diff --git a/docs/security/auth-enforcement.md b/docs/security/auth-enforcement.md
new file mode 100644
index 0000000..f17e486
--- /dev/null
+++ b/docs/security/auth-enforcement.md
@@ -0,0 +1,45 @@
+# Auth Enforcement
+
+## Scope
+
+This BFF enforces authenticated access on business endpoints using Thalos session validation.
+
+## Protected Endpoints
+
+- `/api/furniture/{furnitureId}/availability`
+- `(GET-only endpoint in this BFF)`
+
+## Anonymous Endpoints
+
+- `/health`
+- `/healthz`
+
+## Session Validation Contract
+
+- BFF requires at least one session cookie:
+  - `thalos_session`
+  - `thalos_refresh`
+- BFF calls Thalos session introspection endpoint:
+  - `GET /api/identity/session/me`
+- Base address configured by:
+  - `ThalosAuth:BaseAddress`
+
+## Error Semantics
+
+Standard auth error payload:
+
+```json
+{
+  "code": "unauthorized|forbidden|session_missing|session_invalid",
+  "message": "human-readable message",
+  "correlationId": "request correlation id"
+}
+```
+
+- `401`: missing or invalid session
+- `403`: permission denied by identity service
+
+## Correlation
+
+- Incoming/outgoing correlation header: `x-correlation-id`
+- Correlation ID is forwarded to Thalos session validation call.
diff --git a/src/Furniture.Bff.Rest/Program.cs b/src/Furniture.Bff.Rest/Program.cs
index a975cd1..f8a589e 100644
--- a/src/Furniture.Bff.Rest/Program.cs
+++ b/src/Furniture.Bff.Rest/Program.cs
@@ -1,3 +1,4 @@
+using System.Net;
 using Core.Blueprint.Common.DependencyInjection;
 using Furniture.Bff.Application.Adapters;
 using Furniture.Bff.Application.DependencyInjection;
@@ -9,6 +10,8 @@ using Furniture.Service.Grpc;
 using Microsoft.Extensions.Primitives;
 
 const string CorrelationHeaderName = "x-correlation-id";
+const string SessionAccessCookieName = "thalos_session";
+const string SessionRefreshCookieName = "thalos_refresh";
 const string CorsPolicyName = "FurnitureBffCors";
 
 var builder = WebApplication.CreateBuilder(args);
@@ -24,6 +27,7 @@ builder.Services.AddHealthChecks();
 builder.Services.AddBlueprintRuntimeCore();
 builder.Services.AddFurnitureBffApplicationRuntime();
 builder.Services.AddScoped<IFurnitureServiceClient, FurnitureServiceGrpcClientAdapter>();
+builder.Services.AddHttpClient("ThalosAuth");
 var allowedOrigins = builder.Configuration.GetSection("FurnitureBff:AllowedOrigins").Get<string[]>() ??
     ["http://localhost:22380", "http://127.0.0.1:22380"];
 builder.Services.AddCors(options =>
@@ -60,8 +64,17 @@ app.Use(async (context, next) =>
 app.MapGet($"{EndpointConventions.ApiPrefix}/{{furnitureId}}/availability", async (
     string furnitureId,
     HttpContext context,
-    IGetFurnitureAvailabilityHandler handler) =>
+    IHttpClientFactory httpClientFactory,
+    IConfiguration configuration,
+    IGetFurnitureAvailabilityHandler handler,
+    CancellationToken ct) =>
 {
+    var authError = await EnforceSessionAsync(context, httpClientFactory, configuration, ct);
+    if (authError is not null)
+    {
+        return authError;
+    }
+
     var request = new GetFurnitureAvailabilityApiRequest(
         furnitureId,
         ResolveCorrelationId(context));
@@ -92,3 +105,75 @@ string ResolveCorrelationId(HttpContext context)
 
     return context.TraceIdentifier;
 }
+
+async Task<IResult?> EnforceSessionAsync(
+    HttpContext context,
+    IHttpClientFactory httpClientFactory,
+    IConfiguration configuration,
+    CancellationToken ct)
+{
+    var correlationId = ResolveCorrelationId(context);
+
+    if (!context.Request.Cookies.ContainsKey(SessionAccessCookieName) &&
+        !context.Request.Cookies.ContainsKey(SessionRefreshCookieName))
+    {
+        return ErrorResponse(StatusCodes.Status401Unauthorized, "session_missing", "No active session.", correlationId);
+    }
+
+    var thalosBaseAddress = configuration["ThalosAuth:BaseAddress"] ?? "http://thalos-bff:8080";
+    using var request = new HttpRequestMessage(
+        HttpMethod.Get,
+        $"{thalosBaseAddress.TrimEnd('/')}/api/identity/session/me");
+
+    request.Headers.TryAddWithoutValidation(CorrelationHeaderName, correlationId);
+    var cookieHeader = BuildForwardCookieHeader(context);
+    if (!string.IsNullOrWhiteSpace(cookieHeader))
+    {
+        request.Headers.TryAddWithoutValidation("Cookie", cookieHeader);
+    }
+
+    using var response = await httpClientFactory.CreateClient("ThalosAuth").SendAsync(request, ct);
+
+    if (response.StatusCode == HttpStatusCode.Forbidden)
+    {
+        return ErrorResponse(StatusCodes.Status403Forbidden, "forbidden", "Permission denied.", correlationId);
+    }
+
+    if (response.StatusCode == HttpStatusCode.Unauthorized)
+    {
+        return ErrorResponse(StatusCodes.Status401Unauthorized, "unauthorized", "Unauthorized request.", correlationId);
+    }
+
+    if (!response.IsSuccessStatusCode)
+    {
+        return ErrorResponse(StatusCodes.Status401Unauthorized, "session_invalid", "Session validation failed.", correlationId);
+    }
+
+    return null;
+}
+
+static string BuildForwardCookieHeader(HttpContext context)
+{
+    var cookies = new List<string>();
+
+    if (context.Request.Cookies.TryGetValue(SessionAccessCookieName, out var accessCookie) &&
+        !string.IsNullOrWhiteSpace(accessCookie))
+    {
+        cookies.Add($"{SessionAccessCookieName}={accessCookie}");
+    }
+
+    if (context.Request.Cookies.TryGetValue(SessionRefreshCookieName, out var refreshCookie) &&
+        !string.IsNullOrWhiteSpace(refreshCookie))
+    {
+        cookies.Add($"{SessionRefreshCookieName}={refreshCookie}");
+    }
+
+    return string.Join("; ", cookies);
+}
+
+static IResult ErrorResponse(int statusCode, string code, string message, string correlationId)
+{
+    return Results.Json(new AuthErrorResponse(code, message, correlationId), statusCode: statusCode);
+}
+
+sealed record AuthErrorResponse(string Code, string Message, string CorrelationId);