Independent security researcher Michael Stepankin has reported a since-patched remote code execution hole in Paypal that could have allowed attackers to hijack production systems.

The critical vulnerability affecting manager.paypal.com revealed overnight was reported 13 December and patched soon after disclosure.

It allowed Stepankin to execute arbitrary shell commands on PayPal web servers through Java object deserialisation opening access to production databases.

"I immediately reported this bug to PayPal security team and it was quickly fixed after that," Stepankin says

"While security testing of manager.paypal.com, my attention was attracted by unusual post form parameter “oldFormData” that looks like a complex object after base64 decoding.

"After some research I realised that it’s a Java serialised object without any signature handled by the application [which] means that you can send serialised object of any existing class to a server and 'readObject' or 'readResolve' method of that class will be called."

Attackers would need to follow the technique disclosed by FoxGlove Security to gain remote code execution.

Stepankin says he used their 'ysoserial' payload generation tool in his attack.

PayPal handed out US$5000 for the bug even though it was a duplicate of a report sent in two days prior by researcher Mark Litchfield. Most bug bounty operators do not pay for duplicates making the payment unusual. ®