Worksheet

F11.8 Writing secure programs

Cyber security · GCSE · OCR J277 1.4.2, AQA 8525 3.6.3, Edexcel 1CP2 5.3.2 · about 15 min

What this lesson is about

Validating input, access levels in code, and testing for security.

• Never trust input
• Access levels in code
• More defensive habits
• Testing for security

Questions 5 marks in all

1. [1 mark]How does a program reduce the risk of SQL injection?

   1. AValidating input and keeping it separate from commands
   2. BRunning faster
   3. CUsing a bigger database
   4. DEncrypting the screen

2. [1 mark]What does 'fail safely' mean?

   1. AIf something goes wrong, stop in a safe state and reveal nothing
   2. BIgnore all errors
   3. CCrash loudly with details
   4. DKeep running whatever happens

3. [1 mark]What is the principle of least access?

   1. AGive each user or program only the access its job needs
   2. BGive everyone admin access
   3. CRemove all access
   4. DShare one account

4. [1 mark]Why should you test with invalid and boundary input?

   1. AAttacks often use unexpected input
   2. BIt makes the code shorter
   3. CValid input never fails
   4. DIt speeds up the program

5. [1 mark]What does this program print?

   def ok(t):
       return t.isdigit() and 0 <= int(t) <= 100
   print(ok('50'), ok('999'), ok('go'))

The task: an access checker

Write allowed(role, action) using the can_do table, returning True or False. For each (user, role, action) in requests, print <user> (<role>) <action>: allowed or : denied. Validate first: if the role is not in can_do, print <user> (<role>) <action>: unknown role instead. At the end print denied or blocked: <n>.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

can_do = {"student": ["view"], "teacher": ["view", "edit"], "admin": ["view", "edit", "delete"]}
# user, role, action
requests = [
    ("Sam", "student", "view"),
    ("Sam", "student", "edit"),
    ("Mr Lee", "teacher", "edit"),
    ("root", "admin", "delete"),
    ("ghost", "hacker", "delete"),
]

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/f11-8-writing-secure-programs/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Add a governor role that can only view. Check a governor cannot delete.
2. Extend safe_speed from the first cell to also reject an empty string and text with spaces.
3. Explain how validating input stops SQL injection.