diff --git a/dotnet/src/Plugins/Plugins.Core/FileIOPlugin.cs b/dotnet/src/Plugins/Plugins.Core/FileIOPlugin.cs
index c67b8a9ad089..c7106e76336a 100644
--- a/dotnet/src/Plugins/Plugins.Core/FileIOPlugin.cs
+++ b/dotnet/src/Plugins/Plugins.Core/FileIOPlugin.cs
@@ -113,7 +113,7 @@ private bool TryGetAllowedFilePath(string path, out string canonicalPath)
         Verify.NotNullOrWhiteSpace(path);
         canonicalPath = string.Empty;
 
-        if (path.StartsWith("\\\\", StringComparison.OrdinalIgnoreCase))
+        if (IsUncOrExtendedPath(path))
         {
             throw new ArgumentException("Invalid file path, UNC paths are not supported.", nameof(path));
         }
@@ -127,6 +127,13 @@ private bool TryGetAllowedFilePath(string path, out string canonicalPath)
 
         canonicalPath = PathUtilities.GetSafeFullPath(path);
 
+        // Re-check after canonicalization: resolving the path could produce a UNC
+        // or extended-path prefix that was not present in the original input.
+        if (IsUncOrExtendedPath(canonicalPath))
+        {
+            throw new ArgumentException("Invalid file path, UNC paths are not supported.", nameof(path));
+        }
+
         if (File.Exists(canonicalPath) && File.GetAttributes(canonicalPath).HasFlag(FileAttributes.ReadOnly))
         {
             // Most environments will throw this with OpenWrite, but running inside docker on Linux will not.
@@ -162,5 +169,20 @@ private bool TryGetAllowedFilePath(string path, out string canonicalPath)
 
         return false;
     }
+
+    /// <summary>
+    /// Determines whether the provided path is a UNC path (e.g., <c>\\server\share</c> or
+    /// <c>//server/share</c>) or an extended-length / device path (e.g., <c>\\?\</c>, <c>\\.\</c>).
+    /// Windows treats any two leading directory separators, in any combination of <c>\</c> and
+    /// <c>/</c>, as such a root, so all combinations are rejected for consistency with the other
+    /// file plugins. Both separator characters are checked explicitly so the result does not
+    /// depend on the current operating system's separator.
+    /// </summary>
+    private static bool IsUncOrExtendedPath(string path)
+    {
+        return path.Length >= 2 && IsDirectorySeparator(path[0]) && IsDirectorySeparator(path[1]);
+    }
+
+    private static bool IsDirectorySeparator(char c) => c is '\\' or '/';
     #endregion
 }
diff --git a/dotnet/src/Plugins/Plugins.UnitTests/Core/FileIOPluginTests.cs b/dotnet/src/Plugins/Plugins.UnitTests/Core/FileIOPluginTests.cs
index 29d5ba6c5d1d..d28cffb2aa43 100644
--- a/dotnet/src/Plugins/Plugins.UnitTests/Core/FileIOPluginTests.cs
+++ b/dotnet/src/Plugins/Plugins.UnitTests/Core/FileIOPluginTests.cs
@@ -144,6 +144,44 @@ public async Task ItCannotReadFromDisallowedFoldersAsync()
         await Assert.ThrowsAsync<ArgumentException>(async () => await plugin.ReadAsync(Path.Combine("", Path.GetRandomFileName())));
     }
 
+    [Theory]
+    [InlineData("\\\\server\\share\\file.txt")]
+    [InlineData("//server/share/file.txt")]
+    [InlineData("\\/server\\share\\file.txt")]
+    [InlineData("/\\server/share/file.txt")]
+    public async Task ItCannotReadFromUncPathsAsync(string uncPath)
+    {
+        // Arrange
+        var plugin = new FileIOPlugin() { AllowedFolders = [Path.GetTempPath()] };
+
+        // Act
+        var exception = await Assert.ThrowsAsync<ArgumentException>(() => plugin.ReadAsync(uncPath));
+
+        // Assert - the UNC guard (not another validation) must be what rejected the path
+        Assert.Contains("UNC paths are not supported", exception.Message, StringComparison.Ordinal);
+    }
+
+    [Theory]
+    [InlineData("\\\\server\\share\\file.txt")]
+    [InlineData("//server/share/file.txt")]
+    [InlineData("\\/server\\share\\file.txt")]
+    [InlineData("/\\server/share/file.txt")]
+    public async Task ItCannotWriteToUncPathsAsync(string uncPath)
+    {
+        // Arrange
+        var plugin = new FileIOPlugin()
+        {
+            AllowedFolders = [Path.GetTempPath()],
+            DisableFileOverwrite = false
+        };
+
+        // Act
+        var exception = await Assert.ThrowsAsync<ArgumentException>(() => plugin.WriteAsync(uncPath, "hello world"));
+
+        // Assert - the UNC guard (not another validation) must be what rejected the path
+        Assert.Contains("UNC paths are not supported", exception.Message, StringComparison.Ordinal);
+    }
+
     [Fact]
     public async Task ItCannotReadThroughSymlinkOutsideAllowedFoldersAsync()
     {