1 files changed, 52 insertions, 0 deletions
diff --git a/src/Agent.Windows/Security/PathPolicy.cs b/src/Agent.Windows/Security/PathPolicy.cs
new file mode 100644
index 0000000..32a949c
--- /dev/null
+++ b/src/Agent.Windows/Security/PathPolicy.cs
@@ -0,0 +1,52 @@
+namespace Agent.Windows.Security;
+
+[Flags]
+public enum FileCapability { None = 0, Read = 1, Write = 2, Execute = 4, Delete = 8 }
+
+public sealed record FilePolicyEntry(string Path, FileCapability Capabilities);
+
+public sealed class PathPolicy(IEnumerable<FilePolicyEntry> entries)
+{
+    private readonly IReadOnlyList<FilePolicyEntry> _entries = entries.Select(entry => entry with { Path = CanonicalPath.Resolve(entry.Path, requireExisting: false) }).ToArray();
+
+    public bool IsAllowed(string path, FileCapability required)
+    {
+        string canonical = CanonicalPath.Resolve(path, requireExisting: required is FileCapability.Read or FileCapability.Execute or FileCapability.Delete);
+        return _entries.Any(entry => IsAtOrBelow(canonical, entry.Path) && (entry.Capabilities & required) == required);
+    }
+
+    public void Demand(string path, FileCapability required)
+    {
+        if (!IsAllowed(path, required)) throw new SandboxPolicyException("policy.deny", $"'{path}' is not approved for {required}.");
+    }
+
+    private static bool IsAtOrBelow(string candidate, string root) =>
+        candidate.Equals(root, StringComparison.OrdinalIgnoreCase) ||
+        candidate.StartsWith(root.TrimEnd(Path.DirectorySeparatorChar) + Path.DirectorySeparatorChar, StringComparison.OrdinalIgnoreCase);
+}
+
+public static class CanonicalPath
+{
+    public static string Resolve(string path, bool requireExisting = true)
+    {
+        if (string.IsNullOrWhiteSpace(path)) throw new SandboxPolicyException("path.invalid", "Path is required.");
+        string fullPath = Path.GetFullPath(path);
+        string? root = Path.GetPathRoot(fullPath);
+        if (string.IsNullOrEmpty(root)) throw new SandboxPolicyException("path.invalid", "Path has no filesystem root.");
+        string current = root;
+        foreach (string segment in fullPath[root.Length..].Split(Path.DirectorySeparatorChar, StringSplitOptions.RemoveEmptyEntries))
+        {
+            current = Path.Combine(current, segment);
+            if (!File.Exists(current) && !Directory.Exists(current)) continue;
+            if ((File.GetAttributes(current) & FileAttributes.ReparsePoint) != 0)
+                throw new SandboxPolicyException("path.reparse", $"Reparse points are not allowed: '{current}'.");
+        }
+        if (requireExisting && !File.Exists(fullPath) && !Directory.Exists(fullPath)) throw new FileNotFoundException("Approved path does not exist.", fullPath);
+        return Path.TrimEndingDirectorySeparator(fullPath);
+    }
+}
+
+public sealed class SandboxPolicyException(string code, string message) : UnauthorizedAccessException(message)
+{
+    public string Code { get; } = code;
+}