Improved the security of the connection between the app window and AI Studio (#1146)

app/MindWork AI Studio/Tools/Security/AppAccessPolicy.cs

@@ -0,0 +1,76 @@
+using System.Security.Cryptography;
+using System.Text;
+
+using Microsoft.AspNetCore.HostFiltering;
+
+namespace AIStudio.Tools.Security;
+
+/// <summary>
+/// Decides which requests may reach the app server.
+/// </summary>
+/// <remarks>
+/// The Rust runtime makes a new token for every start. It gives the token to this server through
+/// an environment variable, and to the app window as the value of the session cookie. Nothing else
+/// on this machine has the cookie, so a request with it comes from the app window.
+/// </remarks>
+/// <param name="appPort">The port the app server listens on.</param>
+/// <param name="uiToken">The token of this start, which the session cookie carries.</param>
+public sealed class AppAccessPolicy(int appPort, string uiToken)
+{
+    /// <summary>
+    /// The name of the session cookie. The Rust runtime sets the cookie under the same name.
+    /// </summary>
+    public const string SESSION_COOKIE_NAME = "ai_studio_session";
+
+    /// <summary>
+    /// The environment variable which carries the token from the Rust runtime to this server.
+    /// </summary>
+    public const string UI_TOKEN_ENVIRONMENT_VARIABLE = "AI_STUDIO_UI_TOKEN";
+
+    /// <summary>
+    /// The host names under which the app server answers. A request for any other name, such as one
+    /// which a website points at the loopback address, gets refused.
+    /// </summary>
+    private static readonly string[] ALLOWED_HOSTS = ["127.0.0.1", "localhost", "[::1]"];
+
+    private readonly HashSet<string> allowedOrigins = ALLOWED_HOSTS.Select(host => $"http://{host}:{appPort}").ToHashSet(StringComparer.OrdinalIgnoreCase);
+
+    private readonly byte[] uiTokenBytes = Encoding.UTF8.GetBytes(uiToken);
+
+    /// <summary>
+    /// Lets the host filtering of ASP.NET Core accept only the host names of the app server.
+    /// </summary>
+    /// <param name="options">The options of the host filtering.</param>
+    public static void ConfigureHostFiltering(HostFilteringOptions options)
+    {
+        options.AllowedHosts = [..ALLOWED_HOSTS];
+        options.AllowEmptyHosts = false;
+        options.IncludeFailureMessage = false;
+    }
+
+    /// <summary>
+    /// Checks whether the given origin is the app itself.
+    /// </summary>
+    /// <param name="origin">The value of the Origin header of a request.</param>
+    /// <returns>True when a page of the app made the request.</returns>
+    public bool IsAppOrigin(string origin) => this.allowedOrigins.Contains(origin);
+
+    /// <summary>
+    /// Checks whether the given request carries the session cookie of this start.
+    /// </summary>
+    /// <remarks>
+    /// The comparison takes the same time no matter where a wrong value differs, so the time of a
+    /// refusal tells nothing about how much of a guess was right.
+    /// </remarks>
+    /// <param name="request">The request.</param>
+    /// <returns>True when the request comes from the app window.</returns>
+    public bool HasSessionCookie(HttpRequest request) => request.Cookies.TryGetValue(SESSION_COOKIE_NAME, out var value) && this.IsUIToken(value);
+
+    /// <summary>
+    /// Checks whether the given value is the token of this start, in a time which does not depend
+    /// on where a wrong value differs.
+    /// </summary>
+    /// <param name="value">The value to check.</param>
+    /// <returns>True when the value is the token.</returns>
+    public bool IsUIToken(string? value) => !string.IsNullOrEmpty(value) && CryptographicOperations.FixedTimeEquals(Encoding.UTF8.GetBytes(value), this.uiTokenBytes);
+}