validation - Dave's Blog

Level 5 of the Stripe CTF revolved around a design issue in an OpenID like protocol.

Code

    def authenticated?(body)
      body =~ /[^\w]AUTHENTICATED[^\w]*$/
    end

...

      if authenticated?(body)
        session[:auth_user] = username
        session[:auth_host] = host
        return "Remote server responded with: #{body}." \
               " Authenticated as #{username}@#{host}!"

Issue

This level is an implementation of a federated identity protocol. You give it an endpoint URI and a username and password, it posts the username and password to the endpoint URI, and if the response is 'AUTHENTICATED' then access is allowed. It is easy to be authenticated on a server you control, but this level requires you to authenticate from the server running the level. This level only talks to stripe CTF servers so the first step is to upload a document to the level 2 server containing the text 'AUTHENTICATED' and we can now authenticate on a level 2 server. Notice that the level 5 server will dump out the content of the endpoint URI and that the regexp it uses to detect the text 'AUTHENTICATED' can match on that dump. Accordingly I uploaded an authenticated file to

https://level02-2.stripe-ctf.com/user-ajvivlehdt/uploads/authenticated

https://level05-1.stripe-ctf.com/user-qtoyekwrod/?pingback=https%3A%2F%2Flevel02-2.stripe-ctf.com%2Fuser-ajvivlehdt%2Fuploads%2Fauthenticated&username=a&password=a

Notes

I didn't see any particular code review red flags, really the issue here is that the regular expression testing for 'AUTHENTICATED' is too permisive and the protocol itself doesn't do enough. The protocol requires only a set piece of common literal text to be returned which makes it easy for a server to accidentally fall into authenticating. Having the endpoint URI have to return variable text based on the input would make it much harder for a server to accidentally authenticate.

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

          $filename = 'secret-combination.txt';
      extract($_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 "
Successfully uploaded your display picture.
";
    }

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:

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'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

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.

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:

More about each level in the future.