug_is_refused_before_Heimdall_is_called()
+    {
+        using var host = new HeimdallHost(appSlug: " ");
+
+        var refused = await host.StartAsync();
+
+        Assert.Equal(ErrorRedirect, refused.Response.Headers.Location!.OriginalString);
+        Assert.Empty(host.Heimdall.StartBodies);
+        Assert.False(refused.Response.Headers.Contains("Set-Cookie"));
+        Assert.Contains("Heimdall start refused: Heimdall is not configured.", string.Join("
+", host.Logs.Lines));
+    }
 }
diff --git a/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs b/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs
index c4992a7..40e024b 100644
--- a/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs
+++ b/tests/Bifrost.Web.Tests/Support/HeimdallTestKit.cs
@@ -347,6 +347,7 @@ public sealed class HeimdallHost : TestWebApplicationFactory
     public const string AppSecret = "test-heimdall-app-secret";
 
     private readonly string _appSecret;
+    private readonly string _appSlug;
     private readonly string _baseUrl;
     private readonly string _publicBaseUrl;
     private readonly bool _discordRoles;
@@ -363,9 +364,11 @@ public sealed class HeimdallHost : TestWebApplicationFactory
         string appSecret = AppSecret,
         string baseUrl = HeimdallShapes.Issuer,
         string publicBaseUrl = "https://localhost",
-        bool discordRoles = true)
+        bool discordRoles = true,
+        string appSlug = "bifrost")
     {
         _appSecret = appSecret;
+        _appSlug = appSlug;
         _baseUrl = baseUrl;
         _publicBaseUrl = publicBaseUrl;
         _discordRoles = discordRoles;
@@ -385,6 +388,7 @@ public sealed class HeimdallHost : TestWebApplicationFactory
             {
                 ["Heimdall:BaseUrl"] = _baseUrl,
                 ["Heimdall:AppSecret"] = _appSecret,
+                ["Heimdall:AppSlug"] = _appSlug,
                 ["Heimdall:DiscordGuildId"] = "guild-1",
                 ["Host:PublicBaseUrl"] = _publicBaseUrl
             });