Paypal Bug Bounty #99 Cross Site Scripting

`Title:  
======  
Paypal Inc Bug Bounty #99 - Filter Bypass & Persistent Web Vulnerability  
  
  
Date:  
=====  
2013-09-20  
  
  
References:  
===========  
http://www.vulnerability-lab.com/get_content.php?id=984  
  
PayPal Security UID: nj1071UU  
  
  
VL-ID:  
=====  
984  
  
  
Common Vulnerability Scoring System:  
====================================  
3.9  
  
  
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.  
Between December 4ñ9, 2010, PayPal services were attacked in a series of denial-of-service attacks organized by Anonymous in retaliation   
for PayPal s decision to freeze the account of WikiLeaks citing terms of use violations over the publication of leaked US diplomatic cables.  
  
(Copy of the Homepage: www.paypal.com) [http://en.wikipedia.org/wiki/PayPal]  
  
  
Abstract:  
=========  
The Vulnerability Laboratory Research Team discovered a filter bypass & persistent web vulnerability in the official PayPal Inc Core Application API.  
  
  
Report-Timeline:  
================  
2013-06-24: Researcher Notification & Coordination (Ateeq ur Rehman Khan)  
2013-06-25: Vendor Notification (PayPal Inc Site Security Team)  
2013-09-10: Vendor Response/Feedback (PayPal Inc Site Security Team)  
2013-09-19: Vendor Fix/Patch (Developer Team - Bug Bounty Program Reward)  
2013-09-20: Public Disclosure (Vulnerability Laboratory)  
  
  
Status:  
========  
Published  
  
  
Affected Products:  
==================  
PayPal Inc  
Product: Core - Common [API] 2013 Q2  
  
  
Exploitation-Technique:  
=======================  
Remote  
  
  
Severity:  
=========  
Medium  
  
  
Details:  
========  
An input filter bypass & persistent script code inject vulnerability is detected in the official PayPal Inc Core Application API.  
A filter bypass vulnerability allows to evade the basic filter validation of a vulnerable application api module.  
A persistent script code inject web vulnerability allows remote attackers to inject script code on the application-side of the affected module.  
  
There is a feature in Paypal that allows its users to Set up custom message for their online customers. Paypal Users can customize   
this message so their online customers can automatically get relevant information if they to file a dispute in the Resolution Center.   
People who will get this message are usually the ones that either bought an item from a user online but said it didn`t arrive, AND OR   
tried to file a dispute in the Resolution Center for items not received or significantly not as described.   
  
It was initially noticed that there is an input box for customer description with character limitation of 1000 alphabets. The researcher   
ran some manual probes and noticed that all payloads were being filtered however only when entered on the very first line. Current filters   
can be bypassed easily If an attacker skips the first line and injects his payloads onto the 2`nd line. Once the input filters are bypassed,   
an attacker can easily inject malicious script code which gets executed successfully. The bugs allow remote attackers to implement/inject   
malicious script code on the application side (persistent) of the paypal web service.  
  
The persistent vulnerability is located in the `Your customer service message` Module of the `My Selling Tools`Section in `My Profile` Settings menu.   
The vulnerability can be exploited by remote attackers with low required user interaction and low privilege paypal application user account.   
Successful exploitation of the vulnerability can lead to session hijacking (customers), account steal via persistent web attack or stable (persistent)   
context manipulation and client side redirects to external domains hosting malware.  
  
Vulnerable Features(s):  
[+] Settings (My Profile)  
  
Vulnerable Section(s):  
[+] My Selling Tools  
  
Vulnerable Module(s):  
[+] Customer service message  
  
Vulnerable Parameter(s):  
[+] cs_description  
  
  
Proof of Concept:  
=================  
The vulnerability can be exploited by remote with low privilege paypal application user account and low required user interaction.   
  
1. Login to a Paypal Account, Goto 'More' and click on "Settings"  
2. Under the 'My Profile' section, click on "My Selling Tools"   
3. On the next page, search for "Customer service message" and click "Update"  
4. Scroll down to the "Your Customer service message" section, click your mouse once in the input box.  
5. To bypass the current security filters, press enter twice so your cursor is on the 'Second' line   
5. Enter the PoC Payload and click on Preview  
6. You should now be able to see a java-script alert box proving the existence of this vulnerability  
  
  
Payload: >"<>"<script>alert(/Vulnerable/)</script>  
  
PoC:  
</div><div id="messageBox"></div><div id="main"><div class="layout1"><p>Please review the transaction details and let   
the seller know why you're opening this dispute.</p><div class="hrline"></div></div><div style="float:left;"><div style="  
width:740px;"><div style="padding:10px 0;"><h3>"/><h1kajsfkjas</h1></h3></div><div style="text-align:center;   
word-wrap:break-word;"></div></div></div><div style="clear:both; padding:8px 0px;">  
<script>alert(/Vulnerable/)</script>  
  
  
Solution:  
=========  
Proper input user sanatization needs to be performed at the web application source code end to block all malicious script code.   
Input filter should be enforced on all 1000 characters and not just the first line of the input box.  
  
The solution is to parse in the `Customer service message` the vulnerable `cs_description` value.  
Parse the input of the vulnerable section recognize to parse also the vulnerable output validation.  
  
  
Risk:  
=====  
The security risk of the filter bypass and persistent script code inject web vulnerability is estimated as medium(+).  
  
  
Credits:  
========  
Vulnerability Laboratory [Research Team] - Ateeq ur Rehman Khan ([email protected])  
  
  
Disclaimer:  
===========  
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