Add path traversal protection to serving files example

examples/src/serving_files.gleam

@@ -2,8 +2,15 @@ import ewe.{type Response}
 import gleam/erlang/process
 import gleam/http/response
 import gleam/option.{None}
+import gleam/string
 import logging
 
+@external(erlang, "filename", "absname")
+fn absname(path: String) -> String
+
+@external(erlang, "filename", "absname_join")
+fn absname_join(dir: String, file: String) -> String
+
 pub fn main() {
   logging.configure()
   logging.set_level(logging.Info)
@@ -19,26 +26,41 @@ pub fn main() {
   process.sleep_forever()
 }
 
+/// Resolves the URL path against the `root` directory and confirms the
+/// result stays inside it. Returns `Error(Nil)` for any traversal attempt.
+///
+fn safe_path(root: String, path: String) -> Result(String, Nil) {
+  let public_dir = absname(root)
+  let relative = string.drop_start(path, 1)
+  let resolved = absname_join(public_dir, relative)
+  case string.starts_with(resolved, public_dir <> "/") {
+    True -> Ok(resolved)
+    False -> Error(Nil)
+  }
+}
+
+fn not_found() -> Response {
+  response.new(404)
+  |> response.set_header("content-type", "text/plain; charset=utf-8")
+  |> response.set_body(ewe.TextData("File not found"))
+}
+
 fn serve_file(path: String) -> Response {
-  // Load file from disk using ewe.file(). This efficiently streams the file
-  // content without loading it entirely into memory.
-  //
-  // In production, make sure to validate paths to prevent directory traversal
-  // attacks! (e.g., requests to "../../../etc/passwd")
-  //
-  case ewe.file("public/" <> path, offset: None, limit: None) {
-    Ok(file) -> {
-      // Using "application/octet-stream" is safe for any file type, but you
-      // may want to specify content-type based on file extension in production.
+  case safe_path("public", path) {
+    Error(_) -> not_found()
+    Ok(safe) ->
+      // Load file from disk using ewe.file(). This efficiently streams the file
+      // content without loading it entirely into memory.
       //
-      response.new(200)
-      |> response.set_header("content-type", "application/octet-stream")
-      |> response.set_body(file)
-    }
-    Error(_) -> {
-      response.new(404)
-      |> response.set_header("content-type", "text/plain; charset=utf-8")
-      |> response.set_body(ewe.TextData("File not found"))
-    }
+      case ewe.file(safe, offset: None, limit: None) {
+        Ok(file) ->
+          // Using "application/octet-stream" is safe for any file type, but you
+          // may want to specify content-type based on file extension in production.
+          //
+          response.new(200)
+          |> response.set_header("content-type", "application/octet-stream")
+          |> response.set_body(file)
+        Error(_) -> not_found()
+      }
   }
 }