allow localhost host to connect to CDP and MCP

src/Server.zig

@@ -908,8 +908,10 @@ test "Client: http handshake host" {
 
     for ([_][]const u8{
         "rebind.evil.com:9583",
-        // Not even the name that resolves to loopback on every machine.
-        "localhost:9583",
+        // Only the exact `localhost:<port>` form is allowed.
+        "localhost",
+        "LOCALHOST:9583",
+        "localhost.evil.com:9583",
     }) |host| {
         var buf: [256]u8 = undefined;
         try assertHTTPError(
@@ -918,6 +920,16 @@ test "Client: http handshake host" {
             try std.fmt.bufPrint(&buf, with_host, .{host}),
         );
     }
+
+    // `localhost:<port>` is hardwired to loopback by browsers, no DNS
+    // lookup involved, so it gets through like an IP literal.
+    {
+        var c = try createTestClient();
+        defer c.deinit();
+        var buf: [256]u8 = undefined;
+        const res = try c.httpRequest(try std.fmt.bufPrint(&buf, with_host, .{"localhost:9583"}));
+        try testing.expect(std.mem.startsWith(u8, res, "HTTP/1.1 101 Switching Protocols\r\n"));
+    }
 }
 
 test "Client: http valid handshake" {

src/cdp/Connection.zig

@@ -495,20 +495,27 @@ pub fn upgrade(self: *Connection, request: []u8) !void {
         } else if (std.ascii.eqlIgnoreCase(key, "host")) {
             const host = value;
             const is_allowed = blk: {
+                // allow literal localhost
+                if (std.mem.startsWith(u8, host, "localhost:")) {
+                    break :blk true;
+                }
+
                 _ = std.Io.net.IpAddress.parseLiteral(host) catch break :blk false;
                 break :blk true;
             };
 
             // Defense in depth against DNS rebinding: an IP literal is the only
             // thing that can legitimately reach us, because no name has to be
-            // resolved to produce one. Any name at all - "localhost" included -
-            // means something answered a DNS lookup with our address, which is
-            // exactly what a rebinding attack looks like. A request without a
-            // Host header isn't from a browser, so it can't be the vector.
+            // resolved to produce one. The one name we accept is
+            // `localhost:<port>`, which browsers hardwire to loopback without
+            // any DNS lookup. Any other name means something answered a DNS
+            // lookup with our address, which is exactly what a rebinding
+            // attack looks like. A request without a Host header isn't from a
+            // browser, so it can't be the vector.
             if (!is_allowed) {
                 log.warn(.cdp, "rejected websocket host", .{
                     .host = host[0..@min(host.len, 64)],
-                    .hint = "connect to the CDP endpoint by IP address",
+                    .hint = "connect to the CDP endpoint by IP address or localhost",
                 });
                 return error.ForbiddenHost;
             }

src/mcp/HttpServer.zig

@@ -403,7 +403,7 @@ fn serve(self: *HttpServer, out: *std.Io.Writer, arena: std.mem.Allocator, reque
     // the head's string memory.
     const session_id = checkHeaders(arena, request) catch |err| switch (err) {
         error.ForbiddenOrigin => return request.respond("Origin not allowed\n", .{ .status = .forbidden, .keep_alive = false }),
-        error.ForbiddenHost => return request.respond("Host not allowed, connect to the MCP endpoint by IP address\n", .{ .status = .forbidden, .keep_alive = false }),
+        error.ForbiddenHost => return request.respond("Host not allowed, connect to the MCP endpoint by IP address or localhost\n", .{ .status = .forbidden, .keep_alive = false }),
         else => return err,
     };
     const keep_alive = request.head.keep_alive;
@@ -457,6 +457,11 @@ fn checkHeaders(arena: std.mem.Allocator, request: *std.http.Server.Request) !?[
             return error.ForbiddenOrigin;
         } else if (std.ascii.eqlIgnoreCase(key, "host")) {
             const is_allowed = blk: {
+                // allow literal localhost
+                if (std.mem.startsWith(u8, value, "localhost:")) {
+                    break :blk true;
+                }
+
                 _ = std.Io.net.IpAddress.parseLiteral(value) catch break :blk false;
                 break :blk true;
             };
@@ -464,7 +469,7 @@ fn checkHeaders(arena: std.mem.Allocator, request: *std.http.Server.Request) !?[
             if (!is_allowed) {
                 log.warn(.mcp, "rejected request host", .{
                     .host = value[0..@min(value.len, 64)],
-                    .hint = "connect to the MCP endpoint by IP address",
+                    .hint = "connect to the MCP endpoint by IP address or localhost",
                 });
                 return error.ForbiddenHost;
             }