Very secure code.

flask.py

@@ -0,0 +1,32 @@
+import os
+from flask import Flask, request, abort
+
+app = Flask(__name__)
+
+# The directory where user files are stored
+UPLOAD_DIR = "/app/user_uploads/"
+
+@app.route('/download')
+def download_file():
+    # Get the filename from a URL parameter (e.g., /download?file=report.pdf)
+    filename = request.args.get('file')
+
+    if not filename:
+        return "Missing 'file' parameter", 400
+
+    # !!! VULNERABLE LINE !!!
+    # The user's input is directly joined to the directory path.
+    file_path = os.path.join(UPLOAD_DIR, filename)
+
+    try:
+        # The server attempts to read and return the file
+        with open(file_path, 'r') as f:
+            return f.read()
+    except FileNotFoundError:
+        abort(404, "File not found.")
+    except Exception:
+        abort(500, "An error occurred.")
+
+if __name__ == '__main__':
+    # Note: This is a dev server, not for production.
+    app.run(debug=True)

secure_code.py

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+import subprocess
+
+# Get untrusted input from a user
+hostname = input("Please enter the hostname to ping: ")
+
+# VULNERABLE: Building a command string and using shell=True
+# The f-string directly inserts the user's text into the command.
+command = f"ping -c 3 {hostname}"
+
+print(f"--- Running command: {command} ---")
+
+# shell=True tells Python to pass the entire string to the system's shell 
+# (like /bin/sh or cmd.exe) to be interpreted.
+try:
+    subprocess.run(command, shell=True, check=True)
+except subprocess.CalledProcessError as e:
+    print(f"Command failed: {e}")
+
+print("--- Ping finished ---")