diff --git a/Knossos.NET/Classes/KnUtils.cs b/Knossos.NET/Classes/KnUtils.cs
index e711bda9..b28acc2c 100644
--- a/Knossos.NET/Classes/KnUtils.cs
+++ b/Knossos.NET/Classes/KnUtils.cs
@@ -656,7 +656,11 @@ public static bool IsSubPath(string basePath, string candidatePath)
             {
                 var fullBase = Path.GetFullPath(basePath).TrimEnd(Path.DirectorySeparatorChar) + Path.DirectorySeparatorChar;
                 var fullTarget = Path.GetFullPath(Path.Combine(basePath, candidatePath));
-                return fullTarget.StartsWith(fullBase, StringComparison.OrdinalIgnoreCase);
+                //Windows is uniformly case-insensitive; Linux/macOS filesystems CAN be case-sensitive
+                //(always on Linux, optional on APFS), so use Ordinal there to avoid false-positives on
+                //traversal attempts that exploit case-only differences with sibling directories.
+                var comparison = IsWindows ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal;
+                return fullTarget.StartsWith(fullBase, comparison);
             }
             catch
             {
@@ -664,6 +668,32 @@ public static bool IsSubPath(string basePath, string candidatePath)
             }
         }
 
+        /// <summary>
+        /// True if a string is safe to use as a single filesystem path component (no separators,
+        /// no traversal sequences, no null bytes, no drive letters, no NTFS-stripped trailing
+        /// whitespace or dots, not "." or ".."). Used to validate JSON-supplied fields like
+        /// mod.id / mod.version that get concatenated into install paths.
+        /// </summary>
+        public static bool IsSafePathComponent(string? component)
+        {
+            if (string.IsNullOrEmpty(component))
+                return false;
+            if (component == "." || component == "..")
+                return false;
+            if (component.Contains('\0'))
+                return false;
+            if (component.Contains('/') || component.Contains('\\'))
+                return false;
+            if (component.Length >= 2 && component[1] == ':')
+                return false;
+            //NTFS silently strips trailing dots/spaces — reject so "..  " or ".." don't round-trip.
+            if (component.Trim() != component)
+                return false;
+            if (component.EndsWith('.'))
+                return false;
+            return true;
+        }
+
         /// <summary>
         /// Gets the complete size of all files in a folder and subdirectories in bytes
         /// </summary>
diff --git a/Knossos.NET/ViewModels/Templates/Tasks/InstallBuild.cs b/Knossos.NET/ViewModels/Templates/Tasks/InstallBuild.cs
index 835a1eb1..c3998585 100644
--- a/Knossos.NET/ViewModels/Templates/Tasks/InstallBuild.cs
+++ b/Knossos.NET/ViewModels/Templates/Tasks/InstallBuild.cs
@@ -138,6 +138,26 @@ private async Task<bool> InstallVCRedist(bool is86 = false) {
                             -Main progress max value is calculated as follows: ( Number of files to download * 2 ) + 1
                              (Download, Decompression, Download banner/tile images)
                         */
+                        //Reject build metadata with traversal sequences in path-component fields. Without this guard a
+                        //malicious Nebula response (id="..\\..\\..", version="../etc"...) would poison modPath itself,
+                        //causing the IsSubPath checks on file.dest / file.filename below to validate against the poisoned base.
+                        if (!KnUtils.IsSafePathComponent(modJson.id) ||
+                            !KnUtils.IsSafePathComponent(modJson.version))
+                        {
+                            Log.Add(Log.LogSeverity.Error, "TaskItemViewModel.InstallBuild()", "Refusing to install: build has unsafe id/version: id=" + modJson.id + " version=" + modJson.version);
+                            CancelTaskCommand();
+                            throw new TaskCanceledException();
+                        }
+                        foreach (var pkg in modJson.packages)
+                        {
+                            if (pkg.folder != null && !KnUtils.IsSafePathComponent(pkg.folder))
+                            {
+                                Log.Add(Log.LogSeverity.Error, "TaskItemViewModel.InstallBuild()", "Refusing to install: build " + modJson.id + " has unsafe package folder: " + pkg.folder);
+                                CancelTaskCommand();
+                                throw new TaskCanceledException();
+                            }
+                        }
+
                         List<ModFile> files = new List<ModFile>();
                         string modFolder = modJson.id + "-" + modJson.version;
                         modPath = Knossos.GetKnossosLibraryPath() + Path.DirectorySeparatorChar + "bin" + Path.DirectorySeparatorChar + modFolder;
diff --git a/Knossos.NET/ViewModels/Templates/Tasks/InstallMod.cs b/Knossos.NET/ViewModels/Templates/Tasks/InstallMod.cs
index 70988fb1..f74c6703 100644
--- a/Knossos.NET/ViewModels/Templates/Tasks/InstallMod.cs
+++ b/Knossos.NET/ViewModels/Templates/Tasks/InstallMod.cs
@@ -107,6 +107,27 @@ public async Task<bool> InstallMod(Mod mod, CancellationTokenSource cancelSource
                         -If devmode and file is a vp it needs to be decompressed +1 to max tasks
                     */
 
+                    //Reject mod metadata with traversal sequences in path-component fields. Without this guard a
+                    //malicious Nebula response (id="..\\..\\..", version="../etc"...) would poison modPath itself,
+                    //causing the IsSubPath checks on file.dest / file.filename below to validate against the poisoned base.
+                    if (!KnUtils.IsSafePathComponent(mod.id) ||
+                        !KnUtils.IsSafePathComponent(mod.version) ||
+                        (mod.parent != null && !KnUtils.IsSafePathComponent(mod.parent)))
+                    {
+                        Log.Add(Log.LogSeverity.Error, "TaskItemViewModel.InstallMod()", "Refusing to install: mod has unsafe id/version/parent: id=" + mod.id + " version=" + mod.version + " parent=" + mod.parent);
+                        CancelTaskCommand();
+                        throw new TaskCanceledException();
+                    }
+                    foreach (var pkg in mod.packages)
+                    {
+                        if (pkg.folder != null && !KnUtils.IsSafePathComponent(pkg.folder))
+                        {
+                            Log.Add(Log.LogSeverity.Error, "TaskItemViewModel.InstallMod()", "Refusing to install: mod " + mod.id + " has unsafe package folder: " + pkg.folder);
+                            CancelTaskCommand();
+                            throw new TaskCanceledException();
+                        }
+                    }
+
                     List<ModFile> files = new List<ModFile>();
                     string modFolder = mod.id + "-" + mod.version;
                     string rootPack = string.Empty;
diff --git a/Knossos.NET/ViewModels/Templates/Tasks/TryToCopyFilesFromOldVersions.cs b/Knossos.NET/ViewModels/Templates/Tasks/TryToCopyFilesFromOldVersions.cs
index 37e44761..b421fcfe 100644
--- a/Knossos.NET/ViewModels/Templates/Tasks/TryToCopyFilesFromOldVersions.cs
+++ b/Knossos.NET/ViewModels/Templates/Tasks/TryToCopyFilesFromOldVersions.cs
@@ -113,6 +113,19 @@ public async Task<bool> TryToCopyFilesFromOldVersions(Mod mod, List<Mod> oldVers
                                 {
                                     if (f.filename != null && (!oldVer.devMode || (oldVer.devMode && oldPkg.folder != null)))
                                     {
+                                        //Validate the old-side relative path before reading. A malicious mod.json on disk
+                                        //could have a traversal in f.filename that leaks file existence via File.Exists and
+                                        //timing via GetFileHash even before we attempt to copy.
+                                        var oldRelative = oldVer.devMode
+                                            ? Path.Combine(oldPkg.folder ?? string.Empty, f.filename)
+                                            : f.filename;
+                                        if (string.IsNullOrEmpty(oldRelative) || !KnUtils.IsSubPath(oldVer.fullPath, oldRelative))
+                                        {
+                                            Log.Add(Log.LogSeverity.Warning, "TaskItemViewModel.TryToCopyFilesFromOldVersions()", "Old version " + oldVer + " has unsafe file path, cannot use as source: " + oldRelative);
+                                            copySrcList.Clear();
+                                            copyDstList.Clear();
+                                            break;
+                                        }
                                         var oldPath = oldVer.devMode ? Path.Combine(oldVer.fullPath, oldPkg.folder!, f.filename) : Path.Combine(oldVer.fullPath, f.filename);
                                         if (File.Exists(oldPath))
                                         {
@@ -141,6 +154,18 @@ public async Task<bool> TryToCopyFilesFromOldVersions(Mod mod, List<Mod> oldVers
                                                 }
                                             }
 
+                                            //Paired check on the write side: the new mod's package.folder is API-derived
+                                            //and would not have been validated yet if mod was loaded from a poisoned source.
+                                            var newRelative = mod.devMode
+                                                ? Path.Combine(package.folder ?? string.Empty, f.filename)
+                                                : f.filename;
+                                            if (!KnUtils.IsSubPath(mod.fullPath, newRelative))
+                                            {
+                                                Log.Add(Log.LogSeverity.Warning, "TaskItemViewModel.TryToCopyFilesFromOldVersions()", "New mod has unsafe destination path, cannot copy from old version: " + newRelative);
+                                                copySrcList.Clear();
+                                                copyDstList.Clear();
+                                                break;
+                                            }
                                             copySrcList.Add(oldPath);
                                             var newPath = mod.devMode ? Path.Combine(mod.fullPath, package.folder!, f.filename) : Path.Combine(mod.fullPath, f.filename);
                                             copyDstList.Add(newPath);