A PayPal stored XSS vulnerability exposed users to hack

Security experts at BitDefender have discovered a PayPal stored XSS vulnerability that could be exploited by hackers run various attacks on PayPal users.

Security researchers at Bitdefender have discovered a PayPal stored XSS vulnerability in the e-payment service that could be exploited by attackers to upload specifically crafted files to hack registered users.

The PayPal stored XSS vulnerability can be used to deliver malicious files in order to carry on a wide range of attacks.

The PayPal store XSS vulnerability affects the way the URLs that transport uploaded files are encrypted, the experts also published a proof-of-concept of the attack to exploit the ‘Create an Invoice’ section of the PayPal platform.

“The problem lies in the way PayPal processes and encrypts URLs that pull uploaded files. Our proof-of-concept using an HTML-formatted XML file, uploading it via the “Create an Invoice” section. Because the “id” parameter for each uploaded file takes the value of a cipher text encoded in base64 and uses the CBC (cipher block chaining) ciphering mode, the attack involved changing a series of bytes from each cipher block in order to force the PayPal server to execute the malicious payload.” states the blog post published by BitDefender.

The researchers were able to manipulate the URL that pulls upload files from PayPal’s servers and force the execution of a malicious payload on PayPal’s server.

The experts created an XML file and uploaded to PayPal’s server, by tampering the URL used to transfer the produced errors and analyzed the server response to discovered the encryption mechanism.

In this way they discovered the full path to the PayPal stored XSS, then they uploaded a second file with a pre-determined name and divided into blocks of 16.

The researcher noticed that each block was changed in output due simple modifications, researchers were able to tune the a response from PayPal that resulted in a link that could be used to attack its users.

Let’s remind that “reflected file download,” vulnerability could be used to serve a file to the victims that appear as originated from the affected platform. The attack is dangerous because victims trust the file due to its origin and with trivial social engineering tricks they could be forced to open it, starting in this way the infection process.

The exploitation of the PayPal XSS flaw only worked in Firefox, the researchers confirmed that it has not been reported in the wild, but due to its disclosure PayPal promptly has issued a fix to solve the problem.

“The huge reach that cyber attackers had access to through this vulnerability was a worrying development for a service that prides itself on security.” said Catalin Cosoi, Chief Security Strategist at Bitdefender. 

Pierluigi Paganini

(Security Affairs – PayPal stored XSS , hacking)