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Paypal Inc - Open Redirect Web Vulnerability



Document Title:
===============
Paypal Inc - Open Redirect Web Vulnerability


References (Source):
====================
http://www.vulnerability-lab.com/get_content.php?id=1598

Vulnerability Magazine: http://magazine.vulnerability-db.com/?q=articles/2015/09/11/paypal-inc-%C2%A0medium-severity-open-redirect-web-vulnerability-fixed


Release Date:
=============
2015-09-11


Vulnerability Laboratory ID (VL-ID):
====================================
1598


Common Vulnerability Scoring System:
====================================
3.3


Product & Service Introduction:
===============================
PayPal is a global e-commerce business allowing payments and money transfers to be made through the Internet. Online money 
transfers serve as electronic alternatives to paying with traditional paper methods, such as checks and money orders. Originally, 
a PayPal account could be funded with an electronic debit from a bank account or by a credit card at the payer s choice. But some 
time in 2010 or early 2011, PayPal began to require a verified bank account after the account holder exceeded a predetermined 
spending limit. After that point, PayPal will attempt to take funds for a purchase from funding sources according to a specified 
funding hierarchy. If you set one of the funding sources as Primary, it will default to that, within that level of the hierarchy 
(for example, if your credit card ending in 4567 is set as the Primary over 1234, it will still attempt to pay money out of your 
PayPal balance, before it attempts to charge your credit card). The funding hierarchy is a balance in the PayPal account; a 
PayPal credit account, PayPal Extras, PayPal SmartConnect, PayPal Extras Master Card or Bill Me Later (if selected as primary 
funding source) (It can bypass the Balance); a verified bank account; other funding sources, such as non-PayPal credit cards.
The recipient of a PayPal transfer can either request a check from PayPal, establish their own PayPal deposit account or request 
a transfer to their bank account.

PayPal is an acquirer, performing payment processing for online vendors, auction sites, and other commercial users, for which it 
charges a fee. It may also charge a fee for receiving money, proportional to the amount received. The fees depend on the currency 
used, the payment option used, the country of the sender, the country of the recipient, the amount sent and the recipient s account 
type. In addition, eBay purchases made by credit card through PayPal may incur extra fees if the buyer and seller use different currencies.

On October 3, 2002, PayPal became a wholly owned subsidiary of eBay. Its corporate headquarters are in San Jose, California, United 
States at eBay s North First Street satellite office campus. The company also has significant operations in Omaha, Nebraska, Scottsdale, 
Arizona, and Austin, Texas, in the United States, Chennai, Dublin, Kleinmachnow (near Berlin) and Tel Aviv. As of July 2007, across 
Europe, PayPal also operates as a Luxembourg-based bank.

On March 17, 2010, PayPal entered into an agreement with China UnionPay (CUP), China s bankcard association, to allow Chinese consumers 
to use PayPal to shop online.PayPal is planning to expand its workforce in Asia to 2,000 by the end of the year 2010.

(Copy of the Homepage: www.paypal.com) [http://en.wikipedia.org/wiki/PayPal]


Abstract Advisory Information:
==============================
The Vulnerability Laboratory Research Team member Ayoub Ait Elmokhtar discovered an Open Redirect Web Vulnerability in the official PayPal Inc Web Application.


Vulnerability Disclosure Timeline:
==================================
2015-09-11: Public Disclosure (Vulnerability Laboratory)


Discovery Status:
=================
Published


Affected Product(s):
====================
PayPal Inc - Email Edg Service


Exploitation Technique:
=======================
Remote


Severity Level:
===============
Medium


Technical Details & Description:
================================
The researcher started the research by the use of different versions of known redirect issues like for instance:

https://www.google.com/search?btnI&q=allinurl:evolution-sec.com

It will redirect you to evolution-sec.com since this is an open redirect vulnerability in Google, since Google allow open redirect and didn`t consider it in scope of Bug Bounty.

So appending this link url and visiting : email-edg.paypal.com/r/VTHX7UY/LP46GM/2OE8AOD/D9NJX6/74IKNJ/2V/h?a=https://www.google.com/search?btnI&q=allinurl:evolution-sec.com

It seems like it will redirect to evolution-sec.com but it won`t .. it will endup with a result which clearly appear to be a filter against Directory Filtering. This assumes that everything after www.domaine.com in this parameter won`t work. Well if we try accessing a subdomaine of google like http://XXX.google.com it will not work and the no redirection takes place.

So this is clearly appear to be some auth rules about redirect parameter rules :
1 - Subdomaine Filter
2 - Directory Filter

The final working POC which Bypass the Directory Filtering :

So this worked: email-edg.paypal.com/r/VTHX7UY/LP46GM/2OE8AOD/D9NJX6/74IKNJ/2V/h?a=https://www.google.com/search?btnI&q=allinurl://evil.com
This worked not:  email-edg.paypal.com/r/VTHX7UY/LP46GM/2OE8AOD/D9NJX6/74IKNJ/2V/h?a=https://www.google.com/search?btnI&q=allinurl:evil.com

You should see the bypass with // which is likely an old python library URL rules bypass.

Oauth documentation say that you shouldn`t accept subdomaines neither directories in the redirect url which appear here as like this redirect URL.


Proof of Concept (PoC):
=======================
The vulnerability can be exploited by remote attackers without low privilege application user account and only low user interaction (click).
For security demonstration or to reproduce the vulnerability follow the provided information and steps below to continue.

PoC:
email-edg.paypal.com/r/VTHX7UY/LP46GM/2OE8AOD/D9NJX6/74IKNJ/2V/h?a=https%3A//www.google.com/search%3FbtnI%26q%3Dallinurl%3A//evil.com

Note: FINAL POC (does not work anylonger because the vulnerability has been patched by paypal.


Security Risk:
==============
The security risk of the open redirect web vulnerability in the paypal online service is estimated as medium. (CVSS 3.3)


Solution:
=========
The vulnerability is already patched by the paypal security team. The issue has been part of the official bug bounty program.
The researcher received a commercial reward for the bug bounty submission to the paypal security team.


Credits & Authors:
==================
Ayoub Ait Elmokhtar  (http://www.vulnerability-lab.com/show.php?user=Ayoub%20Ait%20Elmokhtar)


Disclaimer & Information:
=========================
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