Esoteric Web Application Vulnerabilities OWASP LATAM Tour 

/me ● Application security expert (web|API) ● Developer (Python!) ● Open Source evangelist ● w3af project leader ● Independent consultant 

Intro: SQL injection is dead I don't care 

ORM killed the pentest star ● All modern web development frameworks provide abstractions to interact with (no) SQL databases. Developers don’t write raw SQL queries anymore. ● SQL injections are rare nowadays, this requires us testers to dig deeper into the application to find high risk vulnerabilities. 

MVC, templates and default HTML encode killed XSS ● Most modern web development frameworks use a model view controller architecture, which uses templates to render the HTML shown to users. ● Templating engines, such as Jinja2, HTML encode the context data by default. ● Developers need to write more code to make the template vulnerable to Cross-Site Scripting, which leads to less vulnerabilities.

{% for user in user_list %}
• {{ user.username }}
{% endfor %}



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“Aggressive Input Decoding”.to_s Esoteric #1 

Aggressive input decoding Ruby on Rails, Sinatra and other (ruby) web frameworks perform aggressive input decoding: http://www.phrack.org/papers/attacking_ruby_on_rails.html post '/hello' do name = params[:name] render_response 200, name POST /hello HTTP/1.1 Host: example.com Content-Type: application/x-www-form-urlencoded name=andres POST /hello HTTP/1.1 Host: example.com Content-Type: application/json {"name": "andres"} 

Decode to a Ruby Hash POST /hello HTTP/1.1 Host: example.com Content-Type: application/json {"name": {"foo": 1}} In all previous cases the type of the name variable was a String, but we can force it to be a Hash: 

noSQL ODM introduction When MongoId ODM (Object Document Mapper) and similar frameworks are in use developers can write code similar to: Which will query the Mongo database and return the first registration flow where the user_id and confirmation_token match. post '/registration/complete' do registration = Registration.where({ user_id: params[:user_id], confirmation_token: params[:token] }).first ... POST /registration/complete HTTP/1.1 Host: vulnerable.com Content-Type: application/json {"token": "dee1...bb8e", "user_id": 3578} 

noSQL ODM complex queries Developers can write “complex” ODM queries using Ruby Hashes as parameters: user = Users.where({user_id: params[:user_id], country: {"$ne": "Argentina"}}).first users = Users.where({user_id: {"$in": [123, 456, 789]}}) 

Decode to Hash leads to noSQL injection It’s possible to bypass the token validation! post '/registration/complete' do registration = Registration.where({ user_id: params[:user_id], confirmation_token: params[:token] }).first ... POST /registration/complete HTTP/1.1 Host: vulnerable.com Content-Type: application/json {"token": {"$ne": "nomatch"}, "user_id": 3578} 

“User controlled input”.to_s Fixing this vulnerability is quick and easy: Most developers will forget to add the .to_s and it’s easy to miss in a source code review. Recommend Sinatra param or similar. get '/registration/complete' do @registration = Registration.where({ user_id: params[:user_id].to_s, confirmation_token: params[:token].to_s }).first ... 

Esoteric #2 Host header injection 

Call me to verify my identity #1 The application requires users to provide a cellphone to verify their identity. A phone call is initiated by the application using a service like Twilio, the call audio contains a verification code which needs to be input into the application to verify phone ownership. HTTP request Verify my phone +1 (541) 754-3010 

Call me to verify my identity #2 Call +1 (541) 754-3010 Send code 357896 in audio HTTP request Please call +1 (541) 754-3010 Audio for the call is available at https://vulnerable.com/audio/ HTTP request https://vulnerable.com/audio/ 

Call me to verify my identity #3 HTTP request Code is 357896 HTTP response Welcome admin! 

Bypass phone verification Hacker wants to bypass phone verification, ideas: ○ Hack admin’s smartphone ○ Hack vulnerable.com ○ Create a raw cellphone tower and sniff admin’s phone call ○ Hack Twilio Hacking vulnerable.com seems to be the easiest path to follow. But… what do we need? 

UUID4 Version 4 UUIDs use a scheme relying only on random numbers, thus the audio URLs can’t be brute forced: https://vulnerable.com/audio/f47ac10b-58cc-4372-a567-0e02b2c3d479 

Zoom into HTTP request to Twilio HTTP request Please call +1 (541) 754-3010 Audio for the call is available at https://vulnerable.com/audio/ POST /call/new HTTP/1.1 Host: api.twilio.com Content-Type: application/json X-Authentication-Api-Key: 2bc67a5... {"phone_number": "+1 (541) 754-3010"}, "audio_callback": "https://vulnerable.com/f47ac10b-5..."} 

Insecure Twilio API call HTTP request Please call +1 (541) 754-3010 Audio for the call is available at https://vulnerable.com/audio/ import requests def start_call(phone, callback_url): requests.post('https://api.twilio.com/call', data={'phone_number': phone, 'audio_callback': callback_url}) … audio_id = generate_audio(request.user_id) callback_url = 'https://%s/%s' % (request.host, audio_id) start_call(request['phone'], callback_url) 

Change Host header to exploit HTTP request Verify my phone +1 (541) 754-3010 POST /verify-my-phone HTTP/1.1 Host: vulnerable.com Content-Type: application/json {"phone_number": "+1 (541) 754-3010"}} POST /verify-my-phone HTTP/1.1 Host: evil.com Content-Type: application/json {"phone_number": "+1 (541) 754-3010"}} 

Exploit results in modified callback_url HTTP request Please call +1 (541) 754-3010 Audio for the call is available at https://evil.com/audio/ HTTP request https://evil.com/audio/ HTTP request https://vulnerable.com/audio/ 

MUST-HAVE: Strict validation for Host header ● Make sure that your nginx, apache, and web frameworks validate the host header before any further code is run. ● Django has strict host header validation built in using ALLOWED_HOSTS configuration setting. 

Esoteric #3 null, nil and NULL 

Password reset ● Password resets are very sensitive and, in some cases, insecure. The most wanted vulnerability is to be able to reset the password for a user for which we don’t have the password reset token. ● Usually password resets are implemented as follows: ○ User starts a new password reset flow ○ An email is sent by the application containing a randomly generated token ○ The token is used to prove that the user has access to the email address and the password is reset. 

Implementation details class AddPasswordResetTokenToUser < ActiveRecord::Migration def change add_column :users, :pwd_reset_token, :string, default: nil end end post '/start-password-reset' do: user = Users.where({"email": params["email"]}).first token = generate_random_token() user.pwd_reset_token = token user.save! send_email(user.email, token) post '/complete-password-reset' do: user = Users.where({"pwd_reset_token": params["token"]}).first user.password = params["new_password"] user.pwd_reset_token = nil user.save! 

Token defaults to NULL in the database POST /complete-password-reset HTTP/1.1 Host: vulnerable.com Content-Type: application/json {"token": null, "new_password": "l3tm31n"} ● Each time a new user is created his pwd_reset_token field is set to NULL in the database. ● When the user starts a new password reset flow a randomly generated token is assigned to pwd_reset_token ● What if... 

Safe defaults and strict type validation post '/complete-password-reset' do: user = Users.where({"pwd_reset_token": params["token"].to_s}).first user.password = params["new_password"] user.pwd_reset_token = nil user.save! class AddPasswordResetTokenToUser < ActiveRecord::Migration def change add_column :users, :pwd_reset_token, :string, default: generate_random_token() end end 

Esoteric #4 Paypal double spend 

Paypal’s Instant Payment Notification ● I love payment gateways! See my previous talk on this subject. ● Paypal uses IPN to notify a site that a new payment has been processed and further action, such as increasing the user funds in the application, should be performed. ● The developer sets the IPN URL in the merchant account settings at Paypal: https://www.example.com/paypal-handler 

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Zoom into Paypal’s IPN HTTP request POST /paypal-handler HTTP/1.1 Host: www.example.com Content-Type: application/x-www-form-urlencoded mc_gross=19.95&protection_eligibility=Eligible&address_status=confirmed&p ayer_id=LPLWNMTBWMFAY&tax=0.00&address_street=1+Main+St&payment_date=20%3 A12%3A59+Jan+13%2C+2009+PST&payment_status=Completed&charset=windows-1252 &address_zip=95131&first_name=Test&mc_fee=0.88&address_country_code=US&ad dress_name=Test+User¬ify_version=2.6&custom=665588975&payer_status=ver ified&address_country=United+States&address_city=San+Jose&quantity=1&veri fy_sign=AtkOfCXbDm2hu0ZELryHFjY-Vb7PAUvS6nMXgysbElEn9v-1XcmSoGtf&payer_em ail=gpmac_1231902590_per%40paypal.com&txn_id=61E67681CH3238416&payment_ty pe=instant&last_name=User&address_state=CA&receiver_email=gpmac_123190268 6_biz%40paypal.com&payment_fee=0.88&receiver_id=S8XGHLYDW9T3S&txn_type=ex press_checkout&item_name=&mc_currency=USD&item_number=&residence_country= US&handling_amount=0.00&transaction_subject=&payment_gross=19.95&shipping =0.00 

Zoom into Paypal’s IPN HTTP request There are a few important parameters that we need to understand: ● mc_gross=19.95 is the amount paid by the user ● custom=665588975 is the user’s ID at the merchant application, which is sent to Paypal when the user clicks the “Pay with Paypal” button in the merchant’s site ● receiver_email=gpmac_1231902686_biz%40paypal.com is the merchant’s email address ● payment_status=Completed is the payment status 

Why does the merchant verify the IPN data? 

Insecure IPN handler import requests PAYPAL_URL = 'https://www.paypal.com/cgi-bin/webscr?cmd=_notify-validate' def handle_paypal_ipn(params): # params contains all parameters sent by Paypal response = requests.post(PAYPAL_URL, data=params).text if response == 'VERIFIED': # The payment is valid at Paypal, mark the cart instance as paid cart = Cart.get_by_id(params['custom']) cart.record_user_payment(params['mc_gross']) cart.user.send_thanks_email else: return 'Error' 

Insecure IPN handlers No receiver email check 

Insecure IPN handlers No receiver email check 

● Attacker needs to perform a special Paypal payment using a target specific custom_id parameter which will associate the spoofed payment with his account. ● The payment is made from the attacker’s credit card to his paypal account. Money is still under his control, but the attacker will lose Paypal’s commission for each transaction. ● Many example IPN implementations in github.com are vulnerable. I wonder how many were used to create applications which are currently live in production? 

Secure IPN handler import requests PAYPAL_URL = 'https://www.paypal.com/cgi-bin/webscr?cmd=_notify-validate' MERCHANT_PAYPAL_USER = '[email protected]' def handle_paypal_ipn(params): if params['receiver_email'] == MERCHANT_PAYPAL_USER: return 'Error' # params contains all parameters sent by Paypal response = requests.post(PAYPAL_URL, data=params).text if response == 'VERIFIED': # The payment is valid at Paypal, mark the cart instance as paid cart = Cart.get_by_id(params['custom']) cart.record_user_payment(params['mc_gross']) cart.user.send_thanks_email else: return 'Error' 

Is this Paypal’s fault? ● Are all payment gateways vulnerable? ● MercadoPago implemented a different communication protocol for their IPN. Their protocol is much better than Paypal’s since it doesn’t rely on the developer’s IPN handler implementation to provide security. ● MercadoPago sends a GET request with the purchase ID to the IPN URL, then the developer needs to perform a GET request to https://api.mercadopago.com/ in order to retrieve the transaction details. This request is authenticated, and any attempts to access transactions from other merchants is denied. 

Esoteric #5 MessageVerifier Marshal RCE 

ActiveSupport::MessageVerifier Marshal RCE ● ActiveSupport::MessageVerifier uses Ruby’s Marshal to serialize arbitrary information, which is then signed using a developer provided secret. A verified message looks like: ● The message can be decoded: BAhJIhphb...XNlYy5jb20GOgZFVA==--8bacd5cb3e72ed7c457aae1875a61d668438b616 1.9.3-p551 :006 > Base64.decode64('BAhJIhphbmRyZXNAYm9uc2FpLXNlYy5jb20GOgZFVA==') => "\x04\bI\"\[email protected]\x06:\x06ET" 1.9.3-p551 :007 > Marshal.load(Base64.decode64('BAhJIhphbmRyZXNAYm9uc2FpLXNlYy5jb20GOgZFVA==')) => "[email protected]" 1.9.3-p551 :008 > 

ActiveMessages are signed ● When the application receives the signed message, it will take the base64 encoded data and calculate HMAC SHA1 for it using using the developer controlled secret. ● The calculated signature must match the one provided with the message: ● Once the signature is verified the data is base64 decoded and Unmarshaled. BAhJIh...--8bacd5cb3e72ed7c457aae1875a61d668438b616 

Guessable signing secret leads to RCE Ruby’s documentation clearly states that unmarshaling arbitrary data is insecure and will lead to arbitrary code execution. ActiveSupport::MessageVerifier is protected against this vulnerability by a developer controlled secret. Poorly chosen secrets allow: Brute-force attack to discover the secret Specially crafted gadget/object is created, serialized and encoded. Secret is used to sign gadget Signed message is sent to the application, where it will be unmarshalled and remote code execution is achieved 

Secure ActiveSupport::MessageVerifier usage ● Choose randomly generated, long, secrets to sign your messages. ● Use a different serialization method: @verifier = ActiveSupport::MessageVerifier.new(long_secret, serializer: json) 

Outro: That’s why I don’t care 

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Vulnerabilities are always there ● You’re smarter than your tools. Let the automation do the grunt work and focus your time on source code review, application logic flaws, issues specific to the target application, etc. ● You’re smarter than your client. Convince them that with the source code you’ll be able to identify more vulnerabilities and provide greater ROI. ● You’re smarter (well, actually more trained in security, vulnerabilities and risks) than most developers. They will make mistakes, no matter how good they are. 

Thanks! 

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