PayPal has fixed a serious vulnerability in its back-end management system that could have allowed attackers to execute arbitrary commands on the server and potentially install a backdoor.
The vulnerability is part of a class of bugs that stem from Java object deserialization and which security researchers have warned about a year ago.
In programming languages, serialization is the process of converting data to a binary format for storing it or for sending it over the network. Deserialization is the reverse of that process.
Deserialization is not an issue in itself, but like most processes that involve processing potentially untrusted input, measures need to be taken to ensure that it is performed safely. For example, an attacker could craft a serialized object that includes a Java class that the application accepts and which could be abused for something malicious.