From 87dd0cd756dc84fe4a10e0aab63b52cbe0a066f6 Mon Sep 17 00:00:00 2001
From: Carl Johnsen <carl-johannes@di.ku.dk>
Date: Fri, 9 Jan 2026 10:56:55 +0100
Subject: [PATCH] Incorrect indices passed to the remote synchronization module
 should not record anything in the database, but should instead emit a warning
 to the log

---
 .../Library/Modules/Builtin/RemoteSynchronizationModule.cs | 7 +++++++
 Duplicati/UnitTest/RemoteSynchronizationModuleTests.cs     | 3 ++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/Duplicati/Library/Modules/Builtin/RemoteSynchronizationModule.cs b/Duplicati/Library/Modules/Builtin/RemoteSynchronizationModule.cs
index 24a7609d9..a572df62a 100644
--- a/Duplicati/Library/Modules/Builtin/RemoteSynchronizationModule.cs
+++ b/Duplicati/Library/Modules/Builtin/RemoteSynchronizationModule.cs
@@ -331,6 +331,13 @@ public class RemoteSynchronizationModule : IGenericCallbackModule
     /// <param name="index">The index of the destination.</param>
     private void RecordSyncOperation(int index)
     {
+        // Validate index
+        if (index < 0 || index >= m_destinations.Count)
+        {
+            Logging.Log.WriteWarningMessage(LOGTAG, "RemoteSyncRecordInvalidIndex", null, "Cannot record remote synchronization operation: invalid index {0}.", index);
+            return;
+        }
+
         if (!m_options.TryGetValue("dbpath", out var dbpath) || string.IsNullOrWhiteSpace(dbpath))
             return;
 
diff --git a/Duplicati/UnitTest/RemoteSynchronizationModuleTests.cs b/Duplicati/UnitTest/RemoteSynchronizationModuleTests.cs
index 86d4fb358..1d3535ff4 100644
--- a/Duplicati/UnitTest/RemoteSynchronizationModuleTests.cs
+++ b/Duplicati/UnitTest/RemoteSynchronizationModuleTests.cs
@@ -889,6 +889,7 @@ namespace Duplicati.UnitTest
         [Category("RemoteSync")]
         public void TestRecordSyncOperation_WithDifferentIndex()
         {
+            // Incorrect index should not record, but throw a warning.
             var module = new RemoteSynchronizationModule();
             var options = new Dictionary<string, string>
             {
@@ -910,7 +911,7 @@ namespace Duplicati.UnitTest
                 WHERE ""Description"" = 'Rsync 2'
             ";
             var count = (long)cmd.ExecuteScalar();
-            Assert.AreEqual(1, count);
+            Assert.AreEqual(0, count);
         }
 
         [Test]