diff --git a/scripts/Invoke-M4DetectorShadow.ps1 b/scripts/Invoke-M4DetectorShadow.ps1
index 8545f59..960f0ab 100644
--- a/scripts/Invoke-M4DetectorShadow.ps1
+++ b/scripts/Invoke-M4DetectorShadow.ps1
@@ -114,10 +114,17 @@ function Write-Utf8NoBom([string]$Path, [string]$Value) {
 }
 
 function Get-DirectoryTreeSha256([string]$Root, [object[]]$Files) {
+    $rootPrefix = $Root.TrimEnd("\") + "\"
     $rows = @(
         foreach ($file in $Files) {
+            if (-not $file.FullName.StartsWith(
+                $rootPrefix,
+                [StringComparison]::OrdinalIgnoreCase
+            )) {
+                throw "M4 dependency file escaped its declared root"
+            }
             [pscustomobject]@{
-                Relative = [IO.Path]::GetRelativePath($Root, $file.FullName).Replace("\", "/")
+                Relative = $file.FullName.Substring($rootPrefix.Length).Replace("\", "/")
                 File = $file
             }
         }