Web Programming Step by Step

Lecture 21
Web Security

1. The "security mindset"

security mindset
some basic web attacks
breaking and securing an example page

CSE <= 190M

until now, we have assumed:
- valid user input
- non-malicious users
- nothing will ever go wrong
this is unrealistic!

The real world

in order to write secure code, we must assume:
- invalid input
- evil users
- incompetent users
- everybody is out to get you
- botnets, hackers, script kiddies, KGB, etc. are out there
trust nothing

2. Some basic web attacks

security mindset
some basic web attacks
breaking and securing an example page

HTML injection

a flaw where a user is able to inject arbitrary HTML content into your page

This flaw often exists when a page accepts user input and inserts it bare into the page.
example: magic 8-ball
- https://webster.cs.washington.edu/stepp/security/8ball.html
- What kinds of silly or malicious content can we inject into the page? Why is this bad?

Injecting HTML content

8ball.php?question=<blink>lololol</blink>

injected content can lead to:
- annoyance / confusion
- damage to data on the server
- exposure of private data on the server
- financial gain/loss
- end of the human race as we know it
why is HTML injection bad? It allows others to:
- disrupt the flow/layout of your site
- put words into your mouth
- possibly run malicious code on your users' computers

Cross-site scripting

a flaw where a user is able to inject and execute arbitrary JavaScript code in your page

8ball.php?question=<script type='text/javascript>alert('pwned');</script>

JavaScript is often able to be injected because of a previous HTML injection
example: Lab 4 (Buy-a-Grade)
- https://webster.cs.washington.edu/stepp/security/buyagrade.html
- How can we inject JavaScript code into the page? Why is this bad?
injected script code can:
- masquerade as the original page and trick the user into entering sensitive data
- steal the user's cookies
- masquerade as the user and submit data on their behalf (submit forms, click buttons, etc.)
- ...

Securing against HTML injection

one idea: disallow harmful characters
- HTML injection is impossible without < >
- can strip those characters from input, or reject the entire request if they are present
better idea: allow them, but escape them

htmlspecialchars returns an HTML-escaped version of a string

$text = "<p>hi 2 u & me</p>";
$text = htmlspecialchars($text);   # "&lt;p&gt;hi 2 u &amp; me&lt;/p&gt;"

SQL injection

a flaw where the user is able to inject arbitrary SQL commands into your query

This flaw often exists when a page accepts user input and inserts it bare into the query.
example: simpsons grade lookup
- https://webster.cs.washington.edu/stepp/security/grades.html
- What kinds of SQL can we inject into the query? Why is this bad?

A SQL injection attack

The query in the Simpsons PHP code is:

$query = "SELECT * FROM users
WHERE username = '$username' AND password = '$password'";

Are there malicious values for the user name and password that we could enter?
Password:

This causes the query to be executed as:

$query = "SELECT * FROM users
WHERE username = '$username' AND password = '' OR '1'='1'";

What will the above query return? Why is this bad?

Too true...

injected SQL can:
- change the query to output others' data (revealing private information)
- insert a query to modify existing data (increase bank account balance)
- delete existing data (; DROP TABLE students; -- )
- bloat the query to slow down the server (JOIN a JOIN b JOIN c ... )


			
			
			
			
				Securing against SQL injection
				
				
					similar to securing against HTML injection, escape the string before you include it in your query
				
				
				
					
						
							mysql_real_escape_string
						
						
							returns a SQL-escaped version of a string
						
					
				

				$username = mysql_real_escape_string($_REQUEST["username"]);
$password = mysql_real_escape_string($_REQUEST["password"]);
$query = "SELECT name, ssn, dob FROM users
WHERE username = '$username' AND password = '$password'";



				
					replaces ' with \', etc.
				
			
			
			
			
			
				3. Breaking and securing an example page
				
				
					PHP/SQL review

					some basic web attacks
					breaking and securing an example page
				
			



			
				Practice problem: Hack Marty's turnin

				
				
					How can we break this page? 

						https://webster.cs.washington.edu/stepp/security/turnin/
					
					
					We want to cheat on Homework Assignment 7, Song.java.  We want to find a way to submit a perfect working solution without doing any real work.
					
					We got a low grade on a past assignment, so if possible, we want to set our past grades to be higher than they are now.
					
					Our enemy is fellow classmate Felix Chu.  We want to find out his personal information (password, email, student ID, grade, etc.).
					
					We don't like the course instructor, Marty Stepp.  We want to make the turnin page print an embarrassing message about him.

Web Programming Step by Step, Lecture 21

Web Security