feat: harden gateway reply remap and leaf loop transparency

src/NATS.Server/Gateways/ReplyMapper.cs

@@ -4,6 +4,10 @@ public static class ReplyMapper
 {
     private const string GatewayReplyPrefix = "_GR_.";
 
+    public static bool HasGatewayReplyPrefix(string? subject)
+        => !string.IsNullOrWhiteSpace(subject)
+            && subject.StartsWith(GatewayReplyPrefix, StringComparison.Ordinal);
+
     public static string? ToGatewayReply(string? replyTo, string localClusterId)
     {
         if (string.IsNullOrWhiteSpace(replyTo))
@@ -16,14 +20,20 @@ public static class ReplyMapper
     {
         restoredReply = string.Empty;
 
-        if (string.IsNullOrWhiteSpace(gatewayReply) || !gatewayReply.StartsWith(GatewayReplyPrefix, StringComparison.Ordinal))
+        if (!HasGatewayReplyPrefix(gatewayReply))
             return false;
 
-        var clusterSeparator = gatewayReply.IndexOf('.', GatewayReplyPrefix.Length);
-        if (clusterSeparator < 0 || clusterSeparator == gatewayReply.Length - 1)
-            return false;
+        var current = gatewayReply!;
+        while (HasGatewayReplyPrefix(current))
+        {
+            var clusterSeparator = current.IndexOf('.', GatewayReplyPrefix.Length);
+            if (clusterSeparator < 0 || clusterSeparator == current.Length - 1)
+                return false;
 
-        restoredReply = gatewayReply[(clusterSeparator + 1)..];
+            current = current[(clusterSeparator + 1)..];
+        }
+
+        restoredReply = current;
         return true;
     }
 }

src/NATS.Server/LeafNodes/LeafLoopDetector.cs

@@ -4,6 +4,9 @@ public static class LeafLoopDetector
 {
     private const string LeafLoopPrefix = "$LDS.";
 
+    public static bool HasLoopMarker(string subject)
+        => subject.StartsWith(LeafLoopPrefix, StringComparison.Ordinal);
+
     public static string Mark(string subject, string serverId)
         => $"{LeafLoopPrefix}{serverId}.{subject}";
 
@@ -13,14 +16,20 @@ public static class LeafLoopDetector
     public static bool TryUnmark(string subject, out string unmarked)
     {
         unmarked = subject;
-        if (!subject.StartsWith(LeafLoopPrefix, StringComparison.Ordinal))
+        if (!HasLoopMarker(subject))
             return false;
 
-        var serverSeparator = subject.IndexOf('.', LeafLoopPrefix.Length);
-        if (serverSeparator < 0 || serverSeparator == subject.Length - 1)
-            return false;
+        var current = subject;
+        while (HasLoopMarker(current))
+        {
+            var serverSeparator = current.IndexOf('.', LeafLoopPrefix.Length);
+            if (serverSeparator < 0 || serverSeparator == current.Length - 1)
+                return false;
 
-        unmarked = subject[(serverSeparator + 1)..];
+            current = current[(serverSeparator + 1)..];
+        }
+
+        unmarked = current;
         return true;
     }
 }

src/NATS.Server/NatsServer.cs

@@ -879,6 +879,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         var replyTo = message.ReplyTo;
         if (ReplyMapper.TryRestoreGatewayReply(replyTo, out var restoredReply))
             replyTo = restoredReply;
+        else if (ReplyMapper.HasGatewayReplyPrefix(replyTo))
+            replyTo = null;
 
         DeliverRemoteMessage(message.Account, message.Subject, replyTo, message.Payload);
     }
@@ -891,6 +893,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         var subject = message.Subject;
         if (LeafLoopDetector.TryUnmark(subject, out var unmarked))
             subject = unmarked;
+        else if (LeafLoopDetector.HasLoopMarker(subject))
+            return;
 
         DeliverRemoteMessage(message.Account, subject, message.ReplyTo, message.Payload);
     }

tests/NATS.Server.Tests/GatewayAdvancedRemapRuntimeTests.cs

@@ -0,0 +1,19 @@
+using NATS.Server.Gateways;
+
+namespace NATS.Server.Tests;
+
+public class GatewayAdvancedRemapRuntimeTests
+{
+    [Fact]
+    public void Transport_internal_reply_and_loop_markers_never_leak_to_client_visible_subjects()
+    {
+        const string clientReply = "_INBOX.123";
+        var nested = ReplyMapper.ToGatewayReply(
+            ReplyMapper.ToGatewayReply(clientReply, "CLUSTER-A"),
+            "CLUSTER-B");
+
+        ReplyMapper.TryRestoreGatewayReply(nested, out var restored).ShouldBeTrue();
+        restored.ShouldBe(clientReply);
+        restored.ShouldNotStartWith("_GR_.");
+    }
+}

tests/NATS.Server.Tests/LeafNodes/LeafLoopTransparencyRuntimeTests.cs

@@ -0,0 +1,18 @@
+using NATS.Server.LeafNodes;
+
+namespace NATS.Server.Tests.LeafNodes;
+
+public class LeafLoopTransparencyRuntimeTests
+{
+    [Fact]
+    public void Transport_internal_reply_and_loop_markers_never_leak_to_client_visible_subjects()
+    {
+        var nested = LeafLoopDetector.Mark(
+            LeafLoopDetector.Mark("orders.created", "S1"),
+            "S2");
+
+        LeafLoopDetector.TryUnmark(nested, out var unmarked).ShouldBeTrue();
+        unmarked.ShouldBe("orders.created");
+        unmarked.ShouldNotStartWith("$LDS.");
+    }
+}