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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

BugBotLab

What this lesson is about

Validating input, access levels in code, and 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
    Answer: A. Never drop untrusted input straight into a query.

  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
    Answer: A. BugBot stops its motors if the program crashes.

  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
    Answer: A. It limits the damage a breach can do.

  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
    Answer: A. A secure program is attacked by its own tests first.

  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'))
    Answer:
    True False False

    50 is valid; 999 is out of range; 'go' is not a number.

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"),
]

The hint students can ask for: Check the role exists before anything else, and say so if it does not. Otherwise look the role up and see whether the action is in the list of things it may do. Count everything that was not allowed.

A solution

from bugbot import *
connect()
can_do = {"student": ["view"], "teacher": ["view", "edit"], "admin": ["view", "edit", "delete"]}
requests = [
    ("Sam", "student", "view"),
    ("Sam", "student", "edit"),
    ("Mr Lee", "teacher", "edit"),
    ("root", "admin", "delete"),
    ("ghost", "hacker", "delete"),
]

def allowed(role, action):
    return action in can_do.get(role, [])

blocked = 0
for user, role, action in requests:
    if role not in can_do:
        print(f"{user} ({role}) {action}: unknown role")
        blocked = blocked + 1
    elif allowed(role, action):
        print(f"{user} ({role}) {action}: allowed")
    else:
        print(f"{user} ({role}) {action}: denied")
        blocked = blocked + 1
print("denied or blocked:", blocked)

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.