Stripe CTF - Input validation (Levels 1 & 2)

Stripe's web security CTF's Level 1 and level 2 of the Stripe CTF had issues with missing input validation solutions described below.

Level 1

Code

    <?php
      $filename = 'secret-combination.txt';
      extract($_GET);
      if (isset($attempt)) {
        $combination = trim(file_get_contents($filename));
        if ($attempt === $combination) {

Issue

The issue here is the usage of the extract php method which extracts name value pairs from the map input parameter and creates corresponding local variables. However this code uses $_GET which contains a map of name value pairs passed in the query of the URI. The expected behavior is to get an attempt variable out, but since no input validation is done I can provide a filename variable and overwrite the value of $filename. Providing an empty string gives an empty string $combination which I can match with an empty string $attempt. So without knowing the combination I can get past the combination check.

Notes

Code review red flag in this case was the direct use of $_GET with no validation. Instead of using extract the developer could try to extract specifically the attempt variable manually without using extract.

Level 2

Code

    $dest_dir = "uploads/";
    $dest = $dest_dir . basename($_FILES["dispic"]["name"]);
    $src = $_FILES["dispic"]["tmp_name"];
    if (move_uploaded_file($src, $dest)) {
      $_SESSION["dispic_url"] = $dest;
      chmod($dest, 0644);
      echo "<p>Successfully uploaded your display picture.</p>";
    }

Issue

This code accepts POST uploads of images but with no validation to ensure it is not an arbitrary file. And even though it uses chmod to ensure the file is not executable, things like PHP don't require a file to be executable in order to run them. Accordingly, one can upload a PHP script, then navigate to that script to run it. My PHP script dumped out the contents of the file we're interested in for this level:

<?php echo file_get_contents("../password.txt"); ?>

Notes

Code review red flags include manual file management, chmod, and use of file and filename inputs without any kind of validation. If this code controlled the filename and ensured that the extension was one of a set of image extensions, this would solve this issue. Due to browser mime sniffing its additionally a good idea to serve a content-type that starts with "image/" for these uploads to ensure browsers treat these as images and not sniff for script or HTML.

Stripe CTF - SQL injections (Levels 0 & 3)

Stripe's web security CTF's level 0 and level 3 had SQL injection solutions described below.

Level 0

Code

app.get('/*', function(req, res) {
  var namespace = req.param('namespace');

  if (namespace) {
    var query = 'SELECT * FROM secrets WHERE key LIKE ? || ".%"';
    db.all(query, namespace, function(err, secrets) {

Issue

There's no input validation on the namespace parameter and it is injected into the SQL query with no encoding applied. This means you can use the '%' character as the namespace which is the wildcard character matching all secrets.

Notes

Code review red flag was using strings to query the database. Additional levels made this harder to exploit by using an API with objects to construct a query rather than strings and by running a query that only returned a single row, only ran a single command, and didn't just dump out the results of the query to the caller.

Level 3

Code

@app.route('/login', methods=['POST'])
def login():
    username = flask.request.form.get('username')
    password = flask.request.form.get('password')

    if not username:
        return "Must provide username\n"

    if not password:
        return "Must provide password\n"

    conn = sqlite3.connect(os.path.join(data_dir, 'users.db'))
    cursor = conn.cursor()

    query = """SELECT id, password_hash, salt FROM users
               WHERE username = '{0}' LIMIT 1""".format(username)
    cursor.execute(query)

    res = cursor.fetchone()
    if not res:
        return "There's no such user {0}!\n".format(username)
    user_id, password_hash, salt = res

    calculated_hash = hashlib.sha256(password + salt)
    if calculated_hash.hexdigest() != password_hash:
        return "That's not the password for {0}!\n".format(username)

Issue

There's little input validation on username before it is used to constrcut a SQL query. There's no encoding applied when constructing the SQL query string which is used to, given a username, produce the hashed password and the associated salt. Accordingly one can make username a part of a SQL query command which ensures the original select returns nothing and provide a new SELECT via a UNION that returns some literal values for the hash and salt. For instance the following in blue is the query template and the red is the username injected SQL code:

SELECT id, password_hash, salt FROM users WHERE username = 'doesntexist' UNION SELECT id, ('5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8') AS password_hash, ('word') AS salt FROM users WHERE username = 'bob' LIMIT 1

In the above I've supplied my own salt and hash such that my salt (word) plus my password (pass) hashed produce the hash I provided above. Accordingly, by providing the above long and interesting looking username and password as 'pass' I can login as any user.

Notes

Code review red flag is again using strings to query the database. Although this level was made more difficult by using an API that returns only a single row and by using the execute method which only runs one command. I was forced to (as a SQL noob) learn the syntax of SELECT in order to figure out UNION and how to return my own literal values.

Stripe Web Security CTF Summary

I was the 546th person to complete Stripe's web security CTF and again had a ton of fun applying my theoretical knowledge of web security issues to the (semi-)real world. As I went through the levels I thought about what red flags jumped out at me (or should have) that I could apply to future code reviews:

Level	Issue	Code Review Red Flags
0	Simple SQL injection	No encoding when constructing SQL command strings. Constructing SQL command strings instead of SQL API
1	extract($_GET);	No input validation.
2	Arbitrary PHP execution	No input validation. Allow file uploads. File permissions modification.
3	Advanced SQL injection	Constructing SQL command strings instead of SQL API.
4	HTML injection, XSS and CSRF	No encoding when constructing HTML. No CSRF counter measures. Passwords stored in plain text. Password displayed on site.
5	Pingback server doesn't need to opt-in	n/a - By design protocol issue.
6	Script injection and XSS	No encoding while constructing script. Deny list (of dangerous characters). Passwords stored in plain text. Password displayed on site.
7	Length extension attack	Custom crypto code. Constructing SQL command string instead of SQL API.
8	Side channel attack	Password handling code. Timing attack mitigation too clever.

More about each level in the future.

Web Security Contest - Stripe CTF

Stripe is running a web security capture the flag - a series of increasingly difficult web security exploit challenges. I've finished it and had a lot of fun. Working on a web browser I knew the theory of these various web based attacks, but this was my first chance to put theory into practice with:

• No adverse consequences
• Knowledge that there is a fun security exploit to find
• Access to the server side source code

Here's a blog post on the CTF behind the scenes setup which has many impressive features including phantom users that can be XSS/CSRF'ed.

I'll have another post on my difficulties and answers for the CTF levels after the contest is over on Wed, but if you're looking for hints, try out the CTF chatroom or the level specific CTF chatroom.

HTTP Compression Documentation Reference

There's a lot of name reuse in HTTP compression so I've made the following to help myself keep it straight.

HTTP Content Coding Token	gzip	deflate	compress
	An encoding format produced by the file compression program "gzip" (GNU zip)	The "zlib" format as described in RFC 1950.	The encoding format produced by the common UNIX file compression program "compress".
Data Format	GZIP file format	ZLIB Compressed Data Format	The compress program's file format
Compression Method	Deflate compression method		LZW
	Deflate consists of LZ77 and Huffman coding

Compress doesn't seem to be supported by popular current browsers, possibly due to its past with patents.

Deflate isn't done correctly all the time. Some servers would send the deflate data format instead of the zlib data format and at least some versions of Internet Explorer expect deflate data format instead of zlib data format.

Application Protocols in Windows 8

In Windows 8 you can still register a desktop application to handle a particular URI scheme, but now you can also register a Metro Win8 application to handle a particular URI scheme. No more manually modifying the registry - now there's pretty UI in VS to handle this.

Changing Windows Live IDs

Use of my old Hotmail account has really snuck up on me as I end up caring more and more about all of the services with which it is associated. The last straw is Windows 8 login, but previous straws include Xbox, Zune, SkyDrive, and my Windows 7 Phone. I like the features and sync'ing associated with the Windows Live ID, but I don't like my old, spam filled, hotmail email address on the Live ID account.

A coworker told me about creating a Live ID from a custom domain, which sounded like just the ticket for me. Following the instructions above I was able to create a new deletethis.net Live ID but the next step of actually using this new Live ID was much more difficult. My first hope was there would be some way to link my new and old Live IDs so as to make them interchangeable. As it turns out there is a way to link Live IDs but all that does is make it easy to switch between accounts on Live Mail, SkyDrive and some other webpages.

Instead one must change over each service or start over depending on the service:

Xbox

In the Xbox 360 system menu you can change the Live ID associated with your gamertag. This worked fine for me and I got an email telling me about the transfer of my Microsoft Points.

Zune

There's no way to do this for the Zune specifically, however changing over your Xbox account also transfers over all your Zune purchased content. I don't have a Zune Pass so I can't confirm that, but all of my previously purchased television shows transferred over successfully.

Windows 7 Phone

To change the main Live ID associated with your phone, reset your phone to factory default and start over. All purchased applications are lost. Had I purchased any applications I would have been pissed, but instead I was just irritated that I had to reset my phone.

Mail

I don't use my Hotmail account for anything and it only sits and collects spam. Accordingly I didn't attempt switching this over.

SkyDrive

I didn't have much in my SkyDrive account. I downloaded all files as a zip and then manually uploaded them to the new account.